Abstract
The introduction of the Reduced height (Rht)-B1b and Rht-D1b semidwarfing genes led to impressive increases in wheat (Triticum aestivum) yields during the Green Revolution. The reduction in stem elongation in varieties containing these alleles is caused by a limited response to the phytohormone gibberellin (GA), resulting in improved resistance to stem lodging and yield benefits through an increase in grain number. Rht-B1 and Rht-D1 encode DELLA proteins, which act to repress GA-responsive growth, and their mutant alleles Rht-B1b and Rht-D1b are thought to confer dwarfism by producing more active forms of these growth repressors. While no semidwarfing alleles of Rht-A1 have been identified, we show that this gene is expressed at comparable levels to the other homeologs and represents a potential target for producing novel dwarfing alleles. In this study, we have characterized additional dwarfing mutations in Rht-B1 and Rht-D1. We show that the severe dwarfism conferred by Rht-B1c is caused by an intragenic insertion, which results in an in-frame 90-bp insertion in the transcript and a predicted 30-amino acid insertion within the highly conserved amino-terminal DELLA domain. In contrast, the extreme dwarfism of Rht-D1c is due to overexpression of the semidwarfing Rht-D1b allele, caused by an increase in gene copy number. We show also that the semidwarfing alleles Rht-B1d and Rht-B1e introduce premature stop codons within the amino-terminal coding region. Yeast two-hybrid assays indicate that these newly characterized mutations in Rht-B1 and Rht-D1 confer "GA-insensitive" dwarfism by producing DELLA proteins that do not bind the GA receptor GA INSENSITIVE DWARF1, potentially compromising their targeted degradation.
Highlights
The introduction of the Reduced height (Rht)-B1b and Rht-D1b semidwarfing genes led to impressive increases in wheat (Triticum aestivum) yields during the Green Revolution
Studies in Arabidopsis (Arabidopsis thaliana) and rice have demonstrated that targeted degradation of DELLAs is initiated by GA binding within a pocket of the GA receptor, GA INSENSITIVE DWARF1 (GID1; UeguchiTanaka et al, 2005; Murase et al, 2008; Shimada et al, 2008)
We provide further evidence that the mutant RHT-1 proteins are unable to interact with the GA receptor, TaGID1, which would result in growth repression that is less responsive to GA
Summary
The introduction of the Reduced height (Rht)-B1b and Rht-D1b semidwarfing genes led to impressive increases in wheat (Triticum aestivum) yields during the Green Revolution. DELLA gain-of-function mutations, which result in reduced GA sensitivity and dwarfism, have been identified in many plant species, including rice and Arabidopsis The majority of these mutations produce amino acid substitutions or deletions in the DELLA/ TVHYNP motifs that affect binding to GID1 (Peng et al, 1997; Dill et al, 2001; Chandler et al, 2002; Asano et al, 2009), but not their transcriptional regulatory activity, which resides in the C-terminal GRAS domain (Silverstone et al, 1998; Chandler et al, 2002; Itoh et al, 2002; Hirano et al, 2010). To determine the importance for the control of stem elongation of Rht-A1, for which no dwarfing alleles have been identified, we have cloned Rht-A1a and demonstrated that it displays a very similar expression profile to Rht-B1a and RhtD1a
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