Abstract
Arabidopsis PIN2 protein directs transport of the phytohormone auxin from the root tip into the root elongation zone. Variation in hormone transport, which depends on a delicate interplay between PIN2 sorting to and from polar plasma membrane domains, determines root growth. By employing a constitutively degraded version of PIN2, we identify brassinolides as antagonists of PIN2 endocytosis. This response does not require de novo protein synthesis, but involves early events in canonical brassinolide signaling. Brassinolide-controlled adjustments in PIN2 sorting and intracellular distribution governs formation of a lateral PIN2 gradient in gravistimulated roots, coinciding with adjustments in auxin signaling and directional root growth. Strikingly, simulations indicate that PIN2 gradient formation is no prerequisite for root bending but rather dampens asymmetric auxin flow and signaling. Crosstalk between brassinolide signaling and endocytic PIN2 sorting, thus, appears essential for determining the rate of gravity-induced root curvature via attenuation of differential cell elongation.
Highlights
Arabidopsis PIN2 protein directs transport of the phytohormone auxin from the root tip into the root elongation zone
Deciphering the molecular basis of PIN sorting is key to our understanding of mechanisms, by which these proteins influence plant growth
By employing a constitutively endocytosed version of Arabidopsis PIN2, we present a detailed characterization of PIN2 crosstalk with brassinolide signaling, and its functional implications for root gravitropism
Summary
Arabidopsis PIN2 protein directs transport of the phytohormone auxin from the root tip into the root elongation zone. PIN exocytotic sorting to distinct polar PM domains[1,2,4,5] is followed by endocytic sorting via clathrin-mediated endocytosis (CME) into trans-Golgi network (TGN) compartments[6] Such cargo is either recycled to PM domains, or passed on to Late Endosomes (LE)/Multivesicular Bodies (MVB) for degradation in the vacuole[5,7]. Cytokinin and the strigolactone agonist rac-GR24 promote PIN endocytic sorting and vacuolar degradation, thereby influencing particular aspects of plant morphogenesis[16,17] Secondary messengers, such as calcium and redox signaling affect PIN sorting during adaptation processes, emphasizing the central role of crosstalk between plant growth regulators and PIN proteins in plant development[18,19]. Brassinolide signaling has been implicated in sorting and steady-state level control of root-specific PIN21,28–30; mechanisms orchestrating such post-transcriptional control and its significance for auxin transport remained obscure
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