Physiological, phytohormonal and molecular responses of soybean to soil drying

Soybean is an important global crop for human and animal nutrition, but its production is affected by environmental stresses such as drought. Crops adapt to these stresses by producing and transporting multiple internal signals (such as phytohormones) between roots and shoots. Understanding the relationships between physiological (water potential and stomatal conductance), biochemical (phytohormones) and gene expression changes can inform cultivar selection and management approaches, offering opportunities to enhance water-limited yields. Soybean (Glycine max (L.) Merr. , genotypes Williams 82 (W82), Jindou 21 (C12), Union (C08), Long Huang 1 (LH1) and Long Huang 2 (LH2)) were grown under soil drying conditions to investigate relationships between leaf xylem sap and leaf tissue ABA concentrations, stomatal conductance and leaf water potential among different genotypes. Stomatal conductance was better explained by variation in leaf xylem sap ABA concentration than leaf tissue ABA concentration or leaf water potential in most of the genotypes studied, which is physiologically important as stomatal closure limits soybean yields. Thus, limited ABA accumulation may be useful as a marker for breeding plants under drought conditions, assuming plants can access sufficient soil moisture at depth. The role of the ABA in root-shoot communication was investigated by exposing plants to a combination of soil drying and stem girdling (which disrupts basipetal phloem transport) to determine the dependence of ABA accumulation on tissue water relations. Shoot-sourced ABA was necessary to allow maximal root ABA accumulation, and maintain root-to-shoot ABA signalling, in response to soil drying. Shoot to root ABA translocation also maintained high stomatal conductance by preventing foliar ABA accumulation under well-watered onditions. However, decreased stomatal conductance (by 20%) of well-watered plants one day after girdling may involve other hormones, induced by wounding effects prior to any ABA (xylem or tissue) accumulation. Within 26 hours of girdling, root ACC concentrations increased 15-fold and leaf ABA, JA and SA concentrations increased 1.5-, 6- and 1.5-fold respectively. In contrast, root GA3, GA4 and ABA concentrations decreased to 0.6-, 0.4- and 0.2-fold respectively. During this time (when there was limited soil drying), only leaf ABA and JA accumulation was highly negatively correlated with stomatal closure. Furthermore, leaf iP and SA concentrations were negatively and positively correlated respectively, and root ACC and ABA concentrations were negatively and positively correlated respectively, with soil moisture. Over the entire experiment in all plants, soil drying induced stomatal closure was negatively, positively and negatively correlated with foliar tZ, GA4 and ABA accumulations respectively, while root GA3, ABA and JA accumulations were negatively correlated with soil water content. Rapid leaf JA accumulation in response to girdling and soil-drying induced ABA accumulation in leaves and roots independently of girdling suggest that both hormones interact to stimulate stomatal closure. Girdling failed to disrupt the positive correlation between root and leaf ABA concentrations, but induced negative correlations between root and leaf JA and tZ concentrations, possibly due to carbohydrate depletion in the roots. Understanding the role of local hormone synthesis versus root-to-shoot signalling in regulating ABA and JA accumulation of each tissue can be facilitated by gene expression analysis via RNA-seq and qRT-PCR. The majority of the ABA biosynthesis, catabolism and signalling genes were upregulated in the roots of girdled plants prior any change in leaf and root water relations, and were sustained as the soil dries. Girdling upregulated the expression level of JA biosynthesis and signalling genes in both roots and leaves. Soil drying up-regulated the ABA biosynthesis and catabolism in roots and leaves, while up-regulating JA biosynthesis and signalling genes in roots and leaves of intact plants. Thus girdling more rapidly increases the number of upregulated genes in the selected (JA and ABA pathway) genes in roots than in leaves. Taken together, this thesis furthers our understanding of relationships between leaf and root phytohormonal communication in co-ordinating physiological responses to soil drying. Further studies of cross-talk between different hormones, including their intermediate metabolites, seems necessary to help understand how plants respond under drought conditions.