The Physiology of Flowering Plants - KHAM PHA MOI

UNRAVELLING PHOTOMORPHOGENESIS 257grown in darkness but were originally isolated on the basis ofincreased anthocyanin accumulation when grown in the light.Many of these mutants (involving lesions in at least 11 differentgenes) show an almost complete photomorphogenic development intotal darkness. They have short hypocotyls, the cotyledons open andtrue leaves form, chloroplasts develop and many light-regulated genesare expressed. It is quite remarkable that these plants can completetheir entire life cycle in darkness, from germination to floweringand seed production, if provided with a suitable nutrient source(e.g. glucose). They do not green under these conditions, however, asthe biosynthesis of chlorophyll includes a light-dependent step. Thedet, cop and fus genes are obviously fundamental in maintaining theseedling in an etiolated state. Other mutations result in only partialphotomorphogenesis in darkness.Some of the genes which have been mutated in these plants havebeen identified and sequenced, providing valuable insights into thisaspect of plant development. The det2 mutant contains a lesion in thegene encoding a cytochrome P450. This class of enzymes plays acentral role in many aspects of secondary metabolism and plantscontain many different cytochrome P450s (at least 30 in Arabidopsis).This particular cytochrome P450 is required for brassinosteroidbiosynthesis (section 7.2.9) and the phenotype of this mutant revertsto that of the wild type if supplied with appropriate brassinosteroidprecursors. Det2 mutants grown in the light are severely dwarfed,indicating that these compounds are important in both dark- andlight-regulated developmental pathways. A number of other dwarfplants have since been found to have lesions in other parts of thebrassinosteroid biosynthetic pathway. Interestingly, wild-type plantstreated with cytokinins also develop in a de-etiolated manner whengrown in darkness, although the significance of this is not yet clear.Other det/cop/fus mutations are thought to be components of a signaltransduction chain involved in both skoto- and photomorphogenesis.The role of some of these is described in more detail in section 10.7.2,and the reader is referred to recent reviews (by Schwechheimer &Deng, and Hardtke & Deng; see Complementary reading for a moredetailed explanation.)10.6 Unravelling photomorphogenesisGiven that plants contain so many light-responsive pathways, theability to inactivate individual components of specific pathwaysselectively provides a powerful tool with which to study these aspectsof plant biology. This approach can be readily extended to examinemultiple pathways by crossing different mutants together and selectingoffspring which have lesions in two, three or even more photoreceptors