Light perception modification

Plant growth and reproduction can be modified by changing light perception. By over- or down-expressing light receptors, it is possible to modify some characters specifically regulated by phytochromes, such as plant development, circadian rhythms, apical dominance, blossom, growth and fruit ripening, photosynthesis products partitioning, development of photosynthetic systems, transpiration control and hormone synthesis (Vince-Prue and Canham 1983; Tucker 1976; Muleo and Thomas 1993; Muleo and Thomas 1997). Several phytochromes are present in the plants, e.g. in Arabidopsis, five phytochromelike coding regions (A–E) have been identified (Sharrock and Quail 1989; Clark et al. 1994), and in tomato there is evidence that more than five are present (Hauser et al. 1995). Since phytochrome genes share considerable sequence homology, the isolation of a large number of gene fragments and cDNAs is rather easy (Robson and Smith 1997). Research using reporter genes with region promoters of phyA and phyB revealed that both promoters are expressed in most tissues except in pollen in which only phyB is expressed. In addition both endogenous and transgenic phytochromes are produced and are exposed to many of the same degradative and signalling mechanisms (Robson and Smith 1997). Phys from several herbaceous plants have been isolated (Robson and Smith, 1997) and recently also from fruit crops (Muleo, pers. comm.) and the expression of transgenes of both phyA and phyB affects a number of responses in both monocotyledonous and dicotyledonous species. The major function of phys in mature plants is the regulation of the ‘shade avoidance syndrome’. The consequence of this phenomenon is that the resources are channelled towards extension growth of stems and petioles to the detriment of storage and reproductive organs. Reduction of shade avoidance syndrome could be a big advantage particularly in a monoculture, including modern orchards in which the plants are placed very close and susceptible to shade each other with high competition for light. PhyA seems to be a major candidate for reducing the response to shade by constitutive expression in plants. Transgenic herbaceous plants over-expressing phyA show short internode, resulting in decrease of stem elongation, reduction of petiole length, increased chlorophyll content, delayed leaf senescence and decrease of apical dominance (Cherry et al. 1991b).

Studies on fruit crops such as cherry rootstock ‘Colt’ and Citrus sp. (Gentile, pers. com.) over-expressing rice phyA are under way. in vitro growing shoots of the cherry rootstock ‘Colt’ over-expressing phyA of rice have demonstrated a reduction of apical dominance with red and far-red light treatments (Muleo and Iacona 1998). This indicates that the excess of red and far-red light, generated in orchards with high-density planting, could modify the distribution of the photosynthesis assimilates among the vegetative growing organs.