A classic example of the light control of seed germination is seen in the Grand Rapids cultivar of lettuce. Such control, and the demonstration of its photoreversibility, was a key milestone in the identification of phytochromes (Borthwick et al. 1952).
Germination has traditionally been considered the result of imbibition and the initiation of cell expansion in the embryo, causing it to break through the seed coat. A recent, elegant analysis (Masubelele et al. 2005), however, has unequivocally demonstrated that the activity of the root apical meristem (RAM) is at the heart of the process. Radicle protrusion is preceded by cell cycle entry in the RAM. Global transcriptome analysis revealed that the expression of 2000 genes changed prior to visible radicle protrusion, with regulators of cell cycle re-entry (six D-type and two A-type cyclins) appearing among them; up-regulation of the levels of one of the D-type cyclins successfully accelerated germination.
Light and the plant hormones gibberellins (GAs) are very well documented triggers of germination. Arabidopsis thaliana seed germination, like that of lettuce, is promoted by light. The process is exquisitely light sensitive, being initiated by fluences of light present in seconds of moonlight, and this is mediated by the phytochrome A (phyA) photoreceptor (Shinomura et al. 1996). The action of this and other phytochromes is mediated by GA, as shown by the inability of GA-deficient plants to germinate in the dark or light. Indeed, phytochrome rapidly up-regulates the transcription of GA3ox1, a gene that converts the last inactive GA into GA4, the active GA in Arabidopsis. This up-regulation is defective in the phyB mutant (Yamaguchi et al. 1998).
The control of GA biosynthesis takes place through a short signalling cascade, using established phytochrome-interacting transcriptional regulators. PIF1/PIL5, one such protein, is directly destabilised upon interaction with active phytochrome, and is itself a negative regulator of the GA biosyn-thetic genes (Oh et al. 2006). In turn, the action of GAs, which directly or indirectly leads to the cell cycle re-entry in the RAM, occurs through the destabilisation of the growth-repressive DELLA proteins. This is shown by the light-independent, constitutive germination of DELLA multiple mutants (Cao et al. 2005).
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