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Review
. 2014 Sep 22:5:449.
doi: 10.3389/fpls.2014.00449. eCollection 2014.

Extracellular signals and receptor-like kinases regulating ROP GTPases in plants

Kaori N Miyawaki  1 , Zhenbiao Yang  2
Affiliations
Review

Extracellular signals and receptor-like kinases regulating ROP GTPases in plants

Kaori N Miyawaki et al. Front Plant Sci. .

Abstract

Rho-like GTPase from plants (ROPs) function as signaling switches that control a wide variety of cellular functions and behaviors including cell morphogenesis, cell division and cell differentiation. The Arabidopsis thaliana genome encodes 11 ROPs that form a distinct single subfamily contrarily to animal or fungal counterparts where multiple subfamilies of Rho GTPases exist. Since Rho proteins bind to their downstream effector proteins only in their GTP-bound "active" state, the activation of ROPs by upstream factor(s) is a critical step in the regulation of ROP signaling. Therefore, it is critical to examine the input signals that lead to the activation of ROPs. Recent findings showed that the plant hormone auxin is an important signal for the activation of ROPs during pavement cell morphogenesis as well as for other developmental processes. In contrast to auxin, another plant hormone, abscisic acid, negatively regulates ROP signaling. Calcium is another emerging signal in the regulation of ROP signaling. Several lines of evidence indicate that plasma membrane localized-receptor like kinases play a critical role in the transmission of the extracellular signals to intracellular ROP signaling pathways. This review focuses on how these signals impinge upon various direct regulators of ROPs to modulate various plant processes.

Keywords: RLK; ROP GTPase; abscisic acid (ABA); auxin; calcium.

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Figures

FIGURE 1
FIGURE 1
Two antagonistic Rho-like GTPase from plant (ROP) pathways regulate pavement cell (PC) interdigitation. (A) Lobes and indentations in Arabidopsis PC. Red arrowheads indicate lobes. Green arrows show indentations. (B) Auxin activates two antagonistic ROP pathways in PC interdigitation (1) Apoplastic auxin localizes to the lobes by PIN1-mediated positive feedback loop. Accumulation of extracellular auxin is generated by the auxin->ROP2->PIN1->auxin feedback loop. (2) Auxin controls ROP2 and ROP6 pathways in an ABP1/TMK-dependent manner. (3) ROP2 activates RIC4 and promotes assembly of fine cortical actin microfilaments in lobe regions. (4) ROP6 activates ROP-interactive CRIB motif containing protein 1 (RIC1) and promotes cortical microtubule (MT) ordering in indenting regions. (5) ROP2-mediated pathway inactivates RIC1-mediated MT bundling. (6) RIC1-mediated MT organization inhibits ROP2/RIC4 interaction.
FIGURE 2
FIGURE 2
Roles of auxin for PC interdigitation in leaves and clathrin-dependent endocytosis in roots. (A) Schematic view for the promoting effects of auxin on two antagonistic ROP pathways and PIN1-mediated positive feedback loop in PC interdigitation. (B) Schematic view for an inhibitory effect of auxin on clathrin-dependent endocytosis in roots. Left; In the absence of auxin, ABP1 promotes clathrin-dependent endocytosis. Right; Apoplastic auxin binds to ABP1 and activates SPK1/ROP6/RIC1 pathway, resulting in the inhibition of clathrin-dependent endocytosis.
See this image and copyright information in PMC

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