Phytochrome B represses Teosinte Branched1 expression and induces sorghum axillary bud outgrowth in response to light signals

Abstract

Light is one of the environmental signals that regulate the development of shoot architecture. Molecular mechanisms regulating shoot branching by light signals have not been investigated in detail. Analyses of light signaling mutants defective in branching provide insight into the molecular events associated with the phenomenon. It is well documented that phytochrome B (phyB) mutant plants display constitutive shade avoidance responses, including increased plant height and enhanced apical dominance. We investigated the phyB-1 mutant sorghum (Sorghum bicolor) and analyzed the expression of the sorghum Teosinte Branched1 gene (SbTB1), which encodes a putative transcription factor that suppresses bud outgrowth, and the sorghum dormancy-associated gene (SbDRM1), a marker of bud dormancy. Buds are formed in the leaf axils of phyB-1; however, they enter into dormancy soon after their formation. The dormant state of phyB-1 buds is confirmed by the high level of expression of the SbDRM1 gene. The level of SbTB1 mRNA is higher in the buds of phyB-1 compared to wild type, suggesting that phyB mediates the growth of axillary shoots in response to light signals in part by regulating the mRNA abundance of SbTB1. These results are confirmed by growing wild-type seedlings with supplemental far-red light that induces shade avoidance responses. We hypothesize that active phyB (Pfr) suppresses the expression of the SbTB1 gene, thereby inducing bud outgrowth, whereas environmental conditions that inactivate phyB allow increased expression of SbTB1, thereby suppressing bud outgrowth.

Figure 1.

Median longitudinal sections of 4-d-old sorghum shoots. Arrows indicate buds in the axil of the first leaves of phyB-1 (A) and wild-type (B) sorghum seedlings.

Figure 2.

Bud length (A) and seedling height (B) of phyB-1 and wild-type sorghum. Inset shows bud length at 7 DAP, 8 DAP, and 9 DAP (not visible in the main figure). Data are mean ± se; n = 10.

Figure 3.

SbTB1 mRNA abundance in leaf, sheath, root, and bud of sorghum seedlings at 9 DAP. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.

Figure 4.

Sequence similarity among sorghum, pea, and Arabidopsis dormancy-associated proteins (SbDRM1, PsDRM1, and AtDRM1, respectively).

Figure 5.

SbDRM1 mRNA abundance in leaf, sheath, root, and bud of sorghum seedlings at 9 DAP. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.

Figure 6.

A, SbTB1 and SbDRM1 mRNA abundance in the buds from the first leaf axils of phyB-1 and wild-type sorghum seedlings. B and C, Relative quantitation of SbTB1 and SbDRM1 mRNA levels, respectively. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.

Figure 7.

Bud length (A) and seedling height (B) of wild-type sorghum seedlings grown at high and low plant densities. Data are mean ± se; n = 9 or 10.

Figure 8.

A, Abundance of SbTB1 mRNA in axillary buds in the axil of the first leaves of wild-type seedlings grown at high and low densities at 7 DAP and 9 DAP. B, SbTB1 mRNA quantitation relative to buds of seedlings grown at low density at 7 DAP. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.

Figure 9.

A, Abundance of SbDRM1 mRNA in axillary buds in the axil of the first leaves of wild-type seedlings grown at high and low densities at 7 DAP and 9 DAP. B, SbDRM1 mRNA quantitation relative to buds of seedlings grown at low density at 7 DAP. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.

Figure 10.

Effect of supplemental FR light on bud outgrowth (A) and seedling height (B) of wild-type sorghum. Data are mean ± se; n = 7.

Figure 11.

A, Abundance of SbTB1 and SbDRM1 mRNA in axillary buds of wild-type seedlings grown with or without supplemental FR light. B, Relative quantitation of SbTB1 and SbDRM1 mRNA levels. The sorghum ubiquitin gene (SbUBQ) was used as a loading control.