A genome-wide association study identifies novel players in Na and Fe homeostasis in Arabidopsis thaliana under alkaline-salinity stress.

In nature, multiple stress factors occur simultaneously. The screening of natural diversity panels and subsequent Genome-Wide Association Studies (GWAS) is a powerful approach to identify genetic components of various stress responses. Here, the nutritional status variation of a set of 270 natural accessions of Arabidopsis thaliana grown on a natural saline-carbonated soil is evaluated. We report significant natural variation on leaf Na (LNa) and Fe (LFe) concentrations in the studied accessions. Allelic variation in the NINJA and YUC8 genes is associated with LNa diversity, and variation in the ALA3 is associated with LFe diversity. The allelic variation detected in these three genes leads to changes in their mRNA expression and correlates with plant differential growth performance when plants are exposed to alkaline salinity treatment under hydroponic conditions. We propose that YUC8 and NINJA expression patters regulate auxin and jasmonic signaling pathways affecting plant tolerance to alkaline salinity. Finally, we describe an impairment in growth and leaf Fe acquisition associated with differences in root expression of ALA3, encoding a phospholipid translocase active in plasma membrane and the trans Golgi network which directly interacts with proteins essential for the trafficking of PIN auxin transporters, reinforcing the role of phytohormonal processes in regulating ion homeostasis under alkaline salinity.

First Report of Shoot Blight of Grapevine Caused by Sclerotinia sclerotiorum in Illes Balears, Mallorca, Spain.

In 2021, grapevines (Vitis vinifera L.) cv. Callet growing in a commercial vineyard located at Pollença (northeast of the island of Majorca, Spain) showed severe symptoms of shoot blight during spring and early summer, with an incidence of 70%. Symptoms consisted of elongated cankered-like lesions, surrounded by water-soaked darker tissues, that developed at the base or around the middle nodes of the shoot. For fungal isolation, shoot samples with lesions were collected, surface disinfected with 2% NaCl for 90s, rinsed twice with deionized water and placed in Petri plates containing potato dextrose agar (PDA). The plates were incubated at 25°C under 12 h light-darkness for 6 days. Isolations consistently yielded on kind of fungal colonies that produced white mycelium and black spherical to elongated sclerotia (2 to 10 mm in diameter). Morphological characterization was consistent with the description of Sclerotinia sclerotiorum (Lib.) de Bary (Bolton et al. 2006). Three isolates (UIB 118-1, UIB 118-26, and UIB 129-41) were preserved and deposited in the Culture Collection of Microbiology-Faculty of Sciences, University of Balearic Islands, Spain. Genomic DNA was extracted from isolates UIB 118-26 and UIB 129-41 using the EZNA Miniprep Kit (Omega Bio-Tek, Norcross, GA). The internal transcribed spacer (ITS) region of ribosomal DNA, ß-tubulin (BTUB) and calmodulin (CAL) gene regions were amplified using ITS1F-ITS4 (Gardes and Bruns, 1996; White et al. 1990), Bt-2a/Bt-2b (Glass and Donaldson 1995) and CAL228F/CAL737R (Carbone and Kohn 1999) primer sets, respectively. Amplicons were sequenced and deposited in GenBank with accession numbers MZ604647 and MZ604648 for ITS, OK634402 and OK634403 for BTUB and OK634404 and OK634405 for CAL. BLASTn search showed that isolates were >99 % (ITS, BTUB and CAL) identical to S. sclerotiorum GenBank accession no. KF859933, CP017815 and KF871381, respectively. Pathogenicity tests were conducted using eight one-year old grapevines cv. Cabernet Sauvignon. Old and new green shoots were inoculated by inserting a 6-mm plug of mycelium taken from actively growing cultures on PDA into cuts made at the base and at the distal part of each shoot with a sterile scalpel with a total of eight inoculation points per plant. Inoculated wounds were sealed with Parafilm tape to avoid rapid dehydration. Inoculated plants and an equal number of wounded but non-inoculated plants (negative controls) were maintained at 25 ± 1°C for 48 h in plastic containers to ensure a high relative humidity (>90%). After 5 days, the infection girdled and rotted the green new shoots, whereas the older partially lignified shoots developed a localized long brown lesion that reached 16 cm in length. Due to the rotting of the basal part of the petiole, leaves turned gray, wilted, and died, easily detaching from the stem. In advanced stages of the disease, 7 days after infection, branches died and fell with the leaves remained attached (Fig 1 A, B). Reisolations from diseased shoots were successfully performed on PDA to fulfill Koch's postulates. S. slerotiorum was previously reported on grapevine causing shoot blight in Chile (Latorre and Guerrero, 2001), Korea (Jong-Han et al. 2009), California-USA (Boland and Hall, 1994) and Australia (Hall et al. 2002). AlsoS. sclerotiorum was reported among the endophytic mycobiota associated with Vitis vinifera in the Iberian Peninsula (Gonzalez and Tello, 2011) but not as a pathogen causing visible symptoms on that crop. So, this is the first report of the occurrence of S. slerotiorum as a pathogen of grapevines in Spain causing symptoms of canker and shoot blight. This finding highlights a potential risk of this fungal disease for the wine industry in the Mediterranean region and specially for Spain, the country with the largest acreage devoted to grapevines. Although chemical and biological are suitable control strategies, disease management is difficult as sclerotia of Sclerotinia can remain in the soil for up to eight years (Adams and Ayears, 1979), and preventive surveys are greatly recommended as an important epidemiological tool to monitor the epidemiology of disease and identify potential outbreaks of this new pathogen on grapevine in Spain.

