ABSTRACT
The sulphated pentapeptide phytosulphokine (PSK) was identified as a substance that promotes cell division in low-density suspension cultures and has been implicated in various aspects of tissue differentiation in plants. The peptide is derived from PSK precursor proteins that are encoded by small gene families. The physiological roles of PSK are still not clearly defined and little is known about expression of members of the PSK precursor gene family in any plant species. In this study, highly regulated tissue and cell type-specific expression are described for four PSK genes from maize (Zea mays L.) in female and male gametophytes, and during seed development. ZmPSK1 and ZmPSK3 were specifically and differentially expressed in cells of female and male gametophytes and in female and male gametes. In anthers ZmPSK1 or ZmPSK3 transcripts were found, for example, at high levels in secretory tapetal cells which support developing microspores. ZmPSK1 mRNA was abundant in mature pollen including sperm cells. ZmPSK1 and ZmPSK3 transcripts were also detected in egg and central cells of the female gametophyte and ZmPSK1 mRNA was present in synergids, indicating that the PSK peptide probably plays a role during gametogenesis, pollen germination, and fertilization. In developing maize kernels all four ZmPSK genes were expressed, albeit with striking differences in their expression patterns. It is proposed here that PSK is required for numerous but defined processes during gametophyte and early sporophyte development. In general, PSK availability appears to be controlled through transcriptional regulation in a tissue and cell type-specific and development-dependent manner.
