Molecular genetics of reproductive biology in orchids.

Orchids are members of the family Orchidaceae, one of the largest families of flowering plants. There are an estimated 20,000 to 25,000 orchid species, which occupy wide ranges of ecological habitats and exhibit highly specialized morphological, structural, and physiological characteristics (Dressler, 1990). In particular, the most spectacular evolution is shown in reproductive biology. The production of column (a fused structure of stamens and styles) to facilitate pollination is well documented and the co-evolution of orchid flowers and pollinators is well known (van der Pijl and Dodson, 1969). Even more significant but less well-known aspects are the early development and maturation of the pollen grains (packaged as pollinia—pollen grains bound together by viscin threads in masses for effective pollination), the postpollination development and maturation of ovules, the synchronized timing of microand megagametogenesis for effective fertilization along the whole length of placenta, and the release of tens of thousands or millions of immature embryos (globular stage) in mature capsules (Raghavan and Goh, 1994). Without doubt, these various strategies, unique to orchids, contribute to the success of orchid family. During the past decade, intensive molecular studies with the model plant Arabidopsis and with rice (Oryza sativa) have elucidated many gene regulation processes during development, particularly on reproductive biology of flowering. These studies would be further enhanced greatly with the complete sequencing of Arabidopsis and rice genomes and the numerous expressed sequence tag sequencing projects in a wide range of plants. In contrast, few molecular genetic investigations have so far been undertaken on floral transition and subsequent reproductive growth in orchids. This was, in some way, limited by the rather extended and complicated flowering process. The inefficient orchid transformation system also hampered the investigation of gene function and regulation in vivo. However, recent successes in in vitro thin-section techniques for micropropagation and flowering of orchids have not only shortened the orchid juvenile phase from several years to only a few months but also provided more obvious “landmark” events during development (Lakshmanan et al., 1995; Goh, 1996). Also, the improved orchid transformation system (Chia et al., 1994; Yang et al., 1999; Yu et al., 2001) would certainly facilitate studies on gene function. Progress in molecular genetics of orchids can be expected to accelerate in the near future. The unique differences in reproductive biology in orchids as compared with the normal development in Arabidopsis offer distinct advantages to study gene function and evolution in prepollination and post-pollination development.