ABAS1 from soybean is a 1R-subtype MYB transcriptional repressor that enhances ABA sensitivity.

Transcription factors (TFs) help plants respond to environmental stresses by regulating gene expression. Up till now, studies on the MYB family of TFs have mainly focused on the highly abundant R2R3-subtype. While the less well-known 1R-subtype has been generally shown to enhance abscisic acid (ABA) sensitivity by acting as transcriptional activators, the mechanisms of their functions are unclear. Here we identified an ABA sensitivity-associated gene from soybean, ABA-Sensitive 1 (GmABAS1), of the 1R-subtype of MYB. Using the GFP-GmABAS1 fusion protein, we demonstrated that GmABAS1 is localized in the nucleus, and with yeast reporter systems, we showed that it is a transcriptional repressor. We then identified the target gene of GmABAS1 to be Glyma.01G060300, an annotated ABI five-binding protein 3 and showed that GmABAS1 binds to the promoter of Glyma.01G060300 both in vitro and in vivo. Furthermore, Glyma.01G060300 and GmABAS1 exhibited reciprocal expression patterns under osmotic stress, inferring that GmABAS1 is a transcriptional repressor of Glyma.01G060300. As a further confirmation, AtAFP2, an orthologue of Glyma.01G060300, was down-regulated in GmABAS1-transgenic Arabidopsis thaliana, enhancing the plant's sensitivity to ABA. This is the first time a 1R-subtype of MYB from soybean has been reported to enhance ABA sensitivity by acting as a transcriptional repressor.

Redox Systemic Signaling and Induced Tolerance Responses During Soybean-Bradyrhizobium japonicum Interaction: Involvement of Nod Factor Receptor and Autoregulation of Nodulation.

The symbiotic relationship between legumes and nitrogen-fixing rhizobia induces local and systemic responses, which ultimately lead to nodule formation. The autoregulation of nodulation (AON) is a systemic mechanism related to innate immunity that controls nodule development and involves different components ranging from hormones, peptides, receptors to small RNAs. Here, we characterized a rapid systemic redox changes induced during soybean-Bradyrhizobium japonicum symbiotic interaction. A transient peak of reactive oxygen species (ROS) generation was found in soybean leaves after 30 min of root inoculation with B. japonicum. The ROS response was accompanied by changes in the redox state of glutathione and by activation of antioxidant enzymes. Moreover, the ROS peak and antioxidant enzyme activation were abolished in leaves by the addition, in either root or leaf, of DPI, an NADPH oxidase inhibitor. Likewise, these systemic redox changes primed the plant increasing its tolerance to photooxidative stress. With the use of non-nodulating nfr5-mutant and hyper-nodulating nark-mutant soybean plants, we subsequently studied the systemic redox changes. The nfr5-mutant lacked the systemic redox changes after inoculation, whereas the nark-mutant showed a similar redox systemic signaling than the wild type plants. However, neither nfr5- nor nark-mutant exhibited tolerance to photooxidative stress condition. Altogether, these results demonstrated that (i) the early redox systemic signaling during symbiotic interaction depends on a Nod factor receptor, and that (ii) the induced tolerance response depends on the AON mechanisms.

Cell remodeling and subtilase gene expression in the actinorhizal plant Discaria trinervis highlight host orchestration of intercellular Frankia colonization.

Nitrogen-fixing filamentous Frankia colonize the root tissues of its actinorhizal host Discaria trinervis via an exclusively intercellular pathway. Here we present studies aimed at uncovering mechanisms associated with this little-researched mode of root entry, and in particular the extent to which the host plant is an active partner during this process. Detailed characterization of the expression patterns of infection-associated actinorhizal host genes has provided valuable tools to identify intercellular infection sites, thus allowing in vivo confocal microscopic studies of the early stages of Frankia colonization. The subtilisin-like serine protease gene Dt12, as well as its Casuarina glauca homolog Cg12, are specifically expressed at sites of Frankia intercellular colonization of D. trinervis outer root tissues. This is accompanied by nucleo-cytoplasmic reorganization in the adjacent host cells and major remodeling of the intercellular apoplastic compartment. These findings lead us to propose that the actinorhizal host plays a major role in modifying both the size and composition of the intercellular apoplast in order to accommodate the filamentous microsymbiont. The implications of these findings are discussed in the light of the analogies that can be made with the orchestrating role of host legumes during intracellular root hair colonization by nitrogen-fixing rhizobia.

Phosphatidylinositol 3-kinase function at very early symbiont perception: a local nodulation control under stress conditions?