Combined genomics to discover genes associated with tolerance to soil carbonate.

Carbonate-rich soils limit plant performance and crop production. Previously, local adaptation to carbonated soils was detected in wild Arabidopsis thaliana accessions, allowing the selection of two demes with contrasting phenotypes: A1 (carbonate tolerant, c+) and T6 (carbonate sensitive, c-). Here, A1(c+) and T6(c - ) seedlings were grown hydroponically under control (pH 5.9) and bicarbonate conditions (10 mM NaHCO3 , pH 8.3) to obtain ionomic profiles and conduct transcriptomic analysis. In parallel, A1(c+) and T6(c - ) parental lines and their progeny were cultivated on carbonated soil to evaluate fitness and segregation patterns. To understand the genetic architecture beyond the contrasted phenotypes, a bulk segregant analysis sequencing (BSA-Seq) was performed. Transcriptomics revealed 208 root and 2503 leaf differentially expressed genes in A1(c+) versus T6(c - ) comparison under bicarbonate stress, mainly involved in iron, nitrogen and carbon metabolism, hormones and glycosylates biosynthesis. Based on A1(c+) and T6(c - ) genome contrasts and BSA-Seq analysis, 69 genes were associated with carbonate tolerance. Comparative analysis of genomics and transcriptomics discovered a final set of 18 genes involved in bicarbonate stress responses that may have relevant roles in soil carbonate tolerance.

Adaptation to coastal soils through pleiotropic boosting of ion and stress hormone concentrations in wild Arabidopsis thaliana.

Local adaptation in coastal areas is driven chiefly by tolerance to salinity stress. To survive high salinity, plants have evolved mechanisms to specifically tolerate sodium. However, the pathways that mediate adaptive changes in these conditions reach well beyond Na+ . Here we perform a high-resolution genetic, ionomic, and functional study of the natural variation in Molybdenum transporter 1 (MOT1) associated with coastal Arabidopsis thaliana accessions. We quantify the fitness benefits of a specific deletion-harbouring allele (MOT1DEL ) present in coastal habitats that is associated with lower transcript expression and molybdenum accumulation. Analysis of the leaf ionome revealed that MOT1DEL plants accumulate more copper (Cu) and less sodium (Na+ ) than plants with the noncoastal MOT1 allele, revealing a complex interdependence in homeostasis of these three elements. Our results indicate that under salinity stress, reduced MOT1 function limits leaf Na+ accumulation through abscisic acid (ABA) signalling. Enhanced ABA biosynthesis requires Cu. This demand is met in Cu deficient coastal soils through MOT1DEL increasing the expression of SPL7 and the copper transport protein COPT6. MOT1DEL is able to deliver a pleiotropic suite of phenotypes that enhance salinity tolerance in coastal soils deficient in Cu. This is achieved by inducing ABA biosynthesis and promoting reduced uptake or better compartmentalization of Na+ , leading to coastal adaptation.

