Abstract
Root and shoot branches are major determinants of plant form and critical for the effective capture of resources below and above ground. These branches are often maintained at specific angles with respect to gravity, known as gravitropic set point angles (GSAs). We have previously shown that the mechanism permitting the maintenance of non-vertical GSAs is highly auxin-dependent and here we investigate the developmental and environmental regulation of root and shoot branch GSA. We show that nitrogen and phosphorous deficiency have opposing, auxin signalling-dependent effects on lateral root GSA in Arabidopsis: while low nitrate induces less vertical lateral root GSA, phosphate deficiency results in a more vertical lateral root growth angle, a finding that contrasts with the previously reported growth angle response of bean adventitious roots. We find that this root-class-specific discrepancy in GSA response to low phosphorus is mirrored by similar differences in growth angle response to auxin treatment between these root types. Finally we show that both shaded, low red/far-red light conditions and high temperature induce more vertical growth in Arabidopsis shoot branches. We discuss the significance of these findings in the context of efforts to improve crop performance via the manipulation of root and shoot branch growth angle.
Highlights
Root and shoot branches are major determinants of plant form and critical for the effective capture of resources below and above ground
The non-vertical growth phase of an Arabidopsis lateral root does not represent a lack of gravitropic competence but rather the active maintenance of a growth angle that is characteristic of its developmental stage[1,3,6]
The more vertical branch gravitropic set point angles (GSAs) observed in response to treatment with exogenous auxin and in mutants with higher auxin levels led us to examine the possibility that the observed change in GSA as lateral roots grow out could be attributable to increases in endogenous auxin concentration in the root apex
Summary
Root and shoot branches are major determinants of plant form and critical for the effective capture of resources below and above ground. A prominent role for auxin transport and response in the regulation of lateral organ GSA has been described[5,6,7] It has been shown, for example, that treatment with exogenous auxins cause lateral roots in Arabidopsis to grow with a more vertical GSA6,7. In Arabidopsis, a reduction in the ratio of red:far red (R:FR) light leads to a marked increase in hypocotyl and petiole elongation and a corresponding reduction in cotyledon and leaf expansion[8,9] These processes, which form part of the plant’s response to shading by neighbouring plants, require coordinated regulation of distinct developmental processes such as cell proliferation, cell differentiation and direction of cell expansion, all of which are dependent on the action of phytohormones including auxin[10,11]. In young seedlings grown at 29 °C, the most noticeable effect was a drastic increase in hypocotyl and petiole length relative to control plants and again, these effects were shown to be auxin-dependent[12,13,14]
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