HIGH-TEMPERATURE PROMOTES AUXIN-MEDIATED HYPOCOTYL ELONGATION IN ARABIDOPSIS

Physiological studies with excised stem segments have implicated the p lant hormone indole-3-acetic acid (IIA or auxin) in the regulation of cell elongation. Supporting evidence from intact plants has been somew hat more difficult to obtain, however. Here, we report the identificat ion and characterization of an auxin-mediated cell elongation growth r esponse in Arabidopsis thaliana. When grown in the light at high tempe rature (29 degrees C), Arabidopsis seedlings exhibit dramatic hypocoty l elongation compared with seedlings grown at 20 degrees C, This tempe rature-dependent growth response is sharply reduced by mutations in th e auxin response or transport pathways and in seedlings containing red uced levels of free IIA. In contrast, mutants deficient in gibberellin and abscisic acid biosynthesis or in ethylene response are unaffected , Furthermore, we detect a corresponding increase in the level of free IIA in seedlings grown at high temperature, suggesting that temperatu re regulates auxin synthesis or catabolism to mediate this growth resp onse. Consistent with this possibility, high temperature also stimulat es other auxin-mediated processes including auxin-inducible gene expre ssion, Based on these results, we propose that growth at high temperat ure promotes an increase in auxin levels resulting in increased hypoco tyl elongation. These results strongly support the contention that end ogenous auxin promotes cell elongation in intact plants.