ABSTRACT
BACKGROUND AND AIMS: During sexual reproduction in higher angiosperms, the pollen tubes are directed to the ovules in the pistil to deliver sperm cells. This pollen tube attraction is highly species specific, and a group of small secreted proteins, TfCRPs, are necessary for this process in Torenia fournieri. METHODS: A candidate pollen tube attractant protein in Torenia concolor, a related species of T. fournieri, was isolated and the attractant abilities between them were compared. KEY RESULTS: TcCRP1, an orthologous gene of TfCRP1 from T. concolor, is expressed predominantly in the synergid cell. The gene product attracted pollen tubes in a concentration-dependent manner, but attracted fewer pollen tubes from the other species. CONCLUSIONS: The results indicated that this class of CRP proteins is a common pollen tube attractant in Torenia species. The sequence diversity of these proteins is important for species-specific pollen tube attraction.
Subject(s)
Lamiaceae/metabolism , Plant Proteins/metabolism , Pollen Tube/metabolism , Amino Acid Sequence , Crosses, Genetic , Gene Expression Regulation, Plant , Lamiaceae/cytology , Lamiaceae/genetics , Lamiaceae/physiology , Molecular Sequence Data , Organ Specificity/genetics , Ovule/cytology , Ovule/physiology , Plant Proteins/chemistry , Plant Proteins/genetics , Plant Proteins/isolation & purification , Pollen Tube/anatomy & histology , Pollen Tube/cytology , Pollen Tube/growth & development , Sequence Homology, Amino Acid , Species SpecificityABSTRACT
For more than 140 years, pollen tube guidance in flowering plants has been thought to be mediated by chemoattractants derived from target ovules. However, there has been no convincing evidence of any particular molecule being the true attractant that actually controls the navigation of pollen tubes towards ovules. Emerging data indicate that two synergid cells on the side of the egg cell emit a diffusible, species-specific signal to attract the pollen tube at the last step of pollen tube guidance. Here we report that secreted, cysteine-rich polypeptides (CRPs) in a subgroup of defensin-like proteins are attractants derived from the synergid cells. We isolated synergid cells of Torenia fournieri, a unique plant with a protruding embryo sac, to identify transcripts encoding secreted proteins as candidate molecules for the chemoattractant(s). We found two CRPs, abundantly and predominantly expressed in the synergid cell, which are secreted to the surface of the egg apparatus. Moreover, they showed activity in vitro to attract competent pollen tubes of their own species and were named as LUREs. Injection of morpholino antisense oligomers against the LUREs impaired pollen tube attraction, supporting the finding that LUREs are the attractants derived from the synergid cells of T. fournieri.
