Wed. 9 Mar, 2011 15:00 - 16:00
Title: Systems analysis of root gravitropism: new insights into an old hypothesis using a novel auxin reporter
Malcolm Bennett, BBSRC/EPSRC Centre for Plant Integrative Biology, University of Nottingham, UK
Place: KB3B1 "Stora hörsalen", KBC
Gravity represents a critical environmental signal for land plants that profoundly influences their growth and development. Reorientation of Arabidopsis seedlings induces an asymmetric release of the growth regulator auxin from gravity-sensing columella cells at the root apex. The resulting lateral auxin gradient is hypothesised to drive a differential growth response termed root gravitropism; where cell expansion on the lower side of the elongation zone is reduced relative to the upper side, causing the root to bend downwards. Despite representing one of the oldest hypotheses in plant biology, key aspects of this model remain to be validated. For example, how rapidly does the lateral auxin gradient form? If auxin redistribution drives root bending, its gradient should form prior to organ curvature. How long does the lateral auxin gradient persist? Does it exist for the duration of a root gravitropic bending response or for a shorter period? What triggers auxin redistribution to return to equal levels? Finally, which root tissue(s) does auxin target to cause organ curvature?
A major problem in studying the redistribution of auxin in root tissues is the lack of tools to monitor hormone concentrations at high spatio-temporal resolution. We have employed a novel Aux/IAA-based reporter, DII-VENUS (developed by Teva Vernoux, ENS-Lyon), in conjunction with a mathematical model to quantify auxin redistribution following a gravity stimulus. Our multidisciplinary approach detected rapid auxin redistribution to cells on the lower side of the root apex minutes after a gravity stimulus and then a rapid loss of auxin asymmetry as bending roots reached an angle of 40°. Based on this high resolution spatio-temporal information we conclude that auxin functions as the gravitropic effector and a novel ‘tipping point’ mechanism operates to reverse the asymmetric auxin flow at the mid-point of root bending.