Ma Zhaoxue, Wu Wenjuan, Huang Weihua, Huang Jirong
National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, 200032, China.
Photosynth Res. 2015 Dec;126(2-3):301-10. doi: 10.1007/s11120-015-0101-5. Epub 2015 Mar 4.
Chloroplast development is regulated by many biological processes. However, these processes are not fully understood. Leaf variegation mutants have been used as powerful models to elucidate the genetic network of chloroplast development since the degree of leaf variegation is regulated by developmental and environmental cues. The thylakoid formation 1 (thf1) mutant is unique for its variegation in both leaves and cotyledons. Here, we reported a new suppressor gene of thf1 leaf variegation, designated sot8. Map-based cloning and DNA sequencing results showed that a single nucleotide mutation from G to A was detected in the second exon of the gene encoding the ribosomal protein small subunit 9 (PRPS9) in sot8-1, resulting in an amino acid change and a partial loss of PRPS9 function. However, sot8-1 was unable to rescue the thf1 phenotype in low photosystem II activity (Fv/Fm). In addition, we identified two T-DNA insertion mutants defective in plastid-specific ribosomal proteins (PSRPs), psrp2-1, and psrp5-1. Genetic analysis showed that knockdown of PSRP5 expression but not PSRP2 expression suppressed leaf variegation. Northern blotting results showed that precursors of plastid rRNAs over-accumulated in prps9-1 and psrp5-1, indicating that mutations in PRPS9 and PSRP5 cause a defect in rRNA processing. Consistently, inhibition of plastid protein synthesis by spectinomycin led to increased levels of plastid rRNA precursors in wild-type plants, suggesting that rRNA processing and plastid ribosomal assembly are coupled. Taken together, our data indicate that downregulating the expression of specific plastid ribosomal proteins suppresses thf1 leaf variegation, and provide new insights into a role of THF1 in plastid gene expression.
叶绿体的发育受多种生物学过程调控。然而，这些过程尚未完全明晰。由于叶片斑驳程度受发育和环境因素调控，叶片斑驳突变体已成为阐明叶绿体发育遗传网络的有力模型。类囊体形成1（thf1）突变体因其叶片和子叶均出现斑驳而独具特色。在此，我们报道了一个新的thf1叶片斑驳抑制基因，命名为sot8。基于图谱的克隆和DNA测序结果显示，在sot8 - 1中，编码核糖体蛋白小亚基9（PRPS9）的基因第二个外显子中检测到一个从G到A的单核苷酸突变，导致氨基酸改变以及PRPS9功能部分丧失。然而，在低光系统II活性（Fv/Fm）条件下，sot8 - 1无法挽救thf1的表型。此外，我们鉴定出两个质体特异性核糖体蛋白（PSRPs）缺陷的T - DNA插入突变体，即psrp2 - 1和psrp5 - 1。遗传分析表明，敲低PSRP5的表达而非PSRP2的表达可抑制叶片斑驳。Northern杂交结果显示，质体rRNA前体在prps9 - 1和psrp5 - 1中过度积累，表明PRPS9和PSRP5中的突变导致rRNA加工缺陷。同样，壮观霉素抑制质体蛋白质合成导致野生型植物中质体rRNA前体水平升高，这表明rRNA加工与质体核糖体组装是相互关联的。综上所述，我们的数据表明下调特定质体核糖体蛋白的表达可抑制thf1叶片斑驳，并为THF1在质体基因表达中的作用提供了新的见解。
