The Brassica napus boron deficient inflorescence transcriptome resembles a wounding and infection response.

Oilseed rape and other crops of Brassica napus have a high demand for boron (B). Boron deficiencies result in the inhibition of root growth, and eventually premature flower abortion. Understanding the genetic mechanisms underlying flower abortion in B-limiting conditions could provide the basis to enhance B-efficiency and prevent B-deficiency-related yield losses. In this study, we assessed transcriptomic responses to B-deficiency in diverse inflorescence tissues at multiple time points of soil-grown plants that were phenotypically unaffected by B-deficiency until early flowering. Whilst transcript levels of known B transporters were higher in B-deficient samples, these remained remarkably stable as the duration of B-deficiency increased. Meanwhile, GO-term enrichment analysis indicated a growing response resembling that of a pathogen or pest attack, escalating to a huge transcriptome response in shoot heads at mid-flowering. Grouping differentially expressed genes within this tissue into MapMan functional bins indicated enrichment of genes related to wounding, jasmonic acid and WRKY transcription factors. Individual candidate genes for controlling the "flowering-without-seed-setting" phenotype from within MapMan biotic stress bins include those of the metacaspase family, which have been implicated in orchestrating programmed cell death. Overall temporal expression patterns observed here imply a dynamic response to B-deficiency, first increasing expression of B transporters before recruiting various biotic stress-related pathways to coordinate targeted cell death, likely in response to as yet unidentified B-deficiency induced damage-associated molecular patterns (DAMPs). This response indicates new pathways to target and dissect to control B-deficiency-induced flower abortion and to develop more B-efficient crops.

Plant Age and Soil Texture Rather Than the Presence of Root Hairs Cause Differences in Maize Resource Allocation and Root Gene Expression in the Field.

Understanding the biological roles of root hairs is key to projecting their contributions to plant growth and to assess their relevance for plant breeding. The objective of this study was to assess the importance of root hairs for maize nutrition, carbon allocation and root gene expression in a field experiment. Applying wild type and root hairless rth3 maize grown on loam and sand, we examined the period of growth including 4-leaf, 9-leaf and tassel emergence stages, accompanied with a low precipitation rate. rth3 maize had lower shoot growth and lower total amounts of mineral nutrients than wild type, but the concentrations of mineral elements, root gene expression, or carbon allocation were largely unchanged. For these parameters, growth stage accounted for the main differences, followed by substrate. Substrate-related changes were pronounced during tassel emergence, where the concentrations of several elements in leaves as well as cell wall formation-related root gene expression and C allocation decreased. In conclusion, the presence of root hairs stimulated maize shoot growth and total nutrient uptake, but other parameters were more impacted by growth stage and soil texture. Further research should relate root hair functioning to the observed losses in maize productivity and growth efficiency.

Cytokinins as boron deficiency signals to sustain shoot development in boron-efficient oilseed rape.

Boron (B) deficiency is a highly prominent nutrient disorder. While B-efficient accessions have recently been identified in the highly B-demanding crop oilseed rape, it remained unclear which physiological processes underlie B efficiency and which signaling pathways trigger an efficient B-deficiency response. Here, we compared, under three different B supply conditions, two Brassica napus accessions with contrasting B efficiency. Shoot biomass formation, B distribution patterns and metabolic dynamics of different phytohormone species were studied using a combination of mass spectrometry-based analyses and physiological measurements. Our results show that the B-efficient accession CR2267 does not differ from the B-inefficient accession CR2262 in terms of B accumulation and subcellular B-partitioning, although it displays no morphological B-deficiency symptoms under severe B-deficient conditions. Investigating phytohormone metabolism revealed a strong accumulation of cytokinins in CR2267 at a developmental stage when striking B-dependent differences in biomass and organ formation emerge in the two B. napus accessions. In contrast, elevated levels of the stress hormone abscisic acid as well as bioactive auxins, representing functional antagonists of cytokinins in shoots, were detected only in CR2262. Our results indicate that superior B efficiency in CR2267 relies on a higher B utilization efficiency that builds on an earlier and higher cytokinin biosynthesis required for the maintenance of the shoot meristem activity and proper leaf development. We further conclude that an elevated abundance of cytokinins is not a consequence of better plant growth but rather a presumption for better plant growth under low-B conditions.

