Parker Nicole, Wang Yixing, Meinke David
Department of Plant Biology, Ecology, and Evolution, Oklahoma State University, Stillwater, Oklahoma 74078.
Department of Plant Biology, Ecology, and Evolution, Oklahoma State University, Stillwater, Oklahoma 74078
Plant Physiol. 2016 Nov;172(3):1862-1875. doi: 10.1104/pp.16.01291. Epub 2016 Oct 5.
Natural accessions of Arabidopsis (Arabidopsis thaliana) differ in their ability to tolerate a loss of chloroplast translation. These differences can be attributed in part to variation in a duplicated nuclear gene (ACC2) that targets homomeric acetyl-coenzyme A carboxylase (ACCase) to plastids. This functional redundancy allows limited fatty acid biosynthesis to occur in the absence of heteromeric ACCase, which is encoded in part by the plastid genome. In the presence of functional ACC2, tolerant alleles of several nuclear genes, not yet identified, enhance the growth of seedlings and embryos disrupted in chloroplast translation. ACC2 knockout mutants, by contrast, are hypersensitive. Here we describe an expanded search for hypersensitive accessions of Arabidopsis, evaluate whether all of these accessions are defective in ACC2, and characterize genotype-to-phenotype relationships for homomeric ACCase variants identified among 855 accessions with sequenced genomes. Null alleles with ACC2 nonsense mutations, frameshift mutations, small deletions, genomic rearrangements, and defects in RNA splicing are included among the most sensitive accessions examined. By contrast, most missense mutations affecting highly conserved residues failed to eliminate ACC2 function. Several accessions were identified where sensitivity could not be attributed to a defect in either ACC2 or Tic20-IV, the chloroplast membrane channel required for ACC2 uptake. Overall, these results underscore the central role of ACC2 in mediating Arabidopsis response to a loss of chloroplast translation, highlight future applications of this system to analyzing chloroplast protein import, and provide valuable insights into the mutational landscape of an important metabolic enzyme that is highly conserved throughout eukaryotes.
拟南芥（Arabidopsis thaliana）的自然种质在耐受叶绿体翻译缺失的能力上存在差异。这些差异部分可归因于一个重复的核基因（ACC2）的变异，该基因将同聚体乙酰辅酶A羧化酶（ACCase）靶向到质体。这种功能冗余使得在缺乏由质体基因组部分编码的异聚体ACCase的情况下，仍能进行有限的脂肪酸生物合成。在存在功能性ACC2的情况下，几个尚未确定的核基因的耐受等位基因可增强叶绿体翻译受阻的幼苗和胚胎的生长。相比之下，ACC2基因敲除突变体则表现出超敏性。在此，我们描述了对拟南芥超敏种质的扩展搜索，评估所有这些种质是否在ACC2上存在缺陷，并对在855个已测序基因组的种质中鉴定出的同聚体ACCase变体的基因型与表型关系进行了表征。在所检测的最敏感种质中，包括具有ACC2无义突变、移码突变、小缺失、基因组重排以及RNA剪接缺陷的无效等位基因。相比之下，大多数影响高度保守残基的错义突变并未消除ACC2的功能。我们还鉴定出了几个种质，其超敏性不能归因于ACC2或Tic20-IV（ACC2摄取所需的叶绿体膜通道）的缺陷。总体而言，这些结果强调了ACC2在介导拟南芥对叶绿体翻译缺失反应中的核心作用，突出了该系统在分析叶绿体蛋白导入方面的未来应用，并为一种在整个真核生物中高度保守的重要代谢酶的突变格局提供了有价值的见解。
