Drought-adapted leaves are produced even when more water is available in dry tropical forest.

Species in dry environments may adjust their anatomical and physiological behaviors by adopting safer or more efficient strategies. Thus, species distributed across a water availability gradient may possess different phenotypes depending on the specific environmental conditions to which they are subjected. Leaf and vascular tissues are plastic and may vary strongly in response to environmental changes affecting an individual's survival and species distribution. To identify whether and how legumes leaves vary across a water availability gradient in a seasonally dry tropical forest, we quantified leaf construction costs and performed an anatomical study on the leaves of seven legume species. We evaluated seven species, which were divided into three categories of rainfall preference: wet species, which are more abundant in wetter areas; indifferent species, which are more abundant and occur indistinctly under both rainfall conditions; and dry species, which are more abundant in dryer areas. We observed two different patterns based on rainfall preference categories. Contrary to our expectations, wet and indifferent species changed traits in the sense of security when occupying lower rainfall areas, whereas dry species changed some traits when more water was available, such as increasing cuticle and spongy parenchyma thickness, or producing smaller and more numerous stomata. Trischidium molle, the most plastic and wet species, exhibited a similar strategy to the dry species. Our results corroborate the risks to vegetation under future climate change scenarios as stressed species and populations may not endure even more severe conditions.

Dehydration response in Stylosanthes scabra: Transcriptional, biochemical, and physiological modulations.

Stylosanthes scabra, popularly known as stylo, is native to the Brazilian Caatinga semiarid region and stands out as a drought-tolerant shrub forage crop. This work provides information about the plant response during the first 48 h of water deficit, followed by a rehydration treatment. Besides root transcriptomics data, 13 physiological or biochemical parameters were scrutinized. Additionally, RNA-Seq annotated transcripts not associated with the "Viridiplantae" clade were taxonomically categorized. It was found that S. scabra quickly perceives and recovers from the oscillations of the imposed water regime. Physiologically, mechanisms that minimize evapotranspiration or protect the photosynthetic apparatus stood out. Biochemically, it was found that the root tissue invests in synthesizing compounds that can act as osmolytes (proline and sugars), emphasizing the importance of osmoregulation to water deficit acclimation. Consistently, transcriptome and qPCR analyses showed that a set of enriched biological processes with upregulated (UR) transcripts were involved in protective functions against reactive oxygen species or encoding enzymes of important metabolic pathways, which might contribute to S. scabra response to water deficit. Additionally, several UR kinases and transcription factors were identified. Finally, in an innovative approach, some naturally occurring microbial groups (such as Schizosaccharomyces, Bradyrhizobium, etc.) were identified in the S. scabra roots. This study reveals insights into the physiological, biochemical, and molecular mechanisms underlying the S. scabra response to water deficit and provides candidate genes that may be useful in developing drought-tolerant crop varieties through biotechnological applications.

Transcriptome of Cenostigma pyramidale roots, a woody legume, under different salt stress times.

Salinity stress has a significant impact on the gain of plant biomass. Our study provides the first root transcriptome of Cenostigma pyramidale, a tolerant woody legume from a tropical dry forest, under three different salt stress times (30 min, 2 h, and 11 days). The transcriptome was assembled using the RNA sequencing (RNA-Seq) de novo pipeline from GenPipes. We observed 932, 804, and 3157 upregulated differentially expressed genes (DEGs) and 164, 273, and 1332 downregulated DEGs for salt over 30 min, 2 h, and 11 days, respectively. For DEGs annotated with the Viridiplantae clade in the early stress periods, the response to salt stress was mainly achieved by stabilizing homeostasis of such ions like Na+ and K+ , signaling by Ca2+ , transcription factor modulation, water transport, and oxidative stress. For salt stress at 11 days, we observed a higher modulation of transcription factors including the WRKY, MYB, bHLH, NAC, HSF, and AP2-EREBP families, as well as DEGs involved in hormonal responses, water transport, sugar metabolism, proline, and reactive oxygen scavenging mechanisms. Five selected DEGs (K+ transporter, aquaporin, glutathione S-transferase, cyclic nucleotide-gated channel, and superoxide dismutase) were validated by qPCR. Our results indicated that C. pyramidale had an early perception of salt stress modulating ionic channels and transporters, and as the stress progressed, the focus turned to the antioxidant system, aquaporins, and complex hormone responses. The results of this first root transcriptome provide clues on how this native species modulate gene expression to achieve salt stress tolerance.