Root hair curling is an early and essential morphological change required for the success of the symbiotic interaction between legumes and rhizobia. At this stage rhizobia grow as an infection thread within root hairs and are internalized into the plant cells by endocytosis, where the PI3K enzyme plays important roles. Previous observations show that stress conditions affect early stages of the symbiotic interaction, from 2 to 30 min post-inoculation, which we term as very early host responses, and affect symbiosis establishment. Herein, we demonstrated the relevance of the very early host responses for the symbiotic interaction. PI3K and the NADPH oxidase complex are found to have key roles in the microsymbiont recognition response, modulating the apoplastic and intracellular/endosomal ROS induction in root hairs. Interestingly, compared with soybean mutant plants that do not perceive the symbiont, we demonstrated that the very early symbiont perception under sublethal saline stress conditions induced root hair death. Together, these results highlight not only the importance of the very early host-responses on later stages of the symbiont interaction, but also suggest that they act as a mechanism for local control of nodulation capacity, prior to the abortion of the infection thread, preventing the allocation of resources/energy for nodule formation under unfavorable environmental conditions.

Potential Uses of Wild Germplasms of Grain Legumes for Crop Improvement.

Challenged by population increase, climatic change, and soil deterioration, crop improvement is always a priority in securing food supplies. Although the production of grain legumes is in general lower than that of cereals, the nutritional value of grain legumes make them important components of food security. Nevertheless, limited by severe genetic bottlenecks during domestication and human selection, grain legumes, like other crops, have suffered from a loss of genetic diversity which is essential for providing genetic materials for crop improvement programs. Illustrated by whole-genome-sequencing, wild relatives of crops adapted to various environments were shown to maintain high genetic diversity. In this review, we focused on nine important grain legumes (soybean, peanut, pea, chickpea, common bean, lentil, cowpea, lupin, and pigeonpea) to discuss the potential uses of their wild relatives as genetic resources for crop breeding and improvement, and summarized the various genetic/genomic approaches adopted for these purposes.

The seed-borne Southern bean mosaic virus hinders the early events of nodulation and growth in Rhizobium-inoculated Phaseolus vulgaris L.

To simulate seed-borne virus transmission, a noninvasive protocol was designed to infect the radicle of germinating seeds, with 100% effectiveness. Preinfection of 24-h-old black bean (Phaseolus vulgaris L.) radicles by Southern bean mosaic virus (SBMV) followed by Rhizobium inoculation 48h later caused a drastic reduction in root nodulation. Results were attributed to active virus replication within the elongating zone of the radicle at least 32h before Rhizobium inoculation, which elicited severe anatomical malformations; an abnormal accumulation of apoplastic reactive oxygen species in the rhizodermis, cortex, inner cortical and endodermic root cells; the formation of atypical root hair tips and the collapse of 94% of the root hairs in the SBMV-preinfected radicles. Adult SBMV-preinfected plants showed exacerbated virus symptoms and 80% growth reduction ascribed to major virus-induced ultrastructural alterations in the nodules. The accumulation of ureides, α-amino acids and total reducing sugars in the leaves and nodules of SBMV-preinfected plants are indicators of the hindering effects of SBMV infection on N2 fixation and ureide catabolism, causing N starvation. The exogenous addition of 1 or 4µM naringenin, genistein or daidzein did not counteract the deleterious effects of SBMV preinfection on nodulation.

Using genomic information to improve soybean adaptability to climate change.

Climate change has brought severe challenges to agriculture. It is anticipated that there will be a drop in crop yield - including that of soybean - due to climatic stress factors that include drastic fluctuations in temperature, drought, flooding and high salinity. Genomic information on soybean has been accumulating rapidly since initial publication of its reference genome, providing a valuable tool for the improvement of cultivated soybean. Not only are many molecular markers that are associated with important quantitative trait loci now identified, but we also have a more detailed picture of the genomic variations among soybean germplasms, enabling us to utilize these as tools to assist crop breeding. In this review, we will summarize and discuss the currently available soybean genomic approaches, including whole-genome sequencing, sequencing-based genotyping, functional genomics, proteomics, and epigenomics. The information uncovered through these techniques will help further pinpoint important gene candidates and genomic loci associated with adaptive traits, as well as achieving a better understanding of how soybeans cope with the changing climate.

QTLs Regulating the Contents of Antioxidants, Phenolics, and Flavonoids in Soybean Seeds Share a Common Genomic Region.

Soybean seeds are a rich source of phenolic compounds, especially isoflavonoids, which are important nutraceuticals. Our study using 14 wild- and 16 cultivated-soybean accessions shows that seeds from cultivated soybeans generally contain lower total antioxidants compared to their wild counterparts, likely an unintended consequence of domestication or human selection. Using a recombinant inbred population resulting from a wild and a cultivated soybean parent and a bin map approach, we have identified an overlapping genomic region containing major quantitative trait loci (QTLs) that regulate the seed contents of total antioxidants, phenolics, and flavonoids. The QTL for seed antioxidant content contains 14 annotated genes based on the Williams 82 reference genome (Gmax1.01). None of these genes encodes functions that are related to the phenylpropanoid pathway of soybean. However, we found three putative Multidrug And Toxic Compound Extrusion (MATE) transporter genes within this QTL and one adjacent to it (GmMATE1-4). Moreover, we have identified non-synonymous changes between GmMATE1 and GmMATE2, and that GmMATE3 encodes an antisense transcript that expresses in pods. Whether the polymorphisms in GmMATE proteins are major determinants of the antioxidant contents, or whether the antisense transcripts of GmMATE3 play important regulatory roles, awaits further functional investigations.

