Abstract
Gravitropism is a process that allows plant organs to guide their growth relative to the gravity vector. It requires them to sense changes in their orientation and generate a biochemical signal that they transmit to the tissues that drive organ curvature. Trafficking between the plasma membrane and endosomal compartments is important for all of these phases of the gravitropic response. The sedimentation of starch-filled organelles called amyloplasts plays a key role in sensing reorientation, and vacuolar integrity is required for amyloplast sedimentation in shoots. Other proteins associated with the vesicle trafficking pathway contribute to early gravity signal transduction independently of amyloplast sedimentation in both roots and hypocotyls. Phosphatidylinositol signaling, which starts at the plasma membrane and later affects the localization of auxin efflux facilitators, is a likely second messenger in the signal transduction phase of gravitropism. Finally, membrane-localized auxin influx and efflux facilitators contribute to a differential auxin gradient across the gravistimulated organs, which directs root curvature.
Highlights
TO GRAVITROPISM Gravitropism is a dynamic process that involves the perception of an organ’s abnormal orientation within the gravity field, a transduction of the corresponding information into a biochemical signal, the transmission of this signal to a site of response, and organ curvature
Published work discussed in this review, mostly on Arabidopsis, indicates that protein trafficking through the endomembrane system plays a critical role in all of these processes
SOME ENDOMEMBRANE SYSTEM-ASSOCIATED PROTEINS MEDIATE EARLY GRAVITY SIGNAL TRANSDUCTION INDEPENDENTLY OF AMYLOPLAST SEDIMENTATION ALTERED RESPONSE TO GRAVITY 1 (ARG1/RHG) and its paralog ARG1-LIKE 2 (ARL2/GPS4) encode DnaJ-domain-containing peripheral membrane proteins that are necessary for full root and hypocotyl gravitropism (Fukaki et al, 1997; Sedbrook et al, 1999; Boonsirichai et al, 2003; Guan et al, 2003; Luesse et al, 2010)
Summary
TO GRAVITROPISM Gravitropism is a dynamic process that involves the perception of an organ’s abnormal orientation within the gravity field, a transduction of the corresponding information into a biochemical signal, the transmission of this signal to a site of response, and organ curvature. SOME ENDOMEMBRANE SYSTEM-ASSOCIATED PROTEINS MEDIATE EARLY GRAVITY SIGNAL TRANSDUCTION INDEPENDENTLY OF AMYLOPLAST SEDIMENTATION ALTERED RESPONSE TO GRAVITY 1 (ARG1/RHG) and its paralog ARG1-LIKE 2 (ARL2/GPS4) encode DnaJ-domain-containing peripheral membrane proteins that are necessary for full root and hypocotyl gravitropism (Fukaki et al, 1997; Sedbrook et al, 1999; Boonsirichai et al, 2003; Guan et al, 2003; Luesse et al, 2010). PHOSPHATIDYLINOSITOL SIGNALING MEDIATES VESICLE TRAFFICKING, AUXIN GRADIENT FORMATION, AND THE GRAVITROPIC RESPONSE Phosphatidylinositol monophosphate 5-kinase (PIP5K) catalyzes the synthesis of phosphatidylinositol 4,5-bisphosphate (PIP2), a plasma membrane-localized phospholipid.
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