ROPs and ABA Antagonistically Regulate of Cell Wall Patterning and Xylem Differentiation

The precise patterning of secondary walls during root development regulates water and solutes permeability and conductance as well as tissue maturation. Arabidopsis ROP11 has been implicated in secondary wall and negative regulation of ABA responses. Conversely, ABA was shown to down-regulate ROPs. Hence, ROP11 may function as an integrator of ABA signaling and root secondary wall patterning. We show that ROP11 expression is induced during the onset of protoxylem (PX) differentiation and its expression domain then expands to the pericycle and later to the endodermis. Exogenous ABA treatments enhance GFP-ROP11 expression in the endodermis but removal or the root meristem abolishes the ABA effect. In the PX, GFP-ROP11 is localized in an alternating pattern with the secondary cell wall. Induced expression of a constitutively active ROP11 (rop11^CA) under regulation of the ROP11 promoter lead to disruption of secondary wall pattern in the PX, indicating that ROP11 activity domains regulate PX cell wall patterning. The induced expression of rop11^CA also inhibited the meristem stage ABA induced specification of ectopic PX files. In the endodermis, expression of rop11^CA but not wild type ROP11 under constitutive 35S or endodermis specific CASP1 promoters disrupted the integrity of the Casparian strip and lead to salt hypersensitivity. However, when expression of rop11^CA was induced under the ROP11 promoter the Casparian strip remained intact. Furthermore, ABA treatments reduced the association of ROP11 with the plasma membrane, inducing similar localization to inactive dominant negative rop11^DN. Hence, the Casparian strip integrity depend on the timing and levels of ROP11 expression and its activation status. Taken together, these results demonstrate how ROP and ABA signaling fine-tune vascular differentiation and secondary cell wall patterning in the root and indicate that the effect of ROP11 on ABA induced PX differentiation are likely indirect.