Nowadays, extensive attention has been paid to the leaf-color mutation, and certain achievements have been made by studying various organisms, but the mechanism of mutation and the responsible loci have not been fully understood at molecular level, especially in rice. Mutant analysis is a useful approach to illuminate the function of gene in complex biological processes of chloroplast development.
Recently, many reviews about Arabidopsis pigment defective mutants had been reported. immutans (im), variegated1 (var1) and variegated2 (var2) are the typical leaf variegation mutants, and exhibit green- and white-sectored leaves (Yu et al. 2007; Aluru et al. 2006; Sakamoto et al. 2003). However, in this study, we identified a novel yellow-green leaf mutant ygl138 in rice, and defined it as a homogeneous paleness mutant.
By map-based cloning, the YGL138(t (Satoh et al.) gene was finally delimited in a 91.8 kb region on the short arm of chromosome 11. On the same chromosome, six genes: V91984(Dong et al.); tsc12001(Gothandam et al.); OsPPR12005(Zhang et al.); Z12008(Chai et al.); Z22011(Liu et al.) and yl112012), related to the leaf-color mutation have been mapped up to now, but only OsPPR1 gene has been cloned. Among them, V9 and OsPPR1 mutants exhibit almost purely white at their young seedling stage, and leaves emerging at or after transplanting are pale green with white midrib (Satoh et al. 1984; Gothandam et al. 2005); tsc1 mutant is a thermo-sensitive seedling-color mutant, and its thermo-sensitive decreases gradually with the increase of the seedling age (Dong et al. 2001); Z1 mutants exhibit zebra leaves (Zhang et al. and Z22008; Chai et al. 2011); yl11 mutant is a typical chlorophyll deficient mutant, which has yellow leaves at whole growth stages (Liu et al. 2012). According to the previous reports, the six genes do not locate in the fine mapping region of YGL138(t), so we claim that YGL138(t) is a novel gene leading to leaf-color mutation in rice.
By sequencing, we found that, Os11g05552, which was predicted to encode a SRP54 protein and act as a chloroplast precursor, had 18 bp nucleotides deletion in the coding region of ygl138 and led to a frameshift. Furthermore, the mutant phenotype of ygl138 could be restored to the wild-type phenotype by transformation with the wild-type gene. Therefore, the gene Os11g05552 was identified as the YGL138(t) gene.
In higher plants, chloroplast is a complex organelle with a highly organized internal system, and the chloroplast proteome comprises nuclear- and plastome-encoded proteins (Ferro et al. 2010). In order to function correctly, these proteins must be transported to their final destination in the chloroplast (Richter et al. 2010). In this progress, signal recognition particle (SRP) plays an essential role in binding to the signal sequence of the nascent polypeptide emerging from the ribosome and directing the polypeptide toward the proper cellular compartment (Keenan et al. 2001; Halic et al. 2004).
In eukaryotes, the SRP contains six proteins (SRP9, SRP14, SRP19, SRP54, SRP68, SRP72) and a 300-nucleotide RNA (Zwieb et al. 2005). Among them, SRP54 is the only protein subunit conserved in all SRPs, and it is essential for signal sequence recognition and binding at the ribosome, and for the GTP-dependent interaction with the cognate receptor SR. This interaction determines proper transport of the ribosome-nascent chain complex to the protein-translocating channel in the membrane (Egea et al. 2008). SRP54 can also be involved in the co-translational transport of chloroplast-encoded thylakoid proteins, which is able to switch between the co- and post-translational means of interaction with its respective substrate proteins (Franklin and Hoffman 1993; Li et al. 1995; Schünemann 2004). Meanwhile, the conserved SRP54 protein associates with 70S ribosomes to function in the co-translational transport of the plastid-encoded thylakoid membrane protein D1 (Nilsson et al. 1999; Richter et al. 2010). From these reports we can see that SRP54 is a key component in SRP protein and plays prominent roles in chloroplast development.
SRP54 is a basic protein and has a three-domain structure: an N-terminal helical bundle domain, a central GTPase domain and a C-terminal M domain (Marchler-Bauer et al. 2011). The N-terminal domain and the GTPase domain always associate together to constitute a structural and functional catalytic core, while the C-terminal M domain is responsible for the promiscuous recognition of the diverse signal sequences, and this domain is generally less conversed between different SRP homologues and also varies in length (Egea et al. 2008).
Mutant in SRP54 had been isolated in the model dicotyledonous plant Arabidopsis. ffc1-2, which was generated by EMS mutagenesis, always showed pale green leaves phenotype (Amin et al. 1999). This phenomenon was consistent with the reduction of chlorophyll levels in ffc1-2 mutant compared with its wild-type plants (Hutin et al. 2002). However, to our knowledge, no SRP54 has yet been identified in model monocotyledonous plant rice. In our study, Os11g05552, which was predicted to encode a SRP54 protein and act as a chloroplast precursor, was identified as the novel gene YGL138(t) leading to leaf-color mutation in rice. We believe that further efforts on functional characterization of the YGL138(t) gene will contribute to understand the mechanism of SRP54 protein involving in chloroplast development in rice.