Stable Isotope Fractionation of Metals and Metalloids in Plants: A Review.

This work critically reviews stable isotope fractionation of essential (B, Mg, K, Ca, Fe, Ni, Cu, Zn, Mo), beneficial (Si), and non-essential (Cd, Tl) metals and metalloids in plants. The review (i) provides basic principles and methodologies for non-traditional isotope analyses, (ii) compiles isotope fractionation for uptake and translocation for each element and connects them to physiological processes, and (iii) interlinks knowledge from different elements to identify common and contrasting drivers of isotope fractionation. Different biological and physico-chemical processes drive isotope fractionation in plants. During uptake, Ca and Mg fractionate through root apoplast adsorption, Si through diffusion during membrane passage, Fe and Cu through reduction prior to membrane transport in strategy I plants, and Zn, Cu, and Cd through membrane transport. During translocation and utilization, isotopes fractionate through precipitation into insoluble forms, such as phytoliths (Si) or oxalate (Ca), structural binding to cell walls (Ca), and membrane transport and binding to soluble organic ligands (Zn, Cd). These processes can lead to similar (Cu, Fe) and opposing (Ca vs. Mg, Zn vs. Cd) isotope fractionation patterns of chemically similar elements in plants. Isotope fractionation in plants is influenced by biotic factors, such as phenological stages and plant genetics, as well as abiotic factors. Different nutrient supply induced shifts in isotope fractionation patterns for Mg, Cu, and Zn, suggesting that isotope process tracing can be used as a tool to detect and quantify different uptake pathways in response to abiotic stresses. However, the interpretation of isotope fractionation in plants is challenging because many isotope fractionation factors associated with specific processes are unknown and experiments are often exploratory. To overcome these limitations, fundamental geochemical research should expand the database of isotope fractionation factors and disentangle kinetic and equilibrium fractionation. In addition, plant growth studies should further shift toward hypothesis-driven experiments, for example, by integrating contrasting nutrient supplies, using established model plants, genetic approaches, and by combining isotope analyses with complementary speciation techniques. To fully exploit the potential of isotope process tracing in plants, the interdisciplinary expertise of plant and isotope geochemical scientists is required.

Root hairs: the villi of plants.

Strikingly, evolution shaped similar tubular structures at the µm to mm scale in roots of sessile plants and in small intestines of mobile mammals to ensure an efficient transfer of essential nutrients from 'dead matter' into biota. These structures, named root hairs (RHs) in plants and villi in mammals, numerously stretch into the environment, and extremely enlarge root and intestine surfaces. They are believed to forage for nutrients, and mediate their uptake. While the conceptional understanding of plant RH function in hydromineral nutrition seems clear, experimental evidence presented in textbooks is restricted to a very limited number of reference-nutrients. Here, we make an element-by-element journey through the periodic table and link individual nutrient availabilities to the development, structure/shape and function of RHs. Based on recent developments in molecular biology and the identification of mutants differing in number, length or other shape-related characteristics of RHs in various plant species, we present comprehensive advances in (i) the physiological role of RHs for the uptake of specific nutrients, (ii) the developmental and morphological responses of RHs to element availability and (iii) RH-localized nutrient transport proteins. Our update identifies crucial roles of RHs for hydromineral nutrition, mostly under nutrient and/or water limiting conditions, and highlights the influence of certain mineral availabilities on early stages of RH development, suggesting that nutritional stimuli, as deficiencies in P, Mn or B, can even dominate over intrinsic developmental programs underlying RH differentiation.

Genome-wide association mapping identifies HvNIP2;2/HvLsi6 accounting for efficient boron transport in barley.

Boron (B) is an essential mineral element for plant growth, and the seed B pool of crops can be crucial when seedlings need to establish on low-B soils. To date, it is poorly understood how B accumulation in grain crops is genetically controlled. Here, we assessed the genotypic variation of the B concentration in grains of a spring barley (Hordeum vulgare L.) association panel that represents broad genetic diversity. We found a large genetic variation of the grain B concentration and detected in total 23 quantitative trait loci (QTLs) using genome-wide association mapping. HvNIP2;2/HvLsi6, encoding a potential B-transporting membrane protein, mapped closely to a major-effect QTL accounting for the largest proportion of grain B variation. Based on transport studies using heterologous expression systems and gene expression analysis, we demonstrate that HvNIP2;2/HvLsi6 represents a functional B channel and that expression variation in its transcript level associates with root and shoot B concentrations as well as with root dry mass formation under B-deficient conditions.

