Kachroo Aardra, Venugopal Srivathsa C, Lapchyk Ludmila, Falcone Deane, Hildebrand David, Kachroo Pradeep
Department of Plant Pathology, University of Kentucky, Lexington, KY 40546, USA.
Proc Natl Acad Sci U S A. 2004 Apr 6;101(14):5152-7. doi: 10.1073/pnas.0401315101. Epub 2004 Mar 25.
Stearoyl-acyl-carrier-protein-desaturase-mediated conversion of stearic acid (18:0) to oleic acid (18:1) is a key step, which regulates levels of unsaturated fatty acids in cells. We previously showed that stearoyl-acyl-carrier-protein-desaturase mutants ssi2/fab2 carrying a loss-of-function mutation in the plastidial glycerol-3-phosphate (G3P) acyltransferase (act1) have elevated 18:1 levels and are restored in their altered defense signaling. Because G3P is required for the acylation of 18:1 by G3P acyltransferase, it was predicted that reduction of G3P levels should increase 18:1 levels and thereby revert ssi2-triggered phenotypes. Here we show that a mutation in G3P dehydrogenase restores both salicylic acid- and jasmonic acid-mediated phenotypes of ssi2 plants. The G3P dehydrogenase gene was identified by map-based cloning of the ssi2 suppressor mutant rdc8 (gly1-3) and confirmed by epistatic analysis of ssi2 with gly1-1. Restoration of ssi2-triggered phenotypes by the gly1-3 mutation was age-dependent and correlated with the levels of 18:1. Regeneration of G3P pools by glycerol application in ssi2 and ssi2 gly1-3 plants caused a marked reduction in the 18:1 levels, which rendered these plants hypersensitive to glycerol. This hypersensitivity in ssi2 was rescued by the act1 mutation. Furthermore, overexpression of the ACT1 gene resulted in enhanced sensitivity to glycerol. Glycerol application also lowered the 18:1 content in SSI2 plants and converted these into ssi2-mimics. Our results show that 18:1 levels in plastids are regulated by means of acylation with G3P, and a balance between G3P and 18:1 is critical for the regulation of salicylic acid- and jasmonic acid-mediated signaling pathways.
硬脂酰 - 酰基载体蛋白去饱和酶介导的硬脂酸（18:0）向油酸（18:1）的转化是一个关键步骤，它调节细胞中不饱和脂肪酸的水平。我们之前表明，在质体甘油 -3-磷酸（G3P）酰基转移酶（act1）中携带功能丧失突变的硬脂酰 - 酰基载体蛋白去饱和酶突变体ssi2/fab2具有升高的18:1水平，并且其改变的防御信号得以恢复。由于G3P是G3P酰基转移酶对18:1进行酰化所必需的，因此预测G3P水平的降低应会增加18:1水平，从而恢复ssi2引发的表型。在这里我们表明，G3P脱氢酶的突变恢复了ssi2植物中水杨酸和茉莉酸介导的表型。通过对ssi2抑制突变体rdc8（gly1-3）进行图位克隆鉴定出了G3P脱氢酶基因，并通过ssi2与gly1-1的上位性分析得到证实。gly1-3突变对ssi2引发表型的恢复是年龄依赖性的，并且与18:1的水平相关。在ssi2和ssi2 gly1-3植物中通过施加甘油使G3P库再生导致18:1水平显著降低，这使这些植物对甘油高度敏感。ssi2中的这种超敏反应通过act1突变得以挽救。此外，ACT1基因的过表达导致对甘油的敏感性增强。施加甘油也降低了SSI2植物中的18:1含量，并将它们转化为类似ssi2的植物。我们的结果表明，质体中的18:1水平通过与G3P的酰化作用来调节，并且G3P和18:1之间的平衡对于水杨酸和茉莉酸介导的信号通路的调节至关重要。
