Auxin‐mediated statolith production for root gravitropism

Abstract

Summary Root gravitropism is one of the most important processes allowing plant adaptation to the land environment. Auxin plays a central role in mediating root gravitropism, but how auxin contributes to gravitational perception and the subsequent response are still unclear. Here, we showed that the local auxin maximum/gradient within the root apex, which is generated by the PIN directional auxin transporters, regulates the expression of three key starch granule synthesis genes, SS4, PGM and ADG1, which in turn influence the accumulation of starch granules that serve as a statolith perceiving gravity. Moreover, using the cvxIAA‐ccvTIR1 system, we also showed that TIR1‐mediated auxin signaling is required for starch granule formation and gravitropic response within root tips. In addition, axr3 mutants showed reduced auxin‐mediated starch granule accumulation and disruption of gravitropism within the root apex. Our results indicate that auxin‐mediated statolith production relies on the TIR1/AFB‐AXR3‐mediated auxin signaling pathway. In summary, we propose a dual role for auxin in gravitropism: the regulation of both gravity perception and response.

A new wrinkle in our understanding of the role played by auxin in root gravitropism

“…To investigate a potential effect of auxin on the early steps of gravity sensing and/or signal transduction in the root statocytes, Zhang et al . first treated Arabidopsis seedlings with the synthetic auxin α‐naphthalene acetic acid (NAA), with a polar auxin transport inhibitor N ‐1‐naphthylphthalamic acid (NPA), or with the auxin synthesis inhibitor l ‐kynurenin ( l ‐Kyn), and they analyzed the effects of each treatment on auxin accumulation in the root tip (as determined by expression of a DR5‐GFP auxin‐response reporter), starch accumulation in the statocytes, and expression of genes involved in starch metabolism. Treatments leading to increased auxin accumulation in the root tip, such as addition of NAA or NPA to the growth medium, resulted in the formation of larger starch granules in the root statocytes, a process that was accompanied by increased expression of several genes involved in starch biosynthesis, including PHOSPHOGLUCOMUTASE ( PGM ), ADENOSINE DIPHOSPHATE GLUCOSE PYROPHOSPHORYLASE 1 ( ADG1 ) and STARCH SYNTHASE 4 ( SS4 ).…”

“…The plant hormone auxin constitutes a signal that is transmitted from the site of gravity sensing to the site of curvature response (reviewed in Su et al ., ). A ground‐breaking report by Zhang et al . in this issue of New Phytologist (pp. 761–774) now suggests that auxin also functions to regulate gravity perception, doing so by affecting the accumulation of starch within the gravity‐sensing cells of the root cap.…”

“…Model explaining the dual role played by auxin in root gravitropism as established by Zhang et al . in this issue of New Phytologist (pp. 761–774).…”

“…Interestingly, the root statocytes are localized in a region of the root tip that accumulates auxin as a consequence of converging auxin flows and de novo local auxin synthesis (Friml et al ., ). Recognizing this overlap between the auxin‐maximum center in the root tip and the primary site of gravity sensing, Zhang et al . investigate a possible role for auxin in modulating gravity perception. They use a combination of genetic and pharmacological studies to demonstrate a role for auxin in the regulation of starch synthesis in the statocytes, a process that contributes to gravitropic modulation.…”

“…Zhang et al . also show that mutations that alter the flow of auxin toward the root tip ( pin3/4/7 triple mutants) result in the development of smaller starch granules in the statocytes, whereas mutations in PIN2 , which controls the shootward flow of auxin from the root cap to the elongation zone, result in increased expression of starch‐synthesis genes and larger starch granules. Hence, mutations or treatments that affect the auxin‐maximum center at the root tip also alter starch accumulation in the statocytes.…”