The transcription factor WRKY22 is required during cryo-stress acclimation in Arabidopsis shoot tips.

Storage of meristematic tissue at ultra-low temperatures offers a mean to maintain valuable genetic resources from vegetatively reproduced plants. To reveal the biology underlying cryo-stress, shoot tips of the model plant Arabidopsis thaliana were subjected to a standard preservation procedure. A transcriptomic approach was taken to describe the subsequent cellular events which occurred. The cryoprotectant treatment induced the changes in the transcript levels of genes associated with RNA processing and primary metabolism. Explants of a mutant lacking a functional copy of the transcription factor WRKY22 were compromised for recovery. A number of putative downstream targets of WRKY22 were identified, some related to phytohormone-mediated defense, to the osmotic stress response, and to development. There were also alterations in the abundance of transcript produced by genes encoding photosynthesis-related proteins. The wrky22 mutant plants developed an open stomata phenotype in response to their exposure to the cryoprotectant solution. WRKY22 probably regulates a transcriptional network during cryo-stress, linking the explant's defense and osmotic stress responses to changes in its primary metabolism. A model is proposed linking WRKY53 and WRKY70 downstream of the action of WRKY22.

Compatibility of X-ray computed tomography with plant gene expression, rhizosphere bacterial communities and enzyme activities.

Non-invasive X-ray computed tomography (XRCT) is increasingly used in rhizosphere research to visualize development of soil-root interfaces in situ. However, exposing living systems to X-rays can potentially impact their processes and metabolites. In order to evaluate these effects, we assessed the responses of rhizosphere processes 1 and 24 h after a low X-ray exposure (0.81 Gy). Changes in root gene expression patterns occurred 1 h after exposure with down-regulation of cell wall-, lipid metabolism-, and cell stress-related genes, but no differences remained after 24 h. At either time point, XRCT did not affect either root antioxidative enzyme activities or the composition of the rhizosphere bacterial microbiome and microbial growth parameters. The potential activities of leucine aminopeptidase and phosphomonoesterase were lower at 1 h, but did not differ from the control 24 h after exposure. A time delay of 24 h after a low X-ray exposure (0.81 Gy) was sufficient to reverse any effects on the observed rhizosphere systems. Our data suggest that before implementing novel experimental designs involving XRCT, a study on its impact on the investigated processes should be conducted.

Hyperoxidation of mitochondrial peroxiredoxin limits H2 O2 -induced cell death in yeast.

Hydrogen peroxide (H2 O2 ) plays important roles in cellular signaling, yet nonetheless is toxic at higher concentrations. Surprisingly, the mechanism(s) of cellular H2 O2 toxicity remain poorly understood. Here, we reveal an important role for mitochondrial 1-Cys peroxiredoxin from budding yeast, Prx1, in regulating H2 O2 -induced cell death. We show that Prx1 efficiently transfers oxidative equivalents from H2 O2 to the mitochondrial glutathione pool. Deletion of PRX1 abrogates glutathione oxidation and leads to a cytosolic adaptive response involving upregulation of the catalase, Ctt1. Both of these effects contribute to improved cell viability following an acute H2 O2 challenge. By replacing PRX1 with natural and engineered peroxiredoxin variants, we could predictably induce widely differing matrix glutathione responses to H2 O2 . Therefore, we demonstrated a key role for matrix glutathione oxidation in driving H2 O2 -induced cell death. Finally, we reveal that hyperoxidation of Prx1 serves as a switch-off mechanism to limit oxidation of matrix glutathione at high H2 O2 concentrations. This enables yeast cells to strike a fine balance between H2 O2 removal and limitation of matrix glutathione oxidation.

Overexpression of X Intrinsic Protein 1;1 in Nicotiana tabacum and Arabidopsis reduces boron allocation to shoot sink tissues.

