Inhibition of tiller bud outgrowth in the tin mutant of wheat is associated with precocious internode development

Abstract

Tillering (branching) is a major yield component and, therefore, a target for improving the yield of crops. However, tillering is regulated by complex interactions of endogenous and environmental signals, and the knowledge required to achieve optimal tiller number through genetic and agronomic means is still lacking. Regulatory mechanisms may be revealed through physiological and molecular characterization of naturally occurring and induced tillering mutants in the major crops. Here we characterize a reduced tillering (tin, for tiller inhibition) mutant of wheat (Triticum aestivum). The reduced tillering in tin is due to early cessation of tiller bud outgrowth during the transition of the shoot apex from the vegetative to the reproductive stage. There was no observed difference in the development of the main stem shoot apex between tin and the wild type. However, tin initiated internode development earlier and, unlike the wild type, the basal internodes in tin were solid rather than hollow. We hypothesize that tin represents a novel type of reduced tillering mutant associated with precocious internode elongation that diverts sucrose (Suc) away from developing tillers. Consistent with this hypothesis, we have observed upregulation of a gene induced by Suc starvation, downregulation of a Suc-inducible gene, and a reduced Suc content in dormant tin buds. The increased expression of the wheat Dormancy-associated (DRM1-like) and Teosinte Branched1 (TB1-like) genes and the reduced expression of cell cycle genes also indicate bud dormancy in tin. These results highlight the significance of Suc in shoot branching and the possibility of optimizing tillering by manipulating the timing of internode elongation.

Figure 1.

A, Wild-type (WT; left) and tin mutant (right) wheat plants (var Banks). B, The spike of the main stem of WT (left) and tin mutant (right).

Figure 2.

Development of tiller buds and tillers in the first (A), second (B), and third (C) leaf axil of tin and the wild type (WT) at 15, 18, and 21 DAP. Leaf blade (D) and sheath (E) length of the first, second, and third leaves at 21 DAP. The first and second leaves attained their final leaf blade and sheath length by 15 DAP or earlier and the third leaf by 18 DAP. Data are mean ± se; n = 10.

Figure 3.

Shoot apex (A) and internode (B) length of the wild type (WT) and tin at 15, 18, and 21 DAP. Data for A and B are mean ± se; n = 3. Only the internode length at 21 DAP, shown by the asterisk, is statistically significantly different (P value = 0.001) between the wild type and tin. C, Internode development of WT (left) and tin (right) at 28 DAP. Hollow internode of WT (D) and solid internode of tin (E). The first three basal internodes of tin are solid, and the sections of WT and tin are from the third internodes at 5 weeks after planting.

Figure 4.

Internode development in tin plants. A, The main stem of a tin plant, which was stunted during its development and then recovered; an arrow indicates two successive leaves with no separation between their ligules. B, A tin plant that developed normally with distance between ligules of successive leaves. C, Stems of the stunted and recovered shoot (left) and unstunted shoot (right). The shoots are from 5-week-old plants.

Figure 5.

Gene expression and Suc contents in axillary buds of the wild type (WT) and tin. Relative expression levels of (A) Suc starvation-inducible and (B) Suc-inducible genes, and bud weight (C) and Suc contents (D) in buds in the third leaf axil of WT and tin at 15 and 18 DAP. Data for gene expression are mean ± sem; n = 3 biological replicates from two independent experiments. Data for bud weight and Suc are mean ± se; n = 9 or 10 buds for bud weight and from at least four buds for Suc contents.

Figure 6.

Buds on nodes of elongated internodes of tin and the wild type (WT). A, Arrows on the right shoot indicate buds on nodes of elongated internodes of WT, whereas buds are absent on the nodes of tin (left). Sometimes the buds in tin are crushed when they are young, and there may not be any sign of a bud except a mass of disorganized tissues on the grooves of internodes. B, Arrow indicates a bud that was damaged by the stiff elongated internode and enclosing sheath of the parent shoot. The upper half of the bud is visible, whereas the lower was fused into the groove of the internode. C, The buds on nodes of elongated internodes in the WT are floral. The stems are from 5-week-old plants. The sheath was removed to show the buds.

Figure 7.

Suc contents in successively formed axillary buds of the wild type (WT) from the second to sixth leaf axils, sequentially from 12 to 28 DAP. The buds in leaf axils two, three, and four normally grow out and form tillers, whereas the buds in leaf axils five and six do not grow out. Data for Suc contents are means ± se from at least five buds.

Figure 8.

Expression level of cell cycle-related genes in buds in the third leaf axil of the wild type (WT) and tin at (A) 15 and (B) 18 DAP. Data are means ± sem; n = three biological replicates from two independent experiments.

Figure 9.

Expression level of (A) DRM1-like and (B) TB1-like genes in buds in the third leaf axil of the wild type (WT) and tin at 15 and 18 DAP. Data are means ± sem; n = three biological replicates from two independent experiments.

Figure 10.

A, Control (left) and detillered (right) wild type (WT) plants, and (B) main culm ear of control (left) and detillered (right) plants.