Reference genes for quantitative real-time PCR normalization of Cenostigma pyramidale roots under salt stress and mycorrhizal association.

Cenostigma pyramidale is a native legume of the Brazilian semiarid region which performs symbiotic association with arbuscular mycorrhizal fungi (AMF), being an excellent model for studying genes associated with tolerance against abiotic and biotic stresses. In RT-qPCR approach, the use of reference genes is mandatory to avoid incorrect interpretation of the relative expression. This study evaluated the stability of ten candidate reference genes (CRGs) from C. pyramidale root tissues under salt stress (three collection times) and associated with AMF (three different times of salinity). The de novo transcriptome was obtained via RNA-Seq sequencing. Three algorithms were used to calculate the stability of CRGs under different conditions: (i) global (Salt, Salt+AMF, AMF and Control, and collection times), (ii) only non-inoculated plants, and (iii) AMF (only inoculated plants). HAG2, SAC1, aRP3 were the most stable CRGs for global and AMF assays, whereas HAG2, SAC1, RHS1 were the best for salt stress assay. This CRGs were used to validate the relative expression of two up-regulated transcripts in Salt2h (RAP2-3 and PIN8). Our study provides the first set of reference genes for C. pyramidale under salinity and AMF, supporting future researches on gene expression with this species.

Remobilization of leaf Na+ content and use of nonstructural carbohydrates vary depending on the time when salt stress begins in woody species.

Basic mechanisms are known to promote salt tolerance in plants: a delay in Na+ uptake or rapid Na+ remobilization from leaf tissue. We measured dynamics of the Na+/K+ ratio and components of carbon metabolism during the first 72 h after saline stress (200 mM NaCl) began in Cenostigma pyramidale, a woody species, under controlled conditions. Saline stress at two times: one plant group at the beginning of the morning and the other in the evening. Stressed plants had three times more Na+ in leaves than did control plants in the first 24 h. However, in the next few hours, despite new applications of saline solution, the Na+/K+ ratio continued to decline. Several samples, including night treatments, provided evidence that this species uses Na+ recirculation mechanisms to endure salt stress. Effects of salt on the traits evaluated differed depending on the time when stress began. Between the two saline treatments, in the first 24 h after saline stress, gas exchange decreased more strongly in morning-stressed plants, when large amounts of Na+ reached the leaf and K+ left this organ. Nevertheless, when stress was applied in the evening, leaf Na+ remobilization was faster, and the soluble sugar/starch ratio remained greater than did the control. Our data suggested that time of the beginning of salt stress could change the level of damage. Morning-stressed plants synthesized greater amounts of proline, H2O2, and malondialdehyde than did night-stressed plants. We recommend that details regarding the time of stress be taken into consideration in physiological studies.

Different resource-use strategies of invasive and native woody species from a seasonally dry tropical forest under drought stress and recovery.

Exotic plants in semiarid region have developed strategies for efficient use or capture of resources. They have become invasive and outperform native species. To understand which factors could explain the success of invasive woody species in a semiarid region, several physiological traits were analyzed in young plants of two invasive and two native species exposed to different water availability. Invasive plants showed low leaf construction cost, high phosphorus and nitrogen contents, reduced loss of instantaneous energy use efficiency, and smaller specific leaf area when compared to native species. This strategy led to a higher biomass gain and a high root/shoot ratio in both water treatments. After rehydration, invasive plants showed faster recovery and higher rates of CO2 assimilation. This resilience is fundamental for species in semiarid regions, and also increase uptake of nutrients. Maintaining a high photosynthetic rate, whenever there is water availability is a strategy that increases the performance of the species in relation to biomass gain. The low leaf construction cost and the fast recovery of the photosynthetic metabolism of invasive plants after limiting water resources explains the success of these species, and suggests that their potential may increase under prolonged and severe drought seasons.

Daily balance of leaf sugars and amino acids as indicators of common bean (Phaseolus vulgaris L.) metabolic response and drought intensity.

