Chemocyanin, a small basic protein from the lily stigma, induces pollen tube chemotropism

In plant reproduction, pollination is an essential process that delivers the sperm through specialized extracellular matrices (ECM) of the pistil to the ovule. Although specific mechanisms of guidance for pollen tubes through the pistil are not known, the female tissues play a critical role in this event. Many studies have documented the existence of diffusible chemotropic factors in the lily stigma that can induce pollen tube chemotropism in vitro, but no molecules have been isolated to date. In this study, we identified a chemotropic compound from the stigma by use of biochemical methods. We purified a lily stigma protein that is active in an in vitro chemotropism assay by using cation exchange, gel filtration, and HPLC. Tryptic digestion of the protein yielded peptides that identified the protein as a plantacyanin (basic blue protein), and this was confirmed by cloning the cDNA from the lily stigma. Plantacyanins are small cell wall proteins of unknown function. The measured molecular mass by electrospray ionization ion source MS is 9,898 Da, and the molecular mass of the mature protein (calculated from the cDNA) is 9,900.2 Da. Activity of the lily plantacyanin (named chemocyanin) is enhanced in the presence of stigma/stylar cysteine-rich adhesin, previously identified as a pollen tube adhesin in the lily style.