Major Intrinsic Proteins (MIP) are a family of channels facilitating the diffusion of water and/or small solutes across cellular membranes. X Intrinsic Proteins (XIP) form the least characterized MIP subfamily in vascular plants. XIPs are mostly impermeable to water but facilitate the diffusion of hydrogen peroxide, urea and boric acid when expressed in heterologous expression systems. However, their transport capabilities in planta and their impact on plant physiology are still unknown. Here, we demonstrated that overexpression of NtXIP1;1 in Nicotiana tabacum by the En2pPMA4 or the 35S CaMV promoter and in Arabidopsis, which does not contain any XIP gene, by the 35S CaMV promoter, resulted in boron (B)-deficiency symptoms such as death of the shoot apical meristem, infertile flowers, and puckered leaves. Leaf B concentrations in symptomatic tissues and B xylem sap concentrations were lower in the overexpressors than in control plants. Importantly, expression of NtXIP1;1 under the control of the AtNIP5;1 promoter complemented the B deficiency phenotype of the Atnip5;1 knockout mutant, defining its ability to act as a boric acid channel in planta. Protein quantification analysis revealed that NtXIP1;1 was predominantly expressed in young B-demanding tissues and induced under B-deficient conditions. Our results strongly suggest that NtXIP1;1 plays a role in B homeostasis and its tissue-specific expression critically contributes to the distribution of B within tobacco.

Boron demanding tissues of Brassica napus express specific sets of functional Nodulin26-like Intrinsic Proteins and BOR1 transporters.

The sophisticated uptake and translocation regulation of the essential element boron (B) in plants is ensured by two transmembrane transporter families: the Nodulin26-like Intrinsic Protein (NIP) and BOR transporter family. Though the agriculturally important crop Brassica napus is highly sensitive to B deficiency, and NIPs and BORs have been suggested to be responsible for B efficiency in this species, functional information of these transporter subfamilies is extremely rare. Here, we molecularly characterized the NIP and BOR1 transporter family in the European winter-type cv. Darmor-PBY018. Our transport assays in the heterologous oocyte and yeast expression systems as well as in growth complementation assays in planta demonstrated B transport activity of NIP5, NIP6, NIP7 and BOR1 isoforms. Moreover, we provided functional and quantitative evidence that also members of the NIP2, NIP3 and NIP4 groups facilitate the transport of B. A detailed B- and tissue-dependent B-transporter expression map was generated by quantitative polymerase chain reaction. We showed that NIP5 isoforms are highly upregulated under B-deficient conditions in roots, but also in shoot tissues. Moreover, we detected transcripts of several B-permeable NIPs from various groups in floral tissues that contribute to the B distribution within the highly B deficiency-sensitive flowers.

Root system size response of bzh semi-dwarf oilseed rape hybrids to different nitrogen levels in the field.

BACKGROUND AND AIMS: In oilseed rape (Brassica napus) semi-dwarf hybrid varieties from crosses between bzh dwarf and normal-type lines are of increasing interest. They have improved nitrogen (N) uptake, N-utilization and N-use efficiency compared to normal types. This study aimed to elucidate whether these N-related effects can be explained by the bzh shoot growth-type alone or also by differences in root traits. METHODS: Root system size was measured using root electrical capacitance (EC) in field trials with two N levels in two sets of genotypes segregating for the bzh-locus: (1) 108 doubled haploid (DH) test hybrids in two seasons, 2010-2012, and (2) 16 near-isogenic hybrids in the 2016-17 season. Quantitative trait loci (QTL) for root EC were estimated in DH test hybrids. Seedling root architecture parameters were monitored in vitro. KEY RESULTS: In vitro root growth showed a higher root: shoot ratio in bzh semi-dwarf hybrids. Root EC in field trials was higher at high N supply than at zero N fertilization. In most trials semi-dwarf hybrids had higher EC than normal-type hybrids, but they reduced root EC in response to N limitation more than normal types. Root EC was more heritable at the end of flowering (h2 = 0.73) than at the beginning of flowering (h2 = 0.36) in near-isogenic hybrids and had a lower heritability in trials of DH test hybrids (h2 = 0.27). A QTL for root EC in the genomic region of the bzh-locus on linkage group A06 was significant at zero N fertilization. CONCLUSIONS: Root EC proved to be a meaningful method in oilseed rape breeding programmes targeting root system size. The greater reduction of semi-dwarf root EC compared to the normal type under low N supply with simultaneous increase in N efficiency implies that in roots it is not a question of 'the more the merrier' and that the bzh root system reacts highly economically when N is scarce.

