Plant reproduction comprises an interlocking array of developmental pathways which include the formation of the sexual organs, the generation of germ lines
de novo
, and the operation of the mechanisms which regulate epigenetic imprinting and the system of self-incompatibility found in m any angiosperms. Little is known of how these processes are regulated at a molecular level, with the exception of the floral organs which are determined by families of homeotic genes operating in a heterochronic fashion. In dioecious and monoecious plants the expression of these ‘floral’ genes must be modulated by sexdeterm ination sequences, situated in some circumstances on sex chromosomes. Older, physiological data indicate that sex can be determined by growth regulators, particularly gibberellic acid (GA) and cytokinins, and it is possible that sex-determination genes establish local concentrations of growth regulators at the apex, which in turn influence the expression of the homeotic floral genes. Evidence from anther development indicates genes involved in differentiation of the male and female germ lines to be regulated by defined promoter, enhancer, and silencer regions, but few data are available on the sequences directing the initiation and regulation of meiosis; certainly parallels can be drawn with similar events in microorganisms, and useful complementation strategies may be devised, but significant differences do exist between yeasts and higher plants suggesting that more appropriate parallels should be drawn with multicellular eukaryotes such as nematodes. The loci involved in epigenetic imprinting and self-incompatibility are important because they affect both male and female developmental pathways. Nothing is known of the regulatory sequences which direct the epigenetic imprinting of the sperm and central cell genomes, but information is becoming available on the promoter regions of the
S
(incompatibility)-locus. Interestingly, sequences directing expression in male and female tissues are contained within a single 5' stretch within the locus, and these prom oters also induce expression in different cell types in the anther and pistil depending on the type of self-incompatibility involved. Regulation of reproductive development in plants is apparently not very stringent, for there are examples in both male and female germ lines of reversion to an embryonic condition (apomixis and microspore embryogenesis); whether this reflects the highly dedifferentiated state of these cells or differences in the regulation of somatic and reproductive development remains to be determined.
Coevolution of theS-Locus GenesSRK,SLGandSP11/SCRinBrassica oleraceaandB. rapa
