Os05g0349700

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Annotated Information

Function

Yellow-green leaf1 (ygl1), was isolated, which showed yellow-green leaves in young plants with decreased Chl synthesis, increased level of tetrapyrrole intermediates, and delayed chloroplast development.The mutant plant exhibits a yellow-green leaf phenotype, decreased level of Chl, and delayed chloroplast development. The ygl1 mutant was a spontaneous mutant isolated from indica rice ‘Zhenhui 249’, which exhibited a yellow-green leaf phenotype. The ygl1 mutant was slightly smaller than wild type throughout the developmental stage and exhibited reduced levels of Chl a/b as well as carotenoid (Car) content.Leaves of the ygl1 mutant had 20% to 70% reduction of Chl, and 30% to 40% reduction of Car levels compared to those in wild type at different stages, with the most significant differences detectable in 4-week-old plants. The Chl a/b ratio appeared highest at the seedling stage, due likely to the potential of Chl b synthesis in suffering a more severe decline than Chl a. The Chl a/b ratio then declined to eventually reach the wild-type level. Together this suggests that the ygl1 mutant exhibited delayed greening during photomorphogenesis because of slow rates of Chl accumulation. Eventually, mutant plants accumulated substantial quantities of Chl, reaching almost the wildtype levels and becoming slightly yellow with the maturation of leaves.

Expression

Genetic analysis demonstrated that the phenotype of ygl1 was caused by a recessive mutation in a nuclear gene. The ygl1 locus was mapped to chromosome 5 and isolated by map-based cloning. Sequence analysis revealed that it encodes the Chl synthase and its identity was verified by transgenic complementation. A missense mutation was found in a highly conserved residue of YGL1 in the ygl1 mutant, resulting in reduction of the enzymatic activity. YGL1 is constitutively expressed in all tissues, and its expression is not significantly affected in the ygl1 mutant. Interestingly, the mRNA expression of the cab1R gene encoding the Chl a/b-binding protein was severely suppressed in the ygl1 mutant. Moreover, the expression of some nuclear genes associated with Chl biosynthesis or chloroplast development was also affected in ygl1 seedlings. We compared the level of YGL1 transcript in ygl1 mutant and wild-type plants using RT-PCR. Figure 5A showed that YGL1 mRNA was expressed at similar levels in root, leaf sheaths, leaves, and young panicles in both the ygl1 mutant and wild type. We also examined the effect of light and dark growth conditions on the expression of YGL1. No change in transcript levels was observed when ygl1 or wild-type plants were grown under light or dark conditions. Furthermore, no significant differences of YGL1 mRNA levels were observed in the mutant compared to wild type from early to mature stages.These results indicate that the missense mutation of ygl1 does not affect its own mRNA expression. We next addressed the question of whether the ygl1 mutation affected the transcript of other genes associated with Chl biosynthesis, chloroplast development,or photosynthesis. Analysis of mRNA levels using real-time PCR showed that the expression of genes involved in Chl biosynthesis, such as glutamyl tRNA reductase (HEMA1), was reduced by about 40%, and Chlide a oxygenase1 (CAO1) and NADPH:Pchlide oxidoreductase(PORA) were slightly reduced in ygl1 mutant seedlings compared with wild type (Fig. 6). Interestingly, the expression of cab1R, which encodes the lightharvesting Chl a/b-binding protein of PSII was severely suppressed, whereas another cab gene, cab2R, showed only slightly decreased mRNA levels in the ygl1 mutant. The expression levels of plastid genes, psaA and psbA encoding two reaction center polypeptides, and rbcL encoding the large subunit of Rubisco, were not significantly reduced in the ygl1 mutant. However, the expression of the nuclear rbcS gene encoding the small subunit of Rubisco was slightly decreased in the ygl1 mutant. Taken together, it is likely that that the ygl1 mutation affected the transcript of most nuclear genes, such as cab1R, HEMA1, CAO1, etc., but not the expression of plastid-encoded genes including psaA, psbA, and rbcL in the ygl1 mutant.

Evolution

The rice YGL1 was more closely related to Chl synthase from the monocotyledon plant oat than to those of other species. Not surprisingly, YGL1 has a phylogenetically much closer relationship to Chl synthases of the higher plant species than to bacteria proteins. In addition, it is interesting to note that bacteriochlorophyll synthases lack a motif (WAGHDF-197) that exists only in the Chl synthase. Analysis with the transmembrane calculation programs revealed that the ygl1 mutation site occurred at or close to the end of a transmembrane helix.

Labs working on this gene

 1.National Key Facility for Crop Gene Resources and Genetic Improvement, Institute of Crop Science, Chinese Academy of Agricultural Sciences, Beijing, China
 2.National Key Laboratory for Crop Genetics and Germplasm Enhancement, Jiangsu Plant Gene Engineering Research Center, Nanjing Agricultural University, Nanjing, China

References

 1.Wu, Z., Zhang, X., He, B., Diao, L., Sheng, S., Wang, J., & Wan, J. (2007). A chlorophyll-deficient rice mutant with impaired chlorophyllide esterification in chlorophyll biosynthesis. Plant physiology, 145(1), 29-40.
 2.Shalygo, N., Czarnecki, O., Peter, E., & Grimm, B. (2009). Expression of chlorophyll synthase is also involved in feedback-control of chlorophyll biosynthesis. Plant molecular biology, 71(4-5), 425-436.
 3.Wu, Z. M., Zhang, X., Wang, J. L., & Wan, J. M. (2014). Leaf chloroplast ultrastructure and photosynthetic properties of a chlorophyll-deficient mutant of rice. Photosynthetica, 1-6.