Drought-Induced Abscisic Acid Accumulation in Soybean Roots Depends on NCED Gene Expression More Than Shoot-to-Root ABA Transport

Abscisic acid (ABA) is a key phytohormone in plant responses to water deficit. Although there is extensive evidence that roots can synthesise ABA, recent findings suggest that local synthesis in response to dehydration contributes little to the root ABA pool compared to shoot-sourced ABA. To understand how root ABA synthesis and basipetal phloem transport regulate root ABA accumulation, expression of genes encoding ABA biosynthesis and metabolism enzymes was assessed in response to dehydration of detached roots and soil drying of attached roots sourced from intact or girdled soybean plants. Both dehydration and soil drying caused significant ABA accumulation in detached and attached roots, respectively, but less in girdled plants. This indicates root ABA biosynthesis is limited mainly by interrupting the import of ABA precursors and not ABA itself. Lower root ABA concentrations in girdled plants were not due to downregulated NCED gene expression, which was similar (detached roots) or even strongly upregulated (attached roots) in response to dehydration compared to roots from intact plants. This may partially compensate for the lack of ABA precursors. Thus, soybean root ABA accumulation in response to dehydration depends on enhanced ABA biosynthesis more than ABA import from shoots.