Characterization and mapping of a shattering mutant in rice that corresponds to a block of domestication genes

Abstract

Easy shattering reduces yield due to grain loss during harvest in cereals. Shattering is also a hindrance in breeding programs that use wild accessions because the shattering habit is often linked to desirable traits. We characterized a shattering mutant line of rice, Hsh, which was derived from a nonshattering japonica variety, Hwacheong, by N-methyl-N-nitrosourea (MNU) treatment. The breaking tensile strength (BTS) of the grain pedicel was measured using a digital force gauge to evaluate the degree of shattering of rice varieties at 5, 10, 15, 20, 25, 30, 35, and 40 days after heading (DAH). The BTS of Hwacheong did not decrease with increasing DAH, maintaining a level of 180-240 gf, while that of Hsh decreased greatly during 10-20 DAH and finally stabilized at 50 gf. Optical microscopy revealed that Hsh had a well-developed abscission layer similar to the wild rice Oryza nivara (accession IRGC105706), while Hwacheong did not produce an abscission layer, indicating that the shattering of Hsh was caused by differentiation of the abscission layer. On the basis of the BTS value and morphology of the abscission layer of F(1) plants and segregation data in F(2) populations, it was concluded that the easy shattering of Hsh was controlled by the single recessive gene sh-h. The gene sh-h was determined to be located on rice chromosome 7 by bulked segregant analysis. Using 14 SSR markers on rice chromosome 7, the gene sh-h was mapped between the flanking markers RM8262 and RM7161 at distances of 1.6 and 2.0 cM, respectively. An SSR marker Rc17 cosegregated with the gene sh-h. The locus sh-h for shattering was tightly linked to the Rc locus conferring red pericarp, as well as a QTL qSD(s)-7-1 for seed dormancy, implying that this region might represent a domestication block in the evolutionary pathway of rice.

Figure 1.

Schematic of rice pedicel showing abscission layer. P, palea; L, lemma; RA, rachila; SL, sterile lemma; AL, abscission layer; SZ, supporting zone; RG, rudimentary glume; VB, vascular bundle; PE, pedicel.

Figure 2.

Time-course change of grain pedicel BTS of rice varieties. Two panicles from primary tillers of two plants were harvested at each measuring time for each variety. Ten grains from the uppermost part of the panicle were measured for each panicle. Average value is shown in this graph.

Figure 3.

Anatomical structure of grain pedicel tissues showing variation of abscission layer morphology in seven rice varieties. Top photo for each variety: ×100 magnification. Bottom photo for each variety: ×600 magnification. Two samples from each of two plants were observed for each variety. AL, abscission layer; P, pedicel; SL, sterile lemma; VB, vascular bundle.

Figure 4.

Anatomical structure of grain pedicel tissues of F₁ plants. (A) Hsh/Hwacheong. (B) Hsh/Blue&Gundil. (C) Hsh/Milyang23. Top photo in A–C: ×100 magnification. Bottom photo in A–C: ×600 magnification. Two samples from each of two plants were observed for each variety. AL, abscission layer; P, pedicel; SL, sterile lemma; VB, vascular bundle.

Figure 5.

Histogram showing grain pedicel BTS value distribution in two F₂ populations from the Hsh/Blue&Gundil and Hsh/Milyang23 crosses. Average BTS value and the range including average BTS ± standard deviation of parental varieties is shown by solid inverse triangles and thick lines, respectively.

Figure 6.

Examples of BSA using markers RM445 and RM214, which belong to chromosome 7. PCR products of SSR marker primers were run on a 4% denaturing polyacrylamide gel. BG: Blue&Gundil; M23: Milyang23; L1–L3: low-BTS DNA bulks including six plant DNA samples, each selected among 18 F₂ plants having the lowest grain pedicel BTS value; H1–H3: high-BTS DNA bulks including six plant DNA samples, each selected among 18 F₂ plants having the highest BTS value.

Figure 7.

Molecular genetic map of rice chromosome 7 showing the location of the shattering gene sh-h (primer for Rc17: forward 5′-AGTTTGGAGGAAGCAGCAAA-3′; backward 5′-CACCAACTTGCTCTACTTGTGG-3′).