Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, IA 50011, USA.
Plant J. 2012 Jun;70(6):1015-32. doi: 10.1111/j.1365-313X.2012.04942.x. Epub 2012 Mar 31.
Acetoacetyl CoA thiolase (AACT, EC 2.3.1.9) catalyzes the condensation of two acetyl CoA molecules to form acetoacetyl CoA. Two AACT-encoding genes, At5g47720 (AACT1) and At5g48230 (AACT2), were functionally identified in the Arabidopsis genome by direct enzymological assays and functional expression in yeast. Promoter::GUS fusion experiments indicated that AACT1 is primarily expressed in the vascular system and AACT2 is highly expressed in root tips, young leaves, top stems and anthers. Characterization of T-DNA insertion mutant alleles at each AACT locus established that AACT2 function is required for embryogenesis and for normal male gamete transmission. In contrast, plants lacking AACT1 function are completely viable and show no apparent growth phenotypes, indicating that AACT1 is functionally redundant with respect to AACT2 function. RNAi lines that express reduced levels of AACT2 show pleiotropic phenotypes, including reduced apical dominance, elongated life span and flowering duration, sterility, dwarfing, reduced seed yield and shorter root length. Microscopic analysis reveals that the reduced stature is caused by a reduction in cell size and fewer cells, and male sterility is caused by loss of the pollen coat and premature degeneration of the tapetal cells. Biochemical analyses established that the roots of AACT2 RNAi plants show quantitative and qualitative alterations in phytosterol profiles. These phenotypes and biochemical alterations are reversed when AACT2 RNAi plants are grown in the presence of mevalonate, which is consistent with the role of AACT2 in generating the bulk of the acetoacetyl CoA precursor required for the cytosol-localized, mevalonate-derived isoprenoid biosynthetic pathway.
乙酰乙酰辅酶 A 硫解酶（AACT，EC 2.3.1.9）催化两个乙酰辅酶 A 分子缩合形成乙酰乙酰辅酶 A。通过直接酶学测定和在酵母中的功能表达，在拟南芥基因组中鉴定出两个编码 AACT 的基因，At5g47720（AACT1）和 At5g48230（AACT2）。启动子::GUS 融合实验表明，AACT1 主要在维管束系统中表达，AACT2 在根尖、幼叶、顶茎和花药中高度表达。在每个 AACT 基因座的 T-DNA 插入突变等位基因的特征描述中，确立了 AACT2 功能对于胚胎发生和正常雄性配子传递是必需的。相比之下，缺乏 AACT1 功能的植物完全存活且没有明显的生长表型，表明 AACT1 在功能上与 AACT2 功能冗余。表达 AACT2 水平降低的 RNAi 系显示出多效表型，包括顶端优势降低、寿命和开花持续时间延长、不育、矮化、种子产量降低和根长缩短。显微镜分析表明，矮小的原因是细胞大小和细胞数量减少，雄性不育是由于花粉外壁丢失和绒毡层细胞过早退化。生化分析确定，AACT2 RNAi 植物的根在植物甾醇谱中表现出定量和定性的改变。当 AACT2 RNAi 植物在甲羟戊酸存在下生长时，这些表型和生化改变得到逆转，这与 AACT2 在生成细胞质定位的、甲羟戊酸衍生的异戊烯基生物合成途径所需的乙酰乙酰辅酶 A 前体的大部分中起作用是一致的。