Expression of animal anti-apoptotic gene Ced-9 enhances tolerance during Glycine max L.-Bradyrhizobium japonicum interaction under saline stress but reduces nodule formation.

The mechanisms by which the expression of animal cell death suppressors in economically important plants conferred enhanced stress tolerance are not fully understood. In the present work, the effect of expression of animal antiapoptotic gene Ced-9 in soybean hairy roots was evaluated under root hairs and hairy roots death-inducing stress conditions given by i) Bradyrhizobium japonicum inoculation in presence of 50 mM NaCl, and ii) severe salt stress (150 mM NaCl), for 30 min and 3 h, respectively. We have determined that root hairs death induced by inoculation in presence of 50 mM NaCl showed characteristics of ordered process, with increased ROS generation, MDA and ATP levels, whereas the cell death induced by 150 mM NaCl treatment showed non-ordered or necrotic-like characteristics. The expression of Ced-9 inhibited or at least delayed root hairs death under these treatments. Hairy roots expressing Ced-9 had better homeostasis maintenance, preventing potassium release; increasing the ATP levels and controlling the oxidative damage avoiding the increase of reactive oxygen species production. Even when our results demonstrate a positive effect of animal cell death suppressors in plant cell ionic and redox homeostasis under cell death-inducing conditions, its expression, contrary to expectations, drastically inhibited nodule formation even under control conditions.

Redox-related metabolites and gene expression modulated by sugar in sunflower leaves: similarities with Sunflower chlorotic mottle virus-induced symptom.

Sugars are part of an integrated redox system, since they are key regulators of respiration and photosynthesis, and therefore of the levels of reducing power, ATP and ROS. These elements are major determinants of the cellular redox state, which is involved in the perception and regulation of many endogenous and environmental stimuli. Our previous findings suggested that early sugar increase produced during compatible Sunflower chlorotic mottle virus (SuCMoV) infection might modulate chlorotic symptom development through redox state alteration in sunflower. The purpose of this work was to characterize redox-related metabolites and gene expression changes associated with high sugar availability and symptom development induced by SuCMoV. The results show that sugar caused an increase in glutathione, ascorbate, pyridine nucleotides, and ATP. In addition, higher sugar availability reduced hydrogen peroxide and ΦPSII. This finding suggests that high sugar availability would be associated with cellular redox alteration and photoinhibitory process. The expression of the genes analyzed was also strongly affected by sugar, such as the down-regulation of psbA and up-regulation of psbO and cp29. The expression level of cytoplasmic (apx-1 and gr)- and chloroplastic (Fe-sod)-targeted genes was also significantly enhanced in sugar-treated leaves. Therefore, all these responses suggest that sugars induce chloroplastic redox state alteration with photoinhibition process that could be contributing to chlorotic symptom development during SuCMoV infection.

The chlorotic symptom induced by Sunflower chlorotic mottle virus is associated with changes in redox-related gene expression and metabolites.

Systemic infections are commonly associated with changes in host metabolism and gene expression. Sunflower chlorotic mottle virus (SuCMoV) causes systemic infection with sugar increase, photoinhibition and increase in antioxidant enzyme activities before chlorotic symptom appearance in sunflower leaves. The aim of this study was to determine if chlorotic symptom development induced by SuCMoV infection is accompanied by changes in different redox-related metabolites and transcripts. Symptom development was analyzed in the second pair of leaves (systemic infection) at different post-inoculation times: before symptom appearance (BS, 4 dpi), and at an early (ES, 7 dpi) and later stage (LS, 12 dpi) of symptom expression. The results showed that the virus reaches the second pair of leaves at 4 dpi. A positive correlation between chlorotic symptom and number of viral copies was also observed. Changes in hydrogen peroxide, glutathione, pyridine nucleotides and ATP content were observed since symptom appearance (ES, 7 dpi). The expression of some of the genes analyzed was also strongly affected by SuCMoV infection. Specifically, down-regulation of both chloroplast-encoded genes and chloroplast-targeted genes: psbA, rbcS, Cu/Zn sod, Fe sod, phosphoglycolate phosphatase, psbO, psaH and fnr was present, whereas the expression of cytoplasmic-targeted genes, apx1, and Cu/Zn sod was up-regulated. Mitochondrial Mn sod decreased at BS stage and aox decreased only at ES stage. Peroxisomal catalase (cat-2) was lower at BS and LS stages. All these results suggest that SuCMoV infection induces progressive changes in determinants of redox homeostasis associated with chlorotic symptom development.