Stem girdling enhances ABA-induced stomatal closure of phosphorus-deprived soybean plants

Abstract

To investigate whether shoot-to-root communication modulates physiological (leaf water relations and growth), phytohormonal and transcriptomic responses to phosphorus (P) deprivation, soybean (Glycine max L. Merr.) plants received a factorial combination of low P conditions and girdling. Stem phloem tissue was surgically excised from the first internode between the cotyledonary and the first unifoliate leaf, immediately before imposing different P treatments (250 and 5 μM P in hydroponics). Root and leaf expression of genes involved in P homeostasis, sucrose metabolism and transporters, starch metabolism, hormone homeostasis and ABA signalling were analysed, to determine their regulation and relationships to tissue P / ABA concentrations. P deprivation mainly upregulated root and leaf P homeostasis gene expression only in intact plants, acting to maintain foliar P homeostasis. Although girdling suppressed root expression of these genes, it increased root P concentrations and decreased leaf P concentrations. Initially, P deprivation decreased stomatal conductance before leaf water status decreased, irrespective of girdling. Later, stomatal closure of girdled P-deprived plants was greater than intact P-deprived plants at the same, lower leaf water status, coincident with enhanced foliar ABA accumulation and expression of ABA biosynthesis genes. In contrast, P deprivation decreased foliar concentrations of the antitranspirant jasmonic acid and increased root and leaf SA concentrations at the end of the experiment. P deprivation also decreased foliar cytokinin and gibberellin concentrations before leaf growth was inhibited and leaf water status declined, suggesting non-hydraulic growth regulation. Girdling and P deprivation increased foliar sucrose and hexoses, corresponding to increased sucrose invertase and transporter gene expression in the leaves, while decreasing root sucrose and hexoses concentrations and downregulating sucrose metabolism but increasing transporter gene expression. Girdling perturbs multiple root and shoot phytohormonal responses that modulate physiological adaptation to P deprivation.