Abstract
Magnesium protoporphyrin IX methyltransferase (ChlM) catalyzes the formation of magnesium protoporphyrin IX monomethylester (MgPME) from magnesium protoporphyrin IX (MgP) in the chlorophyll synthesis pathway. However, no ChlM gene has yet been identified and studied in monocotyledonous plants. In this study, a spontaneous mutant, yellow-green leaf 18 (ygl18), was isolated from rice (Oryza sativa). This mutant showed yellow-green leaves, decreased chlorophyll level, and climate-dependent growth differences. Map-based cloning of this mutant identified the YGL18 gene LOC_Os06g04150. YGL18 is expressed in green tissues, especially in leaf organs, where it functions in chloroplasts. YGL18 showed an amino-acid sequence similarity to that of ChlM from different photosynthetic organisms. In vitro enzymatic assays demonstrated that YGL18 performed ChlM enzymatic activity, but ygl18 had nearly lost all ChlM activity. Correspondingly, the substrate MgP was largely accumulated while the product MgPME was reduced in ygl18 leaves. YGL18 is required for light-dependent and photoperiod-regulated chlorophyll synthesis. The retarded growth of ygl18 mutant plants was caused by the high light intensity. Moreover, the higher light intensity and longer exposure in high light intensity even made the ygl18 plants be more susceptible to death. Based on these results, it is suggested that YGL18 plays essential roles in light-related chlorophyll synthesis and light intensity–involved plant growth.
Highlights
Chlorophyll is the main component of the photosynthetic pigments found in plants, algae and cyanobacteria (Czarnecki and Grimm, 2012)
The ygl18 phenotypic characterization implied that the YGL18 is probably involved in chlorophyll synthesis and climate-dependent plant growth in rice
Further examination of the YGL18 gene expression revealed it was rapidly upregulated in etiolated wild-type and ygl18 seedlings after illumination (Figure 8C). These results suggested that YGL18 plays an essential role in the lightdependent chlorophyll synthesis during the greening of etiolated plants
Summary
Chlorophyll is the main component of the photosynthetic pigments found in plants, algae and cyanobacteria (Czarnecki and Grimm, 2012). OsChlD was cloned with the spontaneous yellow-green leaf mutant ygl, and the RNAi plants of OsChlD were found to have a lethal phenotype (Deng et al, 2014). OsCRD1 was cloned by using a pale-green leaf mutant m167 and a yellow leaf mutant yl-1, and found to have a dual role in chlorophyll synthesis and photosynthesis capacity (Sheng et al, 2017; Wang et al, 2017). OsPORB, encoding the NADPH:protochlorophyllide oxidoreductase B, was cloned by using the faded green leaf mutant fgl, and it was found essential for chlorophyll synthesis under high light conditions (Sakuraba et al, 2013). OsDVR encodes the divinyl reductase, which catalyzes the conversion of divinyl chlorophyll(ide) a to monovinyl chlorophyll(ide) a; OsDVR was cloned by using the spontaneous yellow-green leaf mutant 824ys (Wang et al, 2010). OsCAO1 was identified through the pale green leaf mutant pgl, impacting leaf senescence and indirectly affecting grain yield and quality (Yang et al, 2016)
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