Date Published:

Abstract:

Patterning of the root xylem into protoxylem (PX) and metaxylem is regulated by auxin-cytokinin signaling and microRNA -mediated suppression of genes encoding Class III HOMEODOMAIN LEU-ZIPPER (HD-ZIPIII) proteins. We found that, in , osmotic stress via core abscisic acid (ABA) signaling in meristematic endodermal cells induces differentiation of PX in radial and longitudinal axes in association with increased expression. Similarly, in tomato, ABA enhanced PX differentiation longitudinally and radially, indicating an evolutionarily conserved mechanism. ABA increased expression of and reduced expression of the repressor ZWILLE (ZLL) (also known as ARGONAUTE10), resulting in reduced levels of all five HD-ZIPIII RNAs. ABA treatments failed to induce additional PX files in a -resistant mutant, , and in and mutants, in which expression is strongly reduced. Thus, ABA regulates xylem patterning and maturation via -regulated expression of HD-ZIPIII mRNAs and associated VND7 levels. In lateral root initials, ABA induced an increase in levels in endodermal precursors and inhibited their reduction in the future quiescent center specifically at pre-emergence stage. Hence, ABA-induced inhibition of lateral root is associated with reduced HD-ZIPIII levels.