A brassinosteroid-hypersensitive mutant of BAK1 indicates that a convergence of photomorphogenic and hormonal signaling modulates phototropism

Abstract

The phototropic response of Arabidopsis (Arabidopsis thaliana) is induced by the phototropin photoreceptors and modulated by the cryptochrome and phytochrome photoreceptors. Downstream of these photoreceptors, asymmetric lateral redistribution of auxin underlies the differential growth, which results in phototropism. Historical physiological evidence and recent analysis of hormone-induced gene expression demonstrate that auxin and brassinosteroid signaling function interdependently. Similarly, in this study we report evidence that interactions between brassinosteroids and auxin signaling modulate phototropic responsiveness. We found that elongated, a previously identified photomorphogenesis mutant, enhances high-light phototropism and represents a unique allele of BAK1/SERK3, a receptor kinase implicated in brassinosteroid perception. Altogether, our results support the hypothesis that phototropic responsiveness is modulated by inputs that influence control of auxin response factor-mediated transcription.

Figure 1.

Photomorphogenic and hormonal mutants showing altered phototropic response under high-fluence rate unilateral blue light. The fluence rate of blue light was 100.0 μmol m⁻² s⁻¹. Vertex curvature (mm⁻¹) was used to measure the magnitude of phototropism. Error bars represent se.

Figure 2.

Photomorphogenic and hormonal mutants showing altered phototropic response under very-low-fluence rate unilateral blue light. The fluence rate of blue light was 0.01 μmol m⁻² s⁻¹. Vertex curvature (mm⁻¹) was used to measure the magnitude of phototropism. Error bars represent se.

Figure 3.

elg is an allele of BAK1/SERK3 (At4g33430). A, elg plants have a point mutation (GGA to GAA) in the fourth exon, 1,370 bp from the transcriptional start site. B, This mutation is predicted to change Asp-122 to an Asn residue in the third LRR of BAK1.

Figure 4.

The elg mutant shows increased sensitivity to brassinolide and IAA. A, Root growth of wild-type seedlings (7 d old) grown on supplemental brassinolide. B, Root growth of 7-d-old wild-type seedlings grown on supplemental IAA. Error bars represent se.

Figure 5.

Supplemental brassinolide enhances high-light phototropism and slightly reduces very-low-light phototropism. A and B, High-light (A; 100 μmol m⁻² s⁻¹) and very-low-light (B; 0.01 μmol m⁻² s⁻¹) phototropic responses of brassionolide (1.0 nm) and mock-treated (ethanol) wild-type (Ler) seedlings. Vertex curvature (mm⁻¹) was used to measure the magnitude of phototropism. Error bars represent se.

Figure 6.

The phototropic responses of bak1-1 mutants and BAK1 overexpressing plants (gBAK1). A, Phototropic responses to high-fluence rates of unilateral blue light (100 μmol m⁻² s⁻¹). B, Phototropic responses to very-low-fluence rates of unilateral blue light (0.01 μmol m⁻² s⁻¹). Vertex curvature (mm⁻¹) was used to measure the magnitude of phototropism. Error bars represent se.

Figure 7.

Partially de-etiolated seedlings have an enhanced high-light phototropic response and slower very-low-light response compared to etiolated seedlings. Partially de-etiolated seedlings were grown under continuous white light (1.0 μmol m⁻² s⁻¹) for 3 d at 23°C prior to blue-light treatment (450 nm ± 50). The responses of etiolated seedlings shown here have been previously reported (Whippo and Hangarter, 2003). A and B, Vertex curvature (mm⁻¹) of etiolated and partially de-etiolated wild-type (Col) hypocotyls under (A) high-fluence rates of unilateral blue light (100.0 μmol m⁻² s⁻¹) or (B) under very-low-fluence rates of unilateral blue light (0.01 μmol m⁻² s⁻¹). Error bars represent se.