The grain color of wheat affects not only the brightness of flour, but also tolerance to preharvest sprouting. Grain color is controlled by dominant R-1 genes located on the long arm of hexaploid wheat chromosomes 3A, 3B, and 3D (R-A1, R-B1, and R-D1, respectively). The red pigment of the grain coat is composed of catechin and proanthocyanidin (PA), which are synthesized via the flavonoid biosynthetic pathway. We isolated the Tamyb10-A1, Tamyb10-B1, and Tamyb10-D1 genes, located on chromosomes 3A, 3B, and 3D, respectively. These genes encode R2R3-type MYB domain proteins, similar to TT2 of Arabidopsis, which controls PA synthesis in testa. In recessive R-A1 lines, two types of Tamyb10-A1 genes: (1) deletion of the first half of the R2-repeat of the MYB region and (2) insertion of a 2.2-kb transposon belonging to the hAT family. The Tamyb10-B1 genes of recessive R-B1 lines had 19-bp deletion, which caused a frame shift in the middle part of the open reading frame. With a transient assay using wheat coleoptiles, we revealed that the Tamyb10 gene in the dominant R-1 allele activated the flavonoid biosynthetic genes. We developed PCR-based markers to detect the dominant/recessive alleles of R-A1, R-B1, and R-D1. These markers proved to be correlated to known R-1 genotypes of 33 varieties except for a mutant with a single nucleotide substitution. Furthermore, double-haploid (DH) lines derived from the cross between red- and white-grained lines were found to necessarily carry functional Tamyb10 gene(s). Thus, PCR-based markers for Tamyb10 genes are very useful to detect R-1 alleles.
Aluminum (Al) resistance in wheat relies on the Al-activated malate efflux from root apices, which appears to be controlled by an Al-activated anion transporter encoded by the ALMT1 gene on chromosome 4DL. Genomic regions upstream and downstream of ALMT1 in 69 wheat lines were characterized to identify patterns that might influence ALMT1 expression. The first 1,000 bp downstream of ALMT1 was conserved among the lines examined apart from the presence of a transposon-like sequence which did not correlate with Al resistance. In contrast, the first 1,000 bp upstream of the ALMT1 coding region was more variable and six different patterns could be discerned (types I-VI). Type I had the simplest structure, while the others had blocks of sequence that were duplicated or triplicated in different arrangements. A pattern emerged among the lines of non-Japanese origin such that the number of repeats in this upstream region was positively correlated with the levels of ALMT1 expression and Al resistance. In contrast, many of the Japanese lines exhibited a large variation in ALMT1 expression and Al resistance despite possessing the same type of upstream region. Although ALMT1 expression was also poorly correlated with Al-activated malate efflux in the Japanese lines, a strong correlation between malate efflux and Al resistance suggested that malate efflux was still the primary mechanism for Al resistance, and that additional genes are involved in the post-transcriptional regulation of ALMT1 function.
The level of grain dormancy and sensitivity to ABA of the embryo, a key factor in grain dormancy, were examined in developing grains of a white-grained wheat line, Novosibirskaya 67 (NS-67), and its red-grained near-isogenic lines (ANK-1A to -1D); a red-grained line, AUS 1490, and its white-grained mutant line (EMS-AUS). ANK lines showed higher levels of grain dormancy than NS-67 at harvest maturity. AUS 1490 grain also showed higher dormancy than EMS-AUS grain. These results suggest that the R gene for grain colour can enhance grain dormancy. However, the dormancy effect conferred by the R gene was not large, suggesting that it plays a minor role in the development of grain dormancy. Water extracts of AUS 1490 and EMS-AUS bran contained germination inhibitors equivalent to 1-10 microM ABA, although there was no difference in the amount of inhibitors between AUS 1490 and EMS-AUS. Thus, the grain colour gene of AUS 1490 did not appear to enhance the level of grain dormancy by accumulating germination inhibitors in its bran. Sensitivity to ABA of embryos was higher in grains collected around harvest-maturity for ANK lines and AUS 1490, compared with NS-67 and EMS-AUS. The R gene might enhance grain dormancy by increasing the sensitivity of embryos to ABA.
;The Purple leaf (Pl) locus of rice (Oryza sativa L.) affects regulation of anthocyanin biosynthesis in various plant tissues. The tissue-specific patterns of anthocyanin pigmentation, together with the syntenic relationship, indicate that the rice Pl locus may play a role in the anthocyanin pathway similar to the maize R/B loci. We isolated two cDNAs showing significant identity to the basic helix-loophelix (bHLH) proteins found in the maize R gene family. OSB1 appeared to be allelic to the previously isolated R homologue, Ra1, but showed a striking difference at the Cterminus because of a 2-bp deletion. Characterization of the corresponding genomic region revealed that the sequence identical to a 5¢ ¢ ¢ ¢-portion of OSB2 existed~10-kb downstream of the OSB1 coding region. OSB2 lacks a conserved C-terminal domain. Restriction fragment length polymorphism analyses using an F 2 population indicate that both genes co-segregate with the purple leaf phenotype. A transient complementation assay showed that the anthocyanin pathway is inducible by OSB1 or OSB2. These results suggest that the Pl w allele may be complex and composed of at least two genes encoding bHLH proteins.
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