The Impact of the Wheat Rht-B1b Semi-Dwarfing Allele on Photosynthesis and Seed Development Under Field Conditions

Abstract

The Reduced Height (Rht) genes formed the basis for the green revolution in wheat by decreasing plant height and increasing productive tillers. There are two current widely used Rht mutant alleles, Rht-B1b and Rht-D1b. Both reduce plant height by 20% and increase seed yield by 5-10%. They are also associated with decreased seed size and protein content. Here, we tested the degree to which Rht-B1b impacts flag leaf photosynthetic rates and carbon and nitrogen partitioning to the flag leaf and grain during grain fill under field conditions using near isogenic lines (NILs) that were either standard height (Rht-B1a) or semi-dwarf (Rht-B1b). The results demonstrate that at anthesis, Rht-B1b reduces flag leaf photosynthetic rate per unit area by 18% and chlorophyll A content by 23%. Rht-B1b significantly reduced grain protein beginning at 14 days post anthesis (DPA) with the greatest difference seen at 21 DPA (12%). Rht-B1b also significantly decreased individual seed weight beginning at 21 DPA and by 15.2% at 28 DPA. Global expression analysis using RNA extracted from developing leaves and stems demonstrated that genes associated with carbon and nitrogen metabolism are not substantially altered by Rht-B1b. From this study, we conclude that Rht-B1b reduces flag leaf photosynthetic rate at flowering while changes in grain composition begin shortly after anthesis.

Keywords: genetics; grain fill; photosynthesis; plant breeding; wheat.

Figure 1

Relative amounts of chlorophyll in flag leaf tissue at anthesis for Rht-B1a (wild-type) and Rht-B1b (mutant) near isogenic lines. ^∗ denotes significance at P-value < 0.05 in comparisons of Rht-B1a vs. Rht-B1b, N = 4, where n represents the composite of five sampling plants per row, error bars represent the standard error.

Figure 2

Abundance of protein and starch quantified in flag leaves over a diurnal period at anthesis for Rht-B1a (wild-type) and Rht-B1b (mutant) near isogenic lines. (A) Leaf protein abundance throughout photoperiod. (B) Leaf starch throughout diurnal period. ^∗ denotes significance at P < 0.05 in comparisons of Rht-B1a vs. Rht-B1b, for (A) N = 4, where n represents the composite of five flag leaf samples taken per row; error bars represent the standard error. For (B) N = 5, where n represents the composite of five flag leaf samples taken per row error bars represent the standard error.

Figure 3

Comparison of grain number and size throughout grain fill for Rht-B1a (wild-type) and Rht-B1b (mutant) near isogenic lines. (A) Number of seeds per head throughout grain fill. (B) Individual seed mass over grain fill. ^∗, ^∗∗, ^∗∗∗ denote P-value at 0.05, 0.01, 0.001, respectively; N = 10, where n represents tissue collected from one plant per row; error bars represent the standard error.