Fluctuating selection on migrant adaptive sodium transporter alleles in coastal Arabidopsis thaliana.

Stressors such as soil salinity and dehydration are major constraints on plant growth, causing worldwide crop losses. Compounding these insults, increasing climate volatility requires adaptation to fluctuating conditions. Salinity stress responses are relatively well understood in Arabidopsis thaliana, making this system suited for the rapid molecular dissection of evolutionary mechanisms. In a large-scale genomic analysis of Catalonian A. thaliana, we resequenced 77 individuals from multiple salinity gradients along the coast and integrated these data with 1,135 worldwide A. thaliana genomes for a detailed understanding of the demographic and evolutionary dynamics of naturally evolved salinity tolerance. This revealed that Catalonian varieties adapted to highly fluctuating soil salinity are not Iberian relicts but instead have immigrated to this region more recently. De novo genome assembly of three allelic variants of the high-affinity K+ transporter (HKT1;1) locus resolved structural variation between functionally distinct alleles undergoing fluctuating selection in response to seasonal changes in soil salinity. Plants harboring alleles responsible for low root expression of HKT1;1 and consequently high leaf sodium (HKT1;1HLS ) were migrants that have moved specifically into areas where soil sodium levels fluctuate widely due to geography and rainfall variation. We demonstrate that the proportion of plants harboring HKT1;1HLS alleles correlates with soil sodium level over time, HKT1;1HLS -harboring plants are better adapted to intermediate levels of salinity, and the HKT1;1HLS allele clusters with high-sodium accumulator accessions worldwide. Together, our evidence suggests that HKT1;1 is under fluctuating selection in response to climate volatility and is a worldwide determinant in adaptation to saline conditions.

Transcriptomics Reveals Fast Changes in Salicylate and Jasmonate Signaling Pathways in Shoots of Carbonate-Tolerant Arabidopsis thaliana under Bicarbonate Exposure.

High bicarbonate concentrations of calcareous soils with high pH can affect crop performance due to different constraints. Among these, Fe deficiency has mostly been studied. The ability to mobilize sparingly soluble Fe is a key factor for tolerance. Here, a comparative transcriptomic analysis was performed with two naturally selected Arabidopsis thaliana demes, the carbonate-tolerant A1(c+) and the sensitive T6(c-). Analyses of plants exposed to either pH stress alone (pH 5.9 vs. pH 8.3) or to alkalinity caused by 10 mM NaHCO3 (pH 8.3) confirmed better growth and nutrient homeostasis of A1(c+) under alkaline conditions. RNA-sequencing (RNA-seq) revealed that bicarbonate quickly (3 h) induced Fe deficiency-related genes in T6(c-) leaves. Contrastingly, in A1(c+), initial changes concerned receptor-like proteins (RLP), jasmonate (JA) and salicylate (SA) pathways, methionine-derived glucosinolates (GS), sulfur starvation, starch degradation, and cell cycle. Our results suggest that leaves of carbonate-tolerant plants do not sense iron deficiency as fast as sensitive ones. This is in line with a more efficient Fe translocation to aerial parts. In A1(c+) leaves, the activation of other genes related to stress perception, signal transduction, GS, sulfur acquisition, and cell cycle precedes the induction of iron homeostasis mechanisms yielding an efficient response to bicarbonate stress.

The arbuscular mycorrhizal mycelium from barley differentially influences various defense parameters in the non-host sugar beet under co-cultivation.

