Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2006
Publication Date: 3/15/2006
Citation: Kebrom, T.H., Burson, B.L., Finlayson, S.A. 2006. Phytochrome b represses teosinte branched 1 expression and induces sorghum axillary bud outgrowth in response to light signals. Plant Physiology. 140:1109-1117.
Interpretive Summary: When grass seeds germinate, the seedlings that emerge from the seed have a central axis that elongates and produces the shoot that eventually develops into a stem or culm. As this shoot grows, leaves emerge from it at designated point along the stem. At the base of each developing leaf, an axillary or lateral bud develops in the axis of where the leaf and the central stem join. These axillary buds have the capacity to germinate or sprout and produce new shoots similar to the primary stem; however, whether or not this happens depends upon genetics, growth hormones, management practices such as closeness of planting and degree of foliar defoliation, and environmental factors such as light, soil fertility, and temperature. In grasses, the axillary buds near the base of the primary stem often produce new shoots. This is commonly called tillering and these new shoots are referred to as tillers. Whether or not these buds produce tillers has a major impact on the architecture or overall form of the grass plant that develops, and this in turn greatly impacts how the grass is used. For example, if the basal branches develop horizontally and grow along the surface of the soil, the plant forms a sod and is often used as a turf grass. In some grasses, only aerial tillers are formed and since these can produce floral heads, this can increase grain yields in thinly seeded cereals such as wheat and oats. Tillering greatly increases forage production in those grasses that are grazed by livestock. Because tillering is so important, this study was conducted to learn more about how light effects the expression of genes that control dormancy and out growth of axillary buds in grain sorghum. It was determined that when sorghum was grown in shade, tillering did not occur because a gene (SbTB1) controlling branching was turned on. However, when sorghum was grown under bright light, tillering or branching was enhanced because the gene (SbTB1) was repressed or turned off. These findings provide insight into the genetic control of axillary bud growth and tillering in grasses.
Technical 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 this 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 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 the 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.