Pollen-Specific Aquaporins NIP4;1 and NIP4;2 Are Required for Pollen Development and Pollination in Arabidopsis thaliana.

In flowers with dry stigmas, pollen development, pollination, and pollen tube growth require spatial and temporal regulation of water and nutrient transport. To better understand the molecular mechanisms involved in reproductive processes, we characterized NIP4;1 and NIP4;2, two pollen-specific aquaporins of Arabidopsis thaliana. NIP4;1 and NIP4;2 are paralogs found exclusively in the angiosperm lineage. Although they have 84% amino acid identity, they displayed different expression patterns. NIP4;1 has low expression levels in mature pollen, while NIP4;2 expression peaks during pollen tube growth. Additionally, NIP4;1pro:GUS flowers showed GUS activity in mature pollen and pollen tubes, whereas NIP4;2pro:GUS flowers only in pollen tubes. Single T-DNA mutants and double artificial microRNA knockdowns had fewer seeds per silique and reduced pollen germination and pollen tube length. Transport assays in oocytes showed NIP4;1 and NIP4;2 function as water and nonionic channels. We also found that NIP4;1 and NIP4;2 C termini are phosphorylated by a pollen-specific CPK that modifies their water permeability. Survival assays in yeast indicated that NIP4;1 also transports ammonia, urea, boric acid, and H2O2 Thus, we propose that aquaporins NIP4;1 and NIP4;2 are exclusive components of the reproductive apparatus of angiosperms with partially redundant roles in pollen development and pollination.

Crystal Structure of an Ammonia-Permeable Aquaporin.

Aquaporins of the TIP subfamily (Tonoplast Intrinsic Proteins) have been suggested to facilitate permeation of water and ammonia across the vacuolar membrane of plants, allowing the vacuole to efficiently sequester ammonium ions and counteract cytosolic fluctuations of ammonia. Here, we report the structure determined at 1.18 Å resolution from twinned crystals of Arabidopsis thaliana aquaporin AtTIP2;1 and confirm water and ammonia permeability of the purified protein reconstituted in proteoliposomes as further substantiated by molecular dynamics simulations. The structure of AtTIP2;1 reveals an extended selectivity filter with the conserved arginine of the filter adopting a unique unpredicted position. The relatively wide pore and the polar nature of the selectivity filter clarify the ammonia permeability. By mutational studies, we show that the identified determinants in the extended selectivity filter region are sufficient to convert a strictly water-specific human aquaporin into an AtTIP2;1-like ammonia channel. A flexible histidine and a novel water-filled side pore are speculated to deprotonate ammonium ions, thereby possibly increasing permeation of ammonia. The molecular understanding of how aquaporins facilitate ammonia flux across membranes could potentially be used to modulate ammonia losses over the plasma membrane to the atmosphere, e.g., during photorespiration, and thereby to modify the nitrogen use efficiency of plants.

Metalloido-porins: Essentiality of Nodulin 26-like intrinsic proteins in metalloid transport.

Metalloids are a group of physiologically important elements ranging from the essential to the highly toxic. Arsenic, antimony, germanium, and tellurium are highly toxic to plants themselves and to consumers of metalloid-contaminated plants. Boron, silicon, and selenium fulfill essential or beneficial functions in plants. However, when present at high concentrations, boron and selenium cause toxicity symptoms that are detrimental to plant fitness and yield. Consequently, all plants require efficient membrane transport systems to control the uptake and extrusion of metalloids into or out of the plant and their distribution within the plant body. Several Nodulin 26-like intrinsic proteins (NIPs) that belong to the aquaporin plant water channel protein family facilitate the diffusion of uncharged metalloid species. Genetic, physiological, and molecular evidence is that NIPs from primitive to higher plants not only transport all environmentally important metalloids, but that these proteins have a major role in the uptake, translocation, and extrusion of metalloids in plants. As most of the metalloid-permeable NIP aquaporins are impermeable or are poorly permeable to water, these NIP channel proteins should be considered as physiologically essential metalloido-porins.