The interactions between arbuscular mycorrhizal fungi (AMF) and non-host species are poorly studied. Particularly scarce is information on members of the Amaranthaceae/Chenopodiaceae family. Sugar beet (Beta vulgaris) plants were co-cultivated with a host species (Hordeum vulgare) in the presence (+AMF) or absence of Rhizophagus intraradices to explore the hypothesis that the presence of an active, pre-established AMF mycelium induces defense responses in the non-host species. Biomass of sugar beet did not respond to the +AMF treatment, while its root exudation of organic acids and phenolic acids was drastically decreased upon co-cultivation with +AMF barley. The most conspicuous effect was observed on a wide range of potential defense parameters being differentially influenced by the +AMF treatment in this non-host species. Antioxidant defense enzymes were activated and the level of endogenous jasmonic acid was elevated accompanied by nitric oxide accumulation and lignin deposition in the roots after long-term +AMF treatment. In contrast, significant reductions in the levels of endogenous salicylic acid and tissue concentration and exudation of phenolic acids indicated that AM fungus hyphae in the substrate did not induce a hypersensitive-type response in the sugar beet roots and downregulated certain chemical defenses. Our results imply that the fitness of this non-host species is not reduced when grown in the presence of an AMF mycelium because of balanced defense costs. Further studies should address the question of whether or not such modulation of defense pattern influences the pest resistance of sugar beet plants under field conditions.

Soil carbonate drives local adaptation in Arabidopsis thaliana.

High soil carbonate limits crop performance especially in semiarid or arid climates. To understand how plants adapt to such soils, we explored natural variation in tolerance to soil carbonate in small local populations (demes) of Arabidopsis thaliana growing on soils differing in carbonate content. Reciprocal field-based transplants on soils with elevated carbonate (+C) and without carbonate (-C) over several years revealed that demes native to (+C) soils showed higher fitness than those native to (-C) soils when both were grown together on carbonate-rich soil. This supports the role of soil carbonate as a driving factor for local adaptation. Analyses of contrasting demes revealed key mechanisms associated with these fitness differences. Under controlled conditions, plants from the tolerant deme A1(+C) native to (+C) soil were more resistant to both elevated carbonate and iron deficiency than plants from the sensitive T6(-C) deme native to (-C) soil. Resistance of A1(+C) to elevated carbonate was associated with higher root extrusion of both protons and coumarin-type phenolics. Tolerant A1(+C) also had better Ca-exclusion than sensitive T6(-C) . We conclude that Arabidopsis demes are locally adapted in their native habitat to soils with moderately elevated carbonate. This adaptation is associated with both enhanced iron acquisition and calcium exclusion.

Snails prefer it sweet: A multifactorial test of the metal defence hypothesis.

Metal defence against insect herbivory in hyperaccumulator plants is well documented. However, there are contradictory results regarding protection against snails. According to the joint effects hypothesis, inorganic and organic defences cooperate in plant protection. To test this hypothesis, we explored the relationships between snail (Cantareus aspersus) feeding and multiple inorganic and organic leaf components in the Cd hyperaccumulator plant Noccaea praecox. Plants grouped by rosette size growing in nutrient solution supplemented or not with 50 µM Cd were offered to the snails. After 3 days of snail feeding, the plants and snails were analysed. In addition to Cd concentrations, we analysed leaves for nutritional factors (sugar and protein), defence-related compounds (glucosinolates, phenolics, tannins, salicylic acid and jasmonate) and essential mineral nutrients. Cadmium concentrations in the snails and in snail excrements were also analysed. Snails preferentially fed on plants grown without Cd. Medium-sized plants exposed to Cd were the least consumed. Snail excrements from this trial weighed less and had higher Cd concentrations than those from other treatments. Cadmium increased salicylate and jasmonate production. A positive relationship between jasmonate levels and the number of attacked leaves was found. Principal component analysis revealed that leaf sugar concentration was the main factor positively affecting snails' leaf consumption, while leaf Cd had a negative but weaker influence. In conclusion, leaf sugar concentration mainly governs snails' feeding preferences. High leaf Cd concentrations do not deter herbivores from attacking leaves, but they do reduce leaf consumption. Our results clearly support the joint effects hypothesis.

How Plants Handle Trivalent (+3) Elements.

