ARF2 Coordinates With PLETHORAs And PINs To Orchestrate ABA-mediated Root Meristem Activity In Arabidopsis

Cell division and differentiation are regulated by several hormones. Abscisic acid (ABA) and auxin are involved in regulation of root growth and development; however, the underlying molecular mechanisms are still poorly understood. AUXIN RESPONSE FATOR (ARF2) is a negative regulator in the ABA response pathway controlling seed germination and primary root growth. PLETHORA (PLT) proteins play crucial roles in determination of root meristem activity. Here, our results shows that hypersensitivity of arf2 to ABA in root growth might be caused by ABA mediated inhibition of quiescent center (QC) division, cell division and cell differentiation in root apical meristem (RAM). ABA suppressed PIN1, PIN3, and PIN7 expression in the root stele and also reduced auxin accumulation in the root apical meristem (RAM) through ARF2 activity. However, in arf2 mutant, the higher auxin accumulation in QC and columella stem cells inhibited QC division, whereas lower auxin level inhibited meristem cell division and elongation in RAM. In compassion with wild type, ABA suppressed the expression of PIN1, PIN3, and PIN7, but induced PIN4 expression in arf2-101 mutants. Knocking out PIN4 gene could partially reversed ABA inhibited root growth of arf2-101.Furthermore, in apical root meristems, ARF2 positively modulates the ABA-mediated expression of PLT1, but might indirectly regulate PLT2 at a post-transcriptional level. Compared with arf2-101, plt1-4, plt2-2, and plt1-4plt2-2, the arf2-101plt1-4 double mutants were more sensitive to ABA, which have even less cell number in meristem zone. Overexpression of PLT2 by using a DEX inducible transgenic line Pro35S:PLT2-GR, largely promoted cell division and completely blocked cell differentiation in arf2-101 root meristem. And this high induction of cell division could be partially reversed by exogenous ABA application. Therefore, ABA inhibits root meristem growth in both ARF2 dependent and independent manners. Furthermore, our data uncover a complex regulatory mechanism in which ARF2 coordinates with PLTs and PINs to modulate ABA-mediated regulation of root meristem in Arabidopsis.