Expression and characterization of plasma membrane aquaporins in stomatal complexes of Zea mays.

Stomata, the microscopic pores on the surface of the aerial parts of plants, are bordered by two specialized cells, known as guard cells, which control the stomatal aperture according to endogenous and environmental signals. Like most movements occurring in plants, the opening and closing of stomata are based on hydraulic forces. During opening, the activation of plasma membrane and tonoplast transporters results in solute accumulation in the guard cells. To re-establish the perturbed osmotic equilibrium, water follows the solutes into the cells, leading to their swelling. Numerous studies have contributed to the understanding of the mechanism and regulation of stomatal movements. However, despite the importance of transmembrane water flow during this process, only a few studies have provided evidence for the involvement of water channels, called aquaporins. Here, we microdissected Zea mays stomatal complexes and showed that members of the aquaporin plasma membrane intrinsic protein (PIP) subfamily are expressed in these complexes and that their mRNA expression generally follows a diurnal pattern. The substrate specificity of two of the expressed ZmPIPs, ZmPIP1;5 and ZmPIP1;6, was investigated by heterologous expression in Xenopus oocytes and yeast cells. Our data show that both isoforms facilitate transmembrane water diffusion in the presence of the ZmPIP2;1 isoform. In addition, both display CO2 permeability comparable to that of the CO2 diffusion facilitator NtAQP1. These data indicate that ZmPIPs may have various physiological roles in stomatal complexes.

A new LxxxA motif in the transmembrane Helix3 of maize aquaporins belonging to the plasma membrane intrinsic protein PIP2 group is required for their trafficking to the plasma membrane.

Aquaporins play important roles in maintaining plant water status under challenging environments. The regulation of aquaporin density in cell membranes is essential to control transcellular water flows. This work focuses on the maize (Zea mays) plasma membrane intrinsic protein (ZmPIP) aquaporin subfamily, which is divided into two sequence-related groups (ZmPIP1s and ZmPIP2s). When expressed alone in mesophyll protoplasts, ZmPIP2s are efficiently targeted to the plasma membrane, whereas ZmPIP1s are retained in the endoplasmic reticulum (ER). A protein domain-swapping approach was utilized to demonstrate that the transmembrane domain3 (TM3), together with the previously identified N-terminal ER export diacidic motif, account for the differential localization of these proteins. In addition to protoplasts, leaf epidermal cells transiently transformed by biolistic particle delivery were used to confirm and refine these results. By generating artificial proteins consisting of a single transmembrane domain, we demonstrated that the TM3 of ZmPIP1;2 or ZmPIP2;5 discriminates between ER and plasma membrane localization, respectively. More specifically, a new LxxxA motif in the TM3 of ZmPIP2;5, which is highly conserved in plant PIP2s, was shown to regulate its anterograde routing along the secretory pathway, particularly its export from the ER.

New insights into the regulation of aquaporins by the arbuscular mycorrhizal symbiosis in maize plants under drought stress and possible implications for plant performance.

The relationship between modulation by arbuscular mycorrhizae (AM) of aquaporin expression in the host plant and changes in root hydraulic conductance, plant water status, and performance under stressful conditions is not well known. This investigation aimed to elucidate how the AM symbiosis modulates the expression of the whole set of aquaporin genes in maize plants under different growing and drought stress conditions, as well as to characterize some of these aquaporins in order to shed further light on the molecules that may be involved in the mycorrhizal responses to drought. The AM symbiosis regulated a wide number of aquaporins in the host plant, comprising members of the different aquaporin subfamilies. The regulation of these genes depends on the watering conditions and the severity of the drought stress imposed. Some of these aquaporins can transport water and also other molecules which are of physiological importance for plant performance. AM plants grew and developed better than non-AM plants under the different conditions assayed. Thus, for the first time, this study relates the well-known better performance of AM plants under drought stress to not only the water movement in their tissues but also the mobilization of N compounds, glycerol, signaling molecules, or metalloids with a role in abiotic stress tolerance. Future studies should elucidate the specific function of each aquaporin isoform regulated by the AM symbiosis in order to shed further light on how the symbiosis alters the plant fitness under stressful conditions.