Plant development and fitness largely depend on the adequate availability of mineral elements in the soil. Most essential nutrients are available and can be membrane transported either as mono or divalent cations or as mono- or divalent anions. Trivalent cations are highly toxic to membranes, and plants have evolved different mechanisms to handle +3 elements in a safe way. The essential functional role of a few metal ions, with the possibility to gain a trivalent state, mainly resides in the ion's redox activity; examples are iron (Fe) and manganese. Among the required nutrients, the only element with +3 as a unique oxidation state is the non-metal, boron. However, plants also can take up non-essential trivalent elements that occur in biologically relevant concentrations in soils. Examples are, among others, aluminum (Al), chromium (Cr), arsenic (As), and antimony (Sb). Plants have evolved different mechanisms to take up and tolerate these potentially toxic elements. This review considers recent studies describing the transporters, and specific and unspecific channels in different cell compartments and tissues, thereby providing a global vision of trivalent element homeostasis in plants.

Salt tolerance mechanisms in three Irano-Turanian Brassicaceae halophytes relatives of Arabidopsis thaliana.

Salt tolerance mechanisms were studied in three Irano-Turanian halophytic species from the Brassicaceae (Lepidium latifolium, L. perfoliatum and Schrenkiella parvula) and compared with the glycophyte Arabidopsis thaliana. According to seed germination under salt stress, L. perfoliatum was the most tolerant species, while L. latifolium and S. parvula were rather susceptible. Contrastingly, based on biomass production L. perfoliatum was more salt sensitive than the other two species. In S. parvula biomass was increased up to 2.8-fold by 100 mM NaCl; no significant growth reduction was observed even when exposed to 400 mM NaCl. Stable activities of antioxidative defense enzymes, nil or negligible accumulation of superoxide anion and hydrogen peroxide, as well as stable membrane integrity in the three halophytes revealed that no oxidative stress occurred in these tolerant species under salt stress. Proline levels increased in response to salt treatment. However, it contributed only by 0.3‒2.0% to the total osmolyte concentration in the three halophytes (at 400 mM NaCl) and even less (0.04%) in the glycophyte, A. thaliana (at 100 mM NaCl). Soluble sugars in all three halophytes and free amino acids pool in S. parvula decreased under salt treatment in contrast to the glycophyte, A. thaliana. The contribution of organic osmolytes to the total osmolyte pool increased by salt treatment in the roots, while decreased in halophyte and glycophyte, A. thaliana leaves. Interestingly, this reduction was compensated by a higher relative contribution of K in the leaves of the halophytes, but of Na in A. thaliana. Taken together, biomass data and biochemical indicators show that S. parvula is more salt tolerant than the two Lepidium species. Our data indicate that L. latifolium, as a perennial halophyte with a large biomass, is highly suitable for both restoration of saline habitats and saline agriculture.

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead.

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO3− may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO2-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C4 and Crassulacean Acid Metabolism (CAM). The presence of HCO3− in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO3− in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO3− transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO3− in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO2/HCO3− in stomatal guard cells. Plant responses to excess soil HCO3− is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO3− uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO3− tolerance in crop plants.

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress.

While the importance of cell type specificity in plant adaptive responses is widely accepted, only a limited number of studies have addressed this issue at the functional level. We have combined electrophysiological, imaging, and biochemical techniques to reveal the physiological mechanisms conferring higher sensitivity of apical root cells to salinity in barley (Hordeum vulgare). We show that salinity application to the root apex arrests root growth in a highly tissue- and treatment-specific manner. Although salinity-induced transient net Na+ uptake was about 4-fold higher in the root apex compared with the mature zone, mature root cells accumulated more cytosolic and vacuolar Na+, suggesting that the higher sensitivity of apical cells to salt is not related to either enhanced Na+ exclusion or sequestration inside the root. Rather, the above differential sensitivity between the two zones originates from a 10-fold difference in K+ efflux between the mature zone and the apical region (much poorer in the root apex) of the root. Major factors contributing to this poor K+ retention ability are (1) an intrinsically lower H+-ATPase activity in the root apex, (2) greater salt-induced membrane depolarization, and (3) a higher reactive oxygen species production under NaCl and a larger density of reactive oxygen species-activated cation currents in the apex. Salinity treatment increased (2- to 5-fold) the content of 10 (out of 25 detected) amino acids in the root apex but not in the mature zone and changed the organic acid and sugar contents. The causal link between the observed changes in the root metabolic profile and the regulation of transporter activity is discussed.

