title: Tomato Pistil Factor STIG1 Promotes in Vivo Pollen Tube Growth by Binding to Phosphatidylinositol 3-Phosphate and the Extracellular Domain of the Pollen Receptor Kinase LePRK2.
abstract: 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.
authors.affiliation: 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.
authors.forename: Wei-Jie
authors.initials: WJ
authors.lastname: Huang
authors.affiliation: 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.
authors.forename: Hai-Kuan
authors.initials: HK
authors.lastname: Liu
authors.affiliation: 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.
authors.forename: Sheila
authors.initials: S
authors.lastname: McCormick
authors.affiliation: 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 whtang@sibs.ac.cn.
authors.forename: Wei-Hua
authors.identifier: http://orcid.org/0000-0002-6167-7870
authors.initials: WH
authors.lastname: Tang