Huang Wei-Jie, Liu Hai-Kuan, McCormick Sheila, Tang Wei-Hua
Shanghai Institutes for Biological Sciences-University of California at Berkeley Center of Molecular Life Sciences, 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 University of the Chinese Academy of Sciences, Institute of Plant Physiology and Ecology, Shanghai 200032, China.
Plant Gene Expression Center, U.S. Department of Agriculture/Agricultural Research Service and Department of Plant and Microbial Biology, University of California at Berkeley, Albany, California 94710.
Plant Cell. 2014 Jun;26(6):2505-2523. doi: 10.1105/tpc.114.123281. Epub 2014 Jun 17.
The speed of pollen tube growth is a major determinant of reproductive success in flowering plants. Tomato (Solanum lycopersicum) STIGMA-SPECIFIC PROTEIN1 (STIG1), a small Cys-rich protein from the pistil, was previously identified as a binding partner of the pollen receptor kinase LePRK2 and shown to promote pollen tube growth in vitro. However, the in vivo function of STIG1 and the underlying mechanism of its promotive effect were unknown. Here, we show that a 7-kD processed peptide of STIG1 is abundant in the stigmatic exudate and accumulates at the pollen tube surface, where it can bind LePRK2. Antisense LePRK2 pollen was less responsive than wild-type pollen to exogenous STIG1 in an in vitro pollen germination assay. Silencing of STIG1 reduced both the in vivo pollen tube elongation rate and seed production. Using partial deletion and point mutation analyses, two regions underlying the promotive activity of the STIG1 processed peptide were identified: amino acids 80 to 83, which interact with LePRK2; and amino acids 88 to 115, which bind specifically to phosphatidylinositol 3-phosphate [PI(3)P]. Furthermore, exogenous STIG1 elevated the overall redox potential of pollen tubes in both PI(3)P-dependent and LePRK2-dependent manners. Our results demonstrate that STIG1 conveys growth-promoting signals acting through the pollen receptor kinase LePRK2, a process that relies on the external phosphoinositide PI(3)P.
花粉管生长速度是开花植物繁殖成功的主要决定因素。番茄（Solanum lycopersicum）柱头特异性蛋白1（STIG1）是一种来自雌蕊的富含半胱氨酸的小蛋白，先前被鉴定为花粉受体激酶LePRK2的结合伴侣，并在体外显示出促进花粉管生长的作用。然而，STIG1在体内的功能及其促进作用的潜在机制尚不清楚。在这里，我们表明STIG1的一个7-kD加工肽在柱头渗出物中含量丰富，并在花粉管表面积累，在那里它可以结合LePRK2。在体外花粉萌发试验中，反义LePRK2花粉对外源STIG1的反应比野生型花粉弱。STIG1基因沉默降低了体内花粉管伸长率和种子产量。通过部分缺失和点突变分析，确定了STIG1加工肽促进活性的两个区域：与LePRK2相互作用的第80至83位氨基酸；以及特异性结合磷脂酰肌醇3-磷酸[PI(3)P]的第88至115位氨基酸。此外，外源STIG1以依赖PI(3)P和依赖LePRK2的方式提高了花粉管的整体氧化还原电位。我们的结果表明，STIG1通过花粉受体激酶LePRK2传递促进生长的信号，这一过程依赖于外部磷酸肌醇PI(3)P。