Zinc hyperaccumulation substitutes for defense failures beyond salicylate and jasmonate signaling pathways of Alternaria brassicicola attack in Noccaea caerulescens.

The hypothesis of metal defense as a substitute for a defective biotic stress signaling system in metal hyperaccumulators was tested using the pathosystem Alternaria brassicicola-Noccaea caerulescens under low (2 µM), medium (12 µM) and high (102 µM) Zn supply. Regardless the Zn supply, N. caerulescens responded to fungal attack with the activation of both HMA4 coding for a Zn transporter, and biotic stress signaling pathways. Salicylate, jasmonate, abscisic acid and indoleacetic acid concentrations, as well as biotic stress marker genes (PDF1.2, CHIB, LOX2, PR1 and BGL2) were activated 24 h upon inoculation. Based on the activation of defense genes 24 h after the inoculation an incompatible fungal-plant interaction could be predicted. Nonetheless, in the longer term (7 days) no effective protection against A. brassicicola was achieved in plants exposed to low and medium Zn supply. After 1 week the biotic stress markers were even further increased in these plants, and this compatible interaction was apparently not caused by a failure in the signaling of the fungal attack, but due to the lack of specificity in the type of the activated defense mechanisms. Only plants receiving high Zn exhibited an incompatible fungal interaction. High Zn accumulation in these plants, possibly in cooperation with high glucosinolate concentrations, substituted for the ineffective defense system and the interaction turned into incompatible. In a threshold-type response, these joint effects efficiently hampered fungal spread and, consequently decreased the biotic stress signaling.

High salinity helps the halophyte Sesuvium portulacastrum in defense against Cd toxicity by maintaining redox balance and photosynthesis.

MAIN CONCLUSION: NaCl alleviates Cd toxicity in Sesvium portulacastrum by maintaining plant water status and redox balance, protecting chloroplasts structure and inducing some potential Cd (2+) chelators as GSH and proline. It has been demonstrated that NaCl alleviates Cd-induced growth inhibition in the halophyte Sesuvium portulacastrum. However, the processes that mediate this effect are still unclear. In this work we combined physiological, biochemical and ultrastructural studies to highlight the effects of salt on the redox balance and photosynthesis in Cd-stressed plants. Seedlings were exposed to different Cd concentrations (0, 25 and 50 µM Cd) combined with low (0.09 mM) (LS), or high (200 mM) NaCl (HS) in hydroponic culture. Plant-water relations, photosynthesis rate, leaf gas exchange, chlorophyll fluorescence, chloroplast ultrastructure, and proline and glutathione concentrations were analyzed after 1 month of treatment. In addition, the endogenous levels of stress-related hormones were determined in plants subjected to 25 µM Cd combined with both NaCl concentrations. In plants with low salt supply (LS), Cd reduced growth, induced plant dehydration, disrupted chloroplast structure and functioning, decreased net CO2 assimilation rate (A) and transpiration rate (E), inhibited the maximum potential quantum efficiency (Fv/Fm) and the quantum yield efficiency (Φ PSII) of PSII, and enhanced the non-photochemical quenching (NPQ). The addition of 200 mM NaCl (HS) to the Cd-containing medium culture significantly mitigated Cd phytotoxicity. Hence, even at similar internal Cd concentrations, HS-Cd plants were less affected by Cd than LS-Cd ones. Hence, 200 mM NaCl significantly alleviates Cd-induced toxicity symptoms, growth inhibition, and photosynthesis disturbances. The cell ultrastructure was better preserved in HS-Cd plants but affected in LS-Cd plants. The HS-Cd plants showed also higher concentrations of reduced glutathione (GSH), proline and jasmonic acid (JA) than the LS-Cd plants. However, under LS-Cd conditions, plants maintained higher concentration of salicylic acid (SA) and abscisic acid (ABA) than the HS-Cd ones. We conclude that in S. portulacastrum alleviation of Cd toxicity by NaCl is related to the modification of GSH and proline contents as well as stress hormone levels thus protecting redox balance and photosynthesis.