Leaf soluble sugar, starch and free amino acid amounts were evaluated in two bean genotypes, Carioca and Ouro Negro, grown in 10 L pots in a greenhouse. This was realized during a single day for Carioca and during ten days of water deficit for both genotypes, at 06:00 and 18:00 h. During the day, an increase in all parameters occurred up to midday, while in the afternoon, carbohydrate amounts varied in opposition to amino acids amounts. Under water deficit, the leaf soluble sugars at 18:00 and their daily balance (amount at 18:00 minus amount at 06:00) increased on the 2(nd) day, with starch and amino acid amounts and their respective daily balances decreasing. On the 8(th) and 10(th) days, the soluble sugar daily balance was negative and the starch daily balance approached zero or was negative, while the amino acid daily balance was positive. During rehydration, soluble sugar daily balance was close to zero and starch daily balance increased, whereas amino acid daily balance was negative. The carbohydrate daily balance, strongly related to free amino acid daily balance, were good indicators for evaluating the stage of water deficit and metabolic capacity to respond to the stress in common bean.

Arbuscular mycorrhizal fungi and foliar phosphorus inorganic supply alleviate salt stress effects in physiological attributes, but only arbuscular mycorrhizal fungi increase biomass in woody species of a semiarid environment.

Salinity may limit plant growth especially in arid and semiarid regions. Arbuscular mycorrhizal fungi (AMF) and the supply of inorganic phosphorus (Pi) could alleviate the negative effects of such stress by improvement in stomatal conductance, photosynthesis and biomass. The aim of this study is to evaluate the ecophysiological performance of Cenostigma pyramidale (Tul.) E. Gagnon & G. P. Lewis (Fabaceae) in a greenhouse under salinity conditions in combination with the supply of AMF and leaf Pi. The experiment was conducted in a factorial design considering two levels of salinity (+NaCl and -NaCl), two levels of AMF (+AMF and -AMF) and two levels of leaf Pi supply (+Pi and -Pi). The variables gas exchange, leaf primary metabolism, dry biomass and nutrients were measured. Plants with AMF under non-saline conditions presented a high photosynthesis and biomass. In saline conditions, AMF promoted lower decrease in photosynthesis, high shoot dry matter and low content of leaf and root Na+ and Cl-. Plants treated with leaf Pi increased biomass and photosynthetic pigments under both conditions and accumulated more Cl- in shoots under salinity conditions. When combined, AMF * Pi increased photosynthesis only in non-saline conditions. Plants under salinity conditions without AMF and Pi had higher decreases in gas exchange and high content of Cl- in roots. Therefore, C. pyramidale plants improved their metabolism under both growth conditions in the presence of AMF, Pi or a combination of both. However, the greatest increases in growth and tolerance to salinity occurred in the isolated presence of AMF.

Arbuscular mycorrhizal fungi improve photosynthetic energy use efficiency and decrease foliar construction cost under recurrent water deficit in woody evergreen species.

Plants suffer recurrent cycles of water deficit in semiarid regions and have several mechanisms to tolerate low water availability. Thus, arbuscular mycorrhizal fungi (AMF) can alleviate deleterious effects of stress. In this study, Cynophalla flexuosa plants, a woody evergreen species from semiarid, when associated with AMF were exposed to two consecutive cycles of water deficit. Leaf primary metabolism, specific leaf area (SLA), leaf construction cost (CC) and photosynthetic energy use efficiency (PEUE) were measured. The maximum stress occurred on seven days (cycle 1) and ten days (cycle 2) after suspending irrigation (photosynthesis close to zero). The rehydration was performed for three days after each maximum stress. In both cycles, plants submitted to water deficit showed reduced gas exchange and leaf relative water content. However, Drought + AMF plants had significantly larger leaf relative water content in cycle 2. At cycle 1, the SLA was larger in non-inoculated plants, while CC was higher in inoculated plants. At cycle 2, Drought + AMF treatment had lower CC and large SLA compared to control, and high PEUE compared to Drought plants. These responses suggest AMFs increase tolerance of C. flexuosa to recurrent water deficit, mainly in cycle 2, reducing the CC, promoting the improvement of SLA and PEUE, leading to higher photosynthetic area. Thus, our result emphasizes the importance of studies on recurrence of water deficit, a common condition in semiarid environments.

Cowpea and abiotic stresses: identification of reference genes for transcriptional profiling by qPCR.

