Tapetum-specific genes: what role do they play in male gametophyte development?

Plant sexual reproduction is a key process in plant development and has a great impact on agriculture. Plant sexual reproduction is established by the interaction between the male gametophyte, producing the sperm cells, and the female gametophyte harbouring the egg cell. The effectivity of this interaction largely depends on the viability and incompatibility properties of the pollen. Therefore, the key role of pollen in the fertilization process justifies a scientific effort to get a better understanding of pollen development, which is also of commercial importance. Development of pollen is initiated inside the anther locule and completed when mature pollen is released. Upon transfer to the female stigma the pollen germinates and subsequently protrudes a tube, containing the sperm cells, towards the ovule where fertilization and seed setting occur (Derksen et al. 1995). Formation of viable pollen is a complex process that depends on accurate execution of developmental programmes in both the sporophytic and gametophytic anther tissues (Kamalay & Goldberg 1980; Mascarenhas 1989, 1990; McCormick 1993; Goldberg et al. 1993). It occurs in the anther in the pollen sac, which is surrounded by a one-cell layer tissue, the tapetum. The tapetum cells are essential for the development of the pollen grain. Tapetal cells arise from the same progenitor cells as the developing male gametophytes and totally enclose the latter during their development (Bhandari 1984; Scott et al. 1991a; Goldberg et al. 1993). This spatial relationship implies that the exchange of nutritional, structural, or regulatory compounds between the sporophytic anther tissues and the developing male gametophytes occurs via the tapetum. The impact of this is exemplified by several cases of male sterility where initial lesions are found in the tapetal tissue and not in the sporogenous cells (Izhar & Frankel 1971; Horner & Rogers 1974; Bino 1985; Grant et al. 1986; Liu et al. 1987; Kaul 1988; Zuberi et al. 1988; Chaudhury et al. 1994; Zhang et al. 1994). Conclusive evidence that the tapetum is essential for pollen development comes from experiments on transgenic Brassica where spatially specific expression of a cytotoxic gene in the tapetal cell layer resulted in destruction of the tapteum and, subsequently, in an arrest of pollen grain development (Mariani et al. 1990).