Differential coupling of gibberellin responses by Rht-B1c suppressor alleles and Rht-B1b in wheat highlights a unique role for the DELLA N-terminus in dormancy

Abstract

During the Green Revolution, substantial increases in wheat (Triticum aestivum) yields were realized, at least in part, through the introduction of the Reduced height (Rht)-B1b and Rht-D1b semi-dwarfing alleles. In contrast to Rht-B1b and Rht-D1b, the Rht-B1c allele is characterized by extreme dwarfism and exceptionally strong dormancy. Recently, 35 intragenic Rht-B1c suppressor alleles were created in the spring wheat cultivar Maringá, and termed overgrowth (ovg) alleles. Here, 14 ovg alleles with agronomically relevant plant heights were reproducibly classified into nine tall and five semi-dwarf alleles. These alleles differentially affected grain dormancy, internode elongation rate, and coleoptile and leaf lengths. The stability of these ovg effects was demonstrated for three ovg alleles in different genetic backgrounds and environments. Importantly, two semi-dwarf ovg alleles increased dormancy, which correlated with improved pre-harvest sprouting (PHS) resistance. Since no negative effects on grain yield or quality were observed, these semi-dwarf ovg alleles are valuable for breeding to achieve adequate height reduction and protection of grain quality in regions prone to PHS. Furthermore, this research highlights a unique role for the first 70 amino acids of the DELLA protein, encoded by the Rht-1 genes, in grain dormancy.

Keywords: DELLA; dormancy; gibberellin (GA); pre-harvest sprouting; suppressor alleles; wheat.

Table 1.

Effect of ovg alleles on elongation in different wheat cultivars, in the greenhouse and field Percentage average difference of phenotypic traits for each ovg allele, relative to the reference allele, are shown. The scale next to the table details the intensity of color saturation for percentage difference.

Fig. 1.

Phenotypic effects of three ovg alleles, compared with the respective reference allele, in Faller and KWS Scirocco. From left to right: reference allele (Rht-B1b for Faller, Rht-B1a for KWS Scirocco), Rht-B1c.22, Rht-B1c.23, and Rht-B1c.26. (A, B) Flowering plants 3 months after sowing; scale bar=10 cm. (C, D) Etiolated coleoptiles when 50% of the second leaf was visible; all leaves were removed on the picture; scale bar=1 cm. (E, F) Ten grains; scale bar=1 cm. (G, H) Sprouted spikes, harvested at physiological maturity from plants grown in the greenhouse; scale bar=1 cm.

Fig. 2.

ovg alleles differentially affect plant elongation in different cultivars in the greenhouse. Plant heights are averages of at least 10 plants. The box indicates the period of first spike emergence (Zadoks 50). Black full line, Rht-B1a; black dashed line, Rht-B1b; black dotted line, Rht-B1c; full green line, Rht-B1c.22; full magenta line, Rht-B1c.26; dashed magenta line, Rht-B1c.23. Results were confirmed in two independent experiments. Representative data from the first experiment are shown.

Table 2.

Effect of ovg alleles on grain size and yield in different wheat cultivars, in the greenhouse and field Percentage average difference of phenotypic traits for each ovg allele, relative to the reference allele, are shown. The scale next to the table details the intensity of color saturation for percentage difference. Greenhouse results were confirmed in at least two independent experiments. Representative data from the first experiment are shown.

Fig. 3.

Effect of ovg alleles on TGW in greenhouse-grown Maringá. Shown are the average values for each allele ±SE (n=10). * indicates significant difference from Rht-B1a or Rht-B1b (P<0.05). Tall alleles are in green and semi-dwarf alleles are in magenta.

Fig. 4.

ovg alleles differentially affect dormancy in different cultivars in the greenhouse and field. Average germination ±SE at time points after harvest when the differences between Rht-B1a or Rht-B1b and the ovg alleles were at their maxima. The number of weeks of after-ripening varied between cultivar and environment, hence germination is shown for Maringá after 11 weeks (A); for KWS Scirocco after 10 weeks (greenhouse) and 3.5 weeks (field) (B); for Faller after 2 weeks (greenhouse) and 3.5 weeks (field) (C); for Crusader at harvest and for EGA Gregory after 1 week (D). * indicates a significant difference from Rht-B1a or Rht-B1b (P<0.05). Tall alleles are in green and semi-dwarf alleles are in magenta. For introgressed lines, results were confirmed in at least two sister line pairs, each derived from an independent BCF₁ plant.

Fig. 5.

Effect of ovg alleles on falling number, independent of PHS, in different cultivars in the greenhouse and field. Average falling numbers ±SE are shown (n=3). * indicates a significant difference (P<0.05) from the reference allele, namely Rht-B1c for Maringá (A); Rht-B1a for KWS Scirocco (B); Rht-B1b for Faller (C), as well as for Crusader and EGA Gregory (D). Tall alleles are in green and semi-dwarf alleles are in magenta. For KWS Scirocco and Faller, results were confirmed in at least two sister line pairs, each derived from an independent BCF₁ plant. For KWS Scirocco, results were additionally confirmed in a field trial in 2014 (Supplementary Fig. S4).

Fig. 6.

Summary of the phenotypic effect of the ovg alleles. Summary of data represented in Table 1 and Figs 3–5. Magenta and green triangles indicate the absolute average value for the semi-dwarf and tall ovg alleles, respectively. The number within the triangle indicates the ovg alleles, requiring the Rht-B1c.- prefix. Black triangles indicate the average phenotypic trait for Rht-B1a and Rht-B1b, designated with ‘a’ and ‘b’, respectively, whereas the black rectangle represents Rht-B1c (‘c’).

Fig. 7.

The position of the ovg second-site mutations on the DELLA protein. (A) Position of the ovg second-site mutations over the different motifs and domains of the DELLA protein. The black box indicates the predicted 30 amino acid Rht-B1c insertion, whereas the black triangle shows the position of the Rht-B1b stop codon. Magenta lines in the LExLE motif represent positions of putative translational reinitiation. Magenta and green triangles indicate the position of the semi-dwarf and tall ovg alleles, respectively. The number within the triangle indicates the ovg alleles, requiring the Rht-B1c.- prefix. The scale bare shows the length of 100 amino acids. (B) Positions of the amino acids altered by the C-terminal ovg mutations on the crystal structure of the OsSCL7 GRAS protein (Li et al., 2016). Carbon atoms of semi-dwarf and tall ovg alleles are shown in magenta and green, respectively. Gray carbons represent the position of Rht-B1c.7 and Rht-B1c.24. Nitrogen and oxygen atoms are colored in blue and red, respectively. The inset shows the amino acids surrounding the Rht-B1c.23 (D371) position, where dashed lines represent hydrogen bonds and the red dot represents a water molecule.