Salinity Is an Agent of Divergent Selection Driving Local Adaptation of Arabidopsis to Coastal Habitats.

Understanding the molecular mechanism of adaptive evolution in plants provides insights into the selective forces driving adaptation and the genetic basis of adaptive traits with agricultural value. The genomic resources available for Arabidopsis (Arabidopsis thaliana) make it well suited to the rapid molecular dissection of adaptive processes. Although numerous potentially adaptive loci have been identified in Arabidopsis, the consequences of divergent selection and migration (both important aspects of the process of local adaptation) for Arabidopsis are not well understood. Here, we use a multiyear field-based reciprocal transplant experiment to detect local populations of Arabidopsis composed of multiple small stands of plants (demes) that are locally adapted to the coast and adjacent inland habitats in northeastern Spain. We identify fitness tradeoffs between plants from these different habitats when grown together in inland and coastal common gardens and also, under controlled conditions in soil excavated from coastal and inland sites. Plants from the coastal habitat also outperform those from inland when grown under high salinity, indicating local adaptation to soil salinity. Sodium can be toxic to plants, and we find its concentration to be elevated in soil and plants sampled at the coast. We conclude that the local adaptation that we observe between adjacent coastal and inland populations is caused by ongoing divergent selection driven by the differential salinity between coastal and inland soils.

Estimation of phenotypic variability in symbiotic nitrogen fixation ability of common bean under drought stress using 15N natural abundance in grain.

Common bean (Phaseolus vulgaris L.) is the most important food legume, cultivated by small farmers and is usually exposed to unfavorable conditions with minimum use of inputs. Drought and low soil fertility, especially phosphorus and nitrogen (N) deficiencies, are major limitations to bean yield in smallholder systems. Beans can derive part of their required N from the atmosphere through symbiotic nitrogen fixation (SNF). Drought stress severely limits SNF ability of plants. The main objectives of this study were to: (i) test and validate the use of 15N natural abundance in grain to quantify phenotypic differences in SNF ability for its implementation in breeding programs of common bean with bush growth habit aiming to improve SNF, and (ii) quantify phenotypic differences in SNF under drought to identify superior genotypes that could serve as parents. Field studies were conducted at CIAT-Palmira, Colombia using a set of 36 bean genotypes belonging to the Middle American gene pool for evaluation in two seasons with two levels of water supply (irrigated and drought stress). We used 15N natural abundance method to compare SNF ability estimated from shoot tissue sampled at mid-pod filling growth stage vs. grain tissue sampled at harvest. Our results showed positive and significant correlation between nitrogen derived from the atmosphere (%Ndfa) estimated using shoot tissue at mid-pod filling and %Ndfa estimated using grain tissue at harvest. Both methods showed phenotypic variability in SNF ability under both drought and irrigated conditions and a significant reduction in SNF ability was observed under drought stress. We suggest that the method of estimating Ndfa using grain tissue (Ndfa-G) could be applied in bean breeding programs to improve SNF ability. Using this method of Ndfa-G, we identified four bean lines (RCB 593, SEA 15, NCB 226 and BFS 29) that combine greater SNF ability with greater grain yield under drought stress and these could serve as potential parents to further improve SNF ability of common bean.

The NPR1-dependent salicylic acid signalling pathway is pivotal for enhanced salt and oxidative stress tolerance in Arabidopsis.

