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Abstract
SummaryGravitropism is an adaptive response that orients plant growth parallel to the gravity vector. Asymmetric distribution of the phytohormone auxin is a necessary prerequisite to the tropic bending both in roots and shoots. During hypocotyl gravitropic response, the PIN3 auxin transporter polarizes within gravity‐sensing cells to redirect intercellular auxin fluxes. First gravity‐induced PIN3 polarization to the bottom cell membranes leads to the auxin accumulation at the lower side of the organ, initiating bending and, later, auxin feedback‐mediated repolarization restores symmetric auxin distribution to terminate bending. Here, we performed a forward genetic screen to identify regulators of both PIN3 polarization events during gravitropic response. We searched for mutants with defective PIN3 polarizations based on easy‐to‐score morphological outputs of decreased or increased gravity‐induced hypocotyl bending. We identified the number of hypocotyl reduced bending (hrb) and hypocotyl hyperbending (hhb) mutants, revealing that reduced bending correlated typically with defective gravity‐induced PIN3 relocation whereas all analyzed hhb mutants showed defects in the second, auxin‐mediated PIN3 relocation. Next‐generation sequencing‐aided mutation mapping identified several candidate genes, including SCARECROW and ACTIN2, revealing roles of endodermis specification and actin cytoskeleton in the respective gravity‐ and auxin‐induced PIN polarization events. The hypocotyl gravitropism screen thus promises to provide novel insights into mechanisms underlying cell polarity and plant adaptive development.
Highlights
Plants exhibit developmental plasticity to adjust to the changing environmental conditions
PIN3 in hypocotyl endodermal cells polarizes to the bottom plasma membrane (PM), which mediates auxin flow to and corresponding auxin accumulation at the lower organ side leading to growth promotion there and hypocotyl bending (Figure 1a,b; Rakusova et al, 2011, 2016)
To identify novel molecular regulators of PIN3 polarity switches during hypocotyl gravitropic response, we designed a forward genetic screen that would allow us to Bending angle (a) 80
Summary
Plants exhibit developmental plasticity to adjust to the changing environmental conditions. Expression and localization pattern, as well as polarity change after tropic stimuli, PIN3 appears to be the main mediator of the lateral directional auxin transport during hypocotyl tropic responses (Friml et al, 2002; Harrison and Masson, 2008; Ding et al, 2011; Rakusova et al, 2011, 2016). PIN3 in hypocotyl endodermal cells polarizes to the bottom PMs, which mediates auxin flow to and corresponding auxin accumulation at the lower organ side leading to growth promotion there and hypocotyl bending (Figure 1a,b; Rakusova et al, 2011, 2016). At the later stage of gravitropic bending, auxin distribution equalizes and hypocotyl terminates bending (Figure 1a,b; Rakusova et al, 2016) This bending termination is triggered by the auxin-mediated feedback regulation of PIN3 polar localization in endodermal cells at the lower side of
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