Patterning of the root xylem into protoxylem (PX) and metaxylem is regulated by auxin-cytokinin signaling and microRNA miR165a/166bmediated suppression of genes encoding Class III HOMEODOMAIN LEU-ZIPPER (HD-ZIPIII) proteins. We found that, in Arabidopsis, 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 VND7 expression. Similarly, in tomato, ABA enhanced PX differentiation longitudinally and radially, indicating an evolutionarily conserved mechanism. ABA increased expression of miR165a/166b and reduced expression of the miR165a/166b 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 miR165a/166b-resistant PHB mutant, phb1-d, and in scr and shr mutants, in which miR165a/166b expression is strongly reduced. Thus, ABA regulates xylem patterning and maturation via miR165a/ 166b-regulated expression of HD-ZIPIII mRNAs and associated VND7 levels. In lateral root initials, ABA induced an increase in miR165a levels in endodermal precursors and inhibited their reduction in the future quiescent center specifically at preemergence stage. Hence, ABA-induced inhibition of lateral root is associated with reduced HD-ZIPIII levels.
Bibliographical noteFunding Information:
This research was supported by the Israel Science Foundation (ISF 827/15, ISF-NCSF 1125/13, ISF-UGA 2739/16) and the Israeli Centers for Research Excellence on Plant Adaptation to Changing Environment (I-CORE 757-12) through grants to S.Y. and by the Mana Center for Plant Research at Tel Aviv University.
© 2019. Published by The Company of Biologists Ltd