High temperature promotes auxin-mediated hypocotyl elongation in Arabidopsis

Abstract

Physiological studies with excised stem segments have implicated the plant hormone indole-3-acetic acid (IAA or auxin) in the regulation of cell elongation. Supporting evidence from intact plants has been somewhat more difficult to obtain, however. Here, we report the identification and characterization of an auxin-mediated cell elongation growth response in Arabidopsis thaliana. When grown in the light at high temperature (29 degreesC), Arabidopsis seedlings exhibit dramatic hypocotyl elongation compared with seedlings grown at 20 degreesC. This temperature-dependent growth response is sharply reduced by mutations in the auxin response or transport pathways and in seedlings containing reduced levels of free IAA. 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 IAA in seedlings grown at high temperature, suggesting that temperature regulates auxin synthesis or catabolism to mediate this growth response. Consistent with this possibility, high temperature also stimulates other auxin-mediated processes including auxin-inducible gene expression. Based on these results, we propose that growth at high temperature promotes an increase in auxin levels resulting in increased hypocotyl elongation. These results strongly support the contention that endogenous auxin promotes cell elongation in intact plants.

Figure 1

Temperature-induced hypocotyl elongation in Arabidopsis. Nine-day-old Arabidopsis seedlings were grown under continuous light at either 20°C (a–e) or 29°C (f–j). Col (a and f); axr1–12 (b and g); 35S-iaaLys (c and h); tir3–1 (d and i); Col grown on medium supplemented with 1 μM NPA (e and j). (Bars = 2 mm.)

Figure 2

Temperature-induced hypocotyl elongation is auxin-dependent. Wild-type (Col), transgenic 35S-iaaLys and mutant lines were grown at 20°C or 29°C under constant light. Hypocotyls were measured from 9-day-old seedlings. Growth medium was supplemented with 1 μM NPA where noted. Data represent mean ± SEM (n = 10).

Figure 3

Hypocotyl elongation over time. Wild-type (Col) and 35S-iaaLys lines were grown at 20°C or 29°C under constant light. Hypocotyls were measured 4, 7, 10, and 13 days after germination. Data represent mean ± SEM (n = 10). Col 20°C, solid line with circles; Col 29°C, broken line with circles; iaaLys 20°C, solid line with triangles; and iaaLys 29°C, broken line with triangles.

Figure 4

Temperature-induced hypocotyl elongation in hormone mutants. Wild-type and mutant lines were grown at 20°C or 29°C under constant light. Hypocotyls were measured from 9-day-old seedlings. The axr1–12, etr1–1, ein2–1, and det2–1 mutations are in the Col background. aba-1 and ga4–1 are in the Ler background. Data represent mean ± SEM (n = 10).

Figure 5

Temperature-induced hypocotyl elongation is a light-specific growth response. Wild-type and mutant lines were grown at 20°C or 29°C in darkness. Hypocotyls were measured from 9-day-old seedlings. Data represent mean ± SEM (n = 10).

Figure 6

High temperature stimulates auxin-inducible gene expression. Wild-type (Col) seedlings carrying a pIAA4-GUS reporter gene were grown for 10 days at 20°C or 29°C on ATS nutrient media and stained for GUS expression. (a) 20°C; (b) 29°C; (c) pIAA4-GUS expression in the roots of 10-day-old wild-type seedlings grown at 29°C (Upper) or 20°C (Lower). LR, lateral root; PR, primary root. (Bars: a and b = 5 mm, c = 0.5 mm.)