Resumen de Systems analysis of root growth: new mechanistic insights using a novel auxin reporter
Leah R. Band, Géraldine Brunoud, Darren M. Wells, Antoine Larrieu, Alistair M. Middleton, Jianyong Sun, Andrew French, Stefan Mairhofer, Susan Zappala, Ethel Mendocilla Sato, Michael Wilson, Benjamin Peret, Marina Oliva, Ranjan Swarup, Geraint Parry, Tom Beeckman, Jon Garibaldi, Mark Estelle, Markus R. Owen, Kris Vissenberg, Charlie Hodgman, John R. King, Jan Traas, Tony Pridmore, Sacha Mooney, Teva Vernoux, Malcolm J. Bennett
Plants ability to sense gravity and respond by modifying root growth is critical for exploring thesoil environment to capture nutrients and water. The mechanisms for gravity perception andresponse have long fascinated scientists since Darwin and Sachs. Seedling reorientation ishypothesised to induce asymmetric release of auxin from gravity-sensing columella cells at theroot apex. The resulting lateral auxin gradient is hypothesised to drive a differential growthresponse termed root gravitropism; where cell expansion on the lower side of the elongationzone is reduced relative to the upper side, causing the root to bend downwards. Despiterepresenting one of the oldest hypotheses in plant biology (1926), key aspects of this modelremain to be validated. For example, how rapidly does the lateral auxin gradient form? If auxinredistribution drives root bending, its gradient should form prior to organ curvature. How longdoes the lateral auxin gradient persist? Does it exist for the duration of a root gravitropicbending response or for a shorter period? What triggers auxin redistribution to return to equallevels? A major problem in studying the redistribution of auxin in root tissues is the lack oftools to monitor hormone concentrations at high spatio-temporal resolution.We have developed a novel Aux/IAA-based reporter, DII-VENUS, in conjunction with amathematical model to quantify auxin redistribution following a gravity stimulus. Ourmultidisciplinary approach detected rapid auxin redistribution to cells on the lower side of theroot apex minutes after a gravity stimulus and then a rapid loss of auxin asymmetry as bendingroots reached an angle of 40°. Based on this high resolution spatio-temporal information weconclude that auxin functions as the gravitropic effector and a novel ‘tipping point’ mechanismoperates to reverse the asymmetric auxin flow at the mid-point of root bending. We concludethat roots have evolved a gravity-sensing mechanism controlling auxin redistribution in anangle-dependent manner, ideally suited to the heterogeneous soil environment in which theygrow.