BACKGROUND: Due to cowpea ability to fix nitrogen in poor soils and relative tolerance to drought and salt stresses, efforts have been directed to identifying genes and pathways that confer stress tolerance in this species. Real-time quantitative PCR (qPCR) has been widely used as the most reliable method to measure gene expression, due to its high accuracy and specificity. In the present study, nine candidate reference genes were rigorously tested for their application in normalization of qPCR data onto roots of four distinct cowpea accessions under two abiotic stresses: root dehydration and salt (NaCl, 100 mM). In addition, the regulation of four target transcripts, under the same referred conditions was also scrutinized. RESULTS: geNorm, NormFinder, BestKeeper, and ΔCt method results indicated a set of three statistically validated RGs for each stress condition: (I) root dehydration (actin, ubiquitin-conjugating enzyme E2 variant 1D, and a Phaseolus vulgaris unknown gene-UNK), and (II) salt (ubiquitin-conjugating enzyme E2 variant 1D, F-box protein, and UNK). The expression profile of the target transcripts suggests that flavonoids are important players in the cowpea response to the abiotic stresses analyzed, since chalcone isomerase and chalcone synthase were up-regulated in the tolerant and sensitive accessions. A lipid transfer protein also participates in the cowpea tolerance mechanisms to root dehydration and salt stress. The referred transcript was up-regulated in the two tolerant accessions and presented no differential expression in the sensitive counterparts. Chitinase B, in turn, generally related to plant defense, was an important target transcript under salt stress, being up-regulated at the tolerant, and down-regulated in the sensitive accession. CONCLUSIONS: Reference genes suitable for qPCR analyses in cowpea under root dehydration and salt stress were identified. This action will lead to a more accurate and reliable analysis of gene expression on this species. Additionally, the results obtained in this study may guide future research on gene expression in cowpea under other abiotic stress types that impose osmotic imbalance. The target genes analyzed, in turn, deserve functional evaluation due to their transcriptional regulation under stresses and biotechnological potential.

Different mechanisms drive the performance of native and invasive woody species in response to leaf phosphorus supply during periods of drought stress and recovery.

The effects of drought stress and leaf phosphorus (Pi) supply on photosynthetic metabolism in woody tropical species are not known, and given the recent global environmental change models that forecast lower precipitation rates and periods of prolonged drought in tropical areas, this type of study is increasingly important. The effects of controlled drought stress and Pi supply on potted young plants of two woody species, Anadenanthera colubrina (native) and Prosopis juliflora (invasive), were determined by analyzing leaf photosynthetic metabolism, biochemical properties and water potential. In the maximum stress, both species showed higher leaf water potential (Ψl) in the treatment drought +Pi when compared with the respective control -Pi. The native species showed higher gas exchange under drought +Pi than under drought -Pi conditions, while the invasive species showed the same values between drought +Pi and -Pi. Drought affected the photochemical part of photosynthetic machinery more in the invasive species than in the native species. The invasive species showed higher leaf amino acid content and a lower leaf total protein content in both Pi treatments with drought. The two species showed different responses to the leaf Pi supply under water stress for several variables measured. In addition, the strong resilience of leaf gas exchange in the invasive species compared to the native species during the recovery period may be the result of higher efficiency of Pi use. The implications of this behavior for the success of this invasive species in semiarid environments are discussed.

Stress tolerance and ecophysiological ability of an invader and a native species in a seasonally dry tropical forest.

Ecophysiological traits of Prosopis juliflora (Sw.) DC. and a phylogenetically and ecologically similar native species, Anadenanthera colubrina (Vell.) Brenan, were studied to understand the invasive species' success in caatinga, a seasonally dry tropical forest ecosystem of the Brazilian Northeast. To determine if the invader exhibited a superior resource-capture or a resource-conservative strategy, we measured biophysical and biochemical parameters in both species during dry and wet months over the course of two years. The results show that P. juliflora benefits from a flexible strategy in which it frequently outperforms the native species in resource capture traits under favorable conditions (e.g., photosynthesis), while also showing better stress tolerance (e.g., antioxidant activity) and water-use efficiency in unfavorable conditions. In addition, across both seasons the invasive has the advantage over the native with higher chlorophyll/carotenoids and chlorophyll a/b ratios, percent N, and leaf protein. We conclude that Prosopis juliflora utilizes light, water and nutrients more efficiently than Anadenanthera colubrina, and suffers lower intensity oxidative stress in environments with reduced water availability and high light radiation.