Impaired Magnesium Protoporphyrin IX Methyltransferase (ChlM) Impedes Chlorophyll Synthesis and Plant Growth in Rice

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 <i>ChlM</i> gene has yet been identified and studied in monocotyledonous plants. In this study, a spontaneous mutant, <i>yellow-green leaf 18</i> (<i>ygl18</i>), was isolated from rice (<i>Oryza sativa</i>). This mutant showed yellow-green leaves, decreased chlorophyll level, and climate-dependent growth differences. Map-based cloning of this mutant identified the <i>YGL18</i> gene LOC_Os06g04150. <i>YGL18</i> 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. <i>In vitro</i> 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 <i>ygl18</i> leaves. <i>YGL18</i> is required for light-dependent and photoperiod-regulated chlorophyll synthesis. The retarded growth of <i>ygl18</i> mutant plants was caused by the high light intensity. Moreover, the higher light intensity and longer exposure in high light intensity even made the <i>ygl18</i> plants be more susceptible to death. Based on these results, it is suggested that <i>YGL18</i> plays essential roles in light-related chlorophyll synthesis and light intensity-involved plant growth.