The role of endogenous salicylic acid (SA) signalling cascades in plant responses to salt and oxidative stresses is unclear. Arabidopsis SA signalling mutants, namely npr1-5 (non-expresser of pathogenesis related gene1), which lacks NPR1-dependent SA signalling, and nudt7 (nudix hydrolase7), which has both constitutively expressed NPR1-dependent and NPR1-independent SA signalling pathways, were compared with the wild type (Col-0) during salt or oxidative stresses. Growth and viability staining showed that, compared with wild type, the npr1-5 mutant was sensitive to either salt or oxidative stress, whereas the nudt7 mutant was tolerant. Acute salt stress caused the strongest membrane potential depolarization, highest sodium and proton influx, and potassium loss from npr1-5 roots in comparison with the wild type and nudt7 mutant. Though salt stress-induced hydrogen peroxide production was lowest in the npr1-5 mutant, the reactive oxygen species (ROS) stress (induced by 1mM of hydroxyl-radical-generating copper-ascorbate mix, or either 1 or 10mM hydrogen peroxide) caused a higher potassium loss from the roots of the npr1-5 mutant than the wild type and nudt7 mutant. Long-term salt exposure resulted in the highest sodium and the lowest potassium concentration in the shoots of npr1-5 mutant in comparison with the wild type and nudt7 mutant. The above results demonstrate that NPR1-dependent SA signalling is pivotal to (i) controlling Na(+) entry into the root tissue and its subsequent long-distance transport into the shoot, and (ii) preventing a potassium loss through depolarization-activated outward-rectifying potassium and ROS-activated non-selective cation channels. In conclusion, NPR1-dependent SA signalling is central to the salt and oxidative stress tolerance in Arabidopsis.

Membrane transporters mediating root signalling and adaptive responses to oxygen deprivation and soil flooding.

This review provides a comprehensive assessment of a previously unexplored topic: elucidating the role that plasma- and organelle-based membrane transporters play in plant-adaptive responses to flooding. We show that energy availability and metabolic shifts under hypoxia and anoxia are critical in regulating membrane-transport activity. We illustrate the high tissue and time dependence of this regulation, reveal the molecular identity of transporters involved and discuss the modes of their regulation. We show that both reduced oxygen availability and accumulation of transition metals in flooded roots result in a reduction in the cytosolic K(+) pool, ultimately determining the cell's fate and transition to programmed cell death (PCD). This process can be strongly affected by hypoxia-induced changes in the amino acid pool profile and, specifically, ϒ-amino butyric acid (GABA) accumulation. It is suggested that GABA plays an important regulatory role, allowing plants to proceed with H2 O2 signalling to activate a cascade of genes that mediate plant adaptation to flooding while at the same time, preventing the cell from entering a 'suicide program'. We conclude that progress in crop breeding for flooding tolerance can only be achieved by pyramiding the numerous physiological traits that confer efficient energy maintenance, cytosolic ion homeostasis, and reactive oxygen species (ROS) control and detoxification.

Kinetics of xylem loading, membrane potential maintenance, and sensitivity of K(+) -permeable channels to reactive oxygen species: physiological traits that differentiate salinity tolerance between pea and barley.

Salt sensitive (pea) and salt tolerant (barley) species were used to understand the physiological basis of differential salinity tolerance in crops. Pea plants were much more efficient in restoring otherwise depolarized membrane potential thereby effectively decreasing K(+) efflux through depolarization-activated outward rectifying potassium channels. At the same time, pea root apex was 10-fold more sensitive to physiologically relevant H2 O2 concentration and accumulated larger amounts of H2 O2 under saline conditions. This resulted in a rapid loss of cell viability in the pea root apex. Barley plants rapidly loaded Na(+) into the xylem; this increase was only transient, and xylem and leaf Na(+) concentration remained at a steady level for weeks. On the contrary, pea plants restricted xylem Na(+) loading during the first few days of treatment but failed to prevent shoot Na(+) elevation in the long term. It is concluded that superior salinity tolerance of barley plants compared with pea is conferred by at least three different mechanisms: (1) efficient control of xylem Na(+) loading; (2) efficient control of H2 O2 accumulation and reduced sensitivity of non-selective cation channels to H2 O2 in the root apex; and (3) higher energy saving efficiency, with less ATP spent to maintain membrane potential under saline conditions.