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.
