Poster: Tropisms

Abs # 416: Role of auxin in the control of lateral root growth in Arabidopsis

Presenter:	Mullen, Jack L., jmullen@bio.indiana.edu
Authors	Mullen, Jack L. (A)   Wolverton, Chris  (B)   Hangarter, Roger P. (A)
Affiliations:	(A): Indiana University, Department of Biology (B): Ohio Wesleyan University, Department of Botany - Microbiology

Research on gravitropism in plants has focused mainly on primary roots and shoots, which typically orient to vertical orientations. However, lateral organs, which are important determinants of plant form, frequently grow at non-vertical orientations. Due to their simple morphology and change in orientation with development, lateral roots of Arabidopsis provide a useful system for studying the maintenance of non-vertical orientation. Arabidopsis lateral roots emerge from the primary at a nearly horizontal orientation and slowly increase their declination over several days before becoming vertical. During this time there is an inverse relation between the orientation and growth rate of the lateral roots. In order to examine the role of auxin, an important growth regulator, in lateral root orientation we examined auxin patterns using DR5 reporter constructs. High reporter expression, presumably indicating high auxin levels was found throughout the root apex, even in newly emergent laterals. While reporter expression was observed in cortical cells of the elongation zone in younger laterals, over time expression became more constricted to the root cap area as the roots developed. When seedlings were gravistimulated by a rotation to horizontal, laterals on both the upper and lower side of the primary responded by curving toward their original orientation. However, laterals on the different flanks of the primary had opposite changes in growth rate. Expression of DR5:GFP was consistent with both of these observations and suggests that asymmetric signals from the gravitropic response in the primary root, such as auxin, may regulate lateral root growth. Further work using mutants impaired in auxin transport may help clarify the role of auxin in the control of lateral root growth.