Endodermal ABA Signaling in the Root Tip Regulates Xylem Maturation and Patterning by Posttranscriptional MicroRNA-Controlled Expression in Stele

Developmental responses of the root vasculature to drought stress have important basic and applied implications. Patterning of the root xylem into a peripheral protoxylem (PX) and central metaxylem (MX) is combinatorically regulated by auxin-cytokinin signaling and the SHORTROOT (SHR), SCARECRAW (SCR), microRNAs miR165a/166b, and Class III HOMEODOMAIN LEU-ZIPPER (HD-ZIPIII) expression network. Abscisic Acid (ABA) is a central regulator of abiotic stress responses in plants. In the root, ABA levels increase rapidly in response to salt and drought stress. We evaluated changes in tissue maturation and patterning and found that osmotic stress and ABA induce the differentiation of PX instead of MX and early secondary wall maturation. The effects of ABA are associated with increased expression of the PX maturation regulator VND7 in the cell division zone MX file and are dependent on core ABA signaling in meristematic endodermal cells. ABA induces expression of miR165a/166b that non-cell autonomously, post-transcriptionally, reduce the levels of all 5 HD-ZIPIII RNAs in the stele. In agreement, ABA fails to repress expression of HD-ZIPIII PHABULOSA (PHB) promoter driven GFP but restricts expression of PHB:GFP fusion protein and fails to induce additional PX files in a miR165a/166b-resistant PHB mutant, phb1-d, and in scr and shr, which regulate miR165a/166b expression. ABA does not induce ectopic expression of the auxin-regulated PX marker AHP6, but induces additional PX files in quadruple auxin receptor mutants and PX differentiation in roots treated with auxinole, auxin signaling inhibitor. Thus, ABA regulates xylem patterning and maturation via miR165a/166b-regulated HD-ZIPIIIs expression and associated VND7 levels.