The catalytic subunit of magnesium-protoporphyrin IX monomethyl ester cyclase forms a chloroplast complex to regulate chlorophyll biosynthesis in rice

Abstract

YGL8 has the dual functions in Chl biosynthesis: one as a catalytic subunit of MgPME cyclase, the other as a core component of FLU-YGL8-LCAA-POR complex in Chl biosynthesis. Magnesium-protoporphyrin IX monomethyl ester (MgPME) cyclase is an essential enzyme involved in chlorophyll (Chl) biosynthesis. However, its roles in regulating Chl biosynthesis are not fully explored. In this study, we isolated a rice mutant yellow-green leaf 8 (ygl8) that exhibited chlorosis phenotype with abnormal chloroplast development in young leaves. As the development of leaves, the chlorotic plants turned green accompanied by restorations in Chl content and chloroplast ultrastructure. Map-based cloning revealed that the ygl8 gene encodes a catalytic subunit of MgPME cyclase. The ygl8 mutation caused a conserved amino acid substitution (Asn182Ser), which was related to the alterations of Chl precursor content. YGL8 was constitutively expressed in various tissues, with more abundance in young leaves and panicles. Furthermore, we showed that expression levels of some nuclear genes associated with Chl biosynthesis were affected in both the ygl8 mutant and YGL8 RNA interference lines. By transient expression in rice protoplasts, we found that N-terminal 40 amino acid residues were enough to localize the YGL8 protein to chloroplast. In vivo experiments demonstrated a physical interaction between YGL8 and a rice chloroplast protein, low chlorophyll accumulation A (OsLCAA). Moreover, bimolecular fluorescence complementation assays revealed that YGL8 also interacted with the other two rice chloroplast proteins, viz. fluorescent (OsFLU1) and NADPH:protochlorophyllide oxidoreductase (OsPORB). These results provide new insights into the roles of YGL8, not only as a subunit with catalytic activity, but as a core component of FLU-YGL8-LCAA-POR complex required for Chl biosynthesis.

Differential alternative polyadenylation contributes to the developmental divergence between two rice subspecies, japonica and indica

“…Among the 14 samples, the APA profile in 60‐day‐old leaf of indica exhibited dramatic differences from japonica (Figures a and a–c). In 60‐day‐old leaf, four genes with 3′ UTR APA site‐switching events were overrepresented in the pathway of porphyrin and chlorophyll metabolism (Table S5), which has a strong effect on chlorophyll content and photosynthesis in rice (Zhang et al ., , ; Kusaba et al ., ; Sato et al ., ; Kong et al ., ). Among them, two genes exhibiting 3′ UTR shortening in indica , chlorina‐9 ( Chl9 ) and yellow‐green leaf 8 ( YGL8 ; validated by qRT‐PCR in Figure S19a,c), had significantly higher expression levels than those in japonica (Figure a,c, Table S8).…”

“…In this study, the most noticeable change in APA was observed in the pathway of porphyrin and chlorophyll metabolism, where Chl9 and YGL8 exhibited 3′ UTR shortening and high expression in indica compared with japonica (Figure a,c). Both genes encode the key catalytic subunit of two essential enzymes for chlorophyll biosynthesis and chloroplast development, and their enhanced expression can promote the chlorophyll content and photosynthetic efficiency in rice (Zhang et al ., , ; Kong et al ., ). Correspondingly, in indica , the chlorophyll content and net photosynthetic rate in 60‐day‐old leaf were significantly higher than in japonica (Figure e,f).…”

A xylan glucuronosyltransferase gene exhibits pleiotropic effects on cellular composition and leaf development in rice

“…To study the genetic and molecular basis of leaf color, more than one hundred of rice mutants associated with leaf colors and chlorophyll content have been identified (http://archive.gramene.org/db/genes/). Several genes have been implicated in the chlorophyll biosynthesis, degradation, and regulation of chloroplast development through analyses of chlorophyll-deficiency mutations, which are typically reflected in leaf color [8][9][10][11] . For example, SGR (Stay Green Rice) gene encodes an ancient protein containing a putative chloroplast transit peptide.…”

Yellow-Leaf 1 encodes a magnesium-protoporphyrin IX monomethyl ester cyclase, involved in chlorophyll biosynthesis in rice (Oryza sativa L.)

“…In addition, we found that transcription level of YL-1 was dramatically reduced in the yl-1 mutant, which may interfere with its enzyme activity (Fig 8). Recently, ygl8 , a novel gene was reported in rice, which also shows defects in Chl synthesis and chloroplast development with chlorisis phenotype [31]. Taken these evidences, we concluded that YL-1 may play a pivotal function during Chl synthesis (Fig 8).…”

“…The photosynthesis and chloroplast morphology phenotypes conferred by mutations in MPEC have mostly not been reported, although recently, ygl8 , a novel MPEC defect mutant in rice, has been identified. This also shows a chlorisis phenotype with faults in Chl synthesis [31]. …”