Phenotypic and transcriptomic characterization of a wheat tall mutant carrying an induced mutation in the C-terminal PFYRE motif of RHT-B1b

Abstract

Background: As central regulators of the gibberellic acid (GA) signaling pathway in plants, DELLA proteins function as growth repressors and affect diverse biological processes. The wheat RHT-B1b and RHT-D1b semi-dwarfing alleles, which encode GA-insensitive DELLA proteins, have been widely adopted in modern wheat varieties to improve lodging tolerance and harvest index. However, the molecular mechanisms by which DELLA modulates these responses in wheat remain largely unknown. Results: We identified a tall tetraploid wheat mutant line carrying an induced missense mutation (E529K) in the PFYRE motif of RHT-B1b that partially suppressed the semi-dwarf phenotype. The height-increasing effect of RHT-B1bE529K relative to RHT-B1b (19 cm or 21% increase) was significantly smaller than the effect of RHT-B1a (33 cm or 34% increase) relative to RHT-B1b in the same field experiment. The RHT-B1bE529K mutation was also associated with length increases in coleoptiles, seedling shoots, and stem internodes relative to the RHT-B1b allele. We detected no significant differences in germination rate, seedling root length, tiller number, flag leaf size, spike length, or yield components. Using RNA-seq, we compared gene expression profiles of plants encoding RHT-B1b and RHT-B1bE529K in coleoptile, first leaf, and elongating peduncles. We detected limited overlap among tissues of the genes differentially regulated by the two genotypes, and more genes upregulated (77%) than downregulated (23%) in RHT-B1bE529K relative to RHT-B1b. These results suggest that the wheat DELLA protein affects the transcriptome in a tissue-specific manner and that the mutation mainly eliminates or reduces repression functions of the RHT-B1 protein. Our study identified distinct sets of potential DELLA direct or indirect target genes involved in cell wall and carbohydrate metabolisms, cell cycle/division, and hormone pathways. Conclusions: We identified the hypomorphic RHT-B1bE529K allele that confers an intermediate plant height and coleoptile elongation. This allele can be useful in rain-fed wheat breeding programs where the strong reduction in height and biomass associated with RHT-B1b has detrimental effects. Transcriptomic characterization of different tissues from the plants encoding RHT-B1bE529K and RHT-B1b provided valuable information for identifying DELLA downstream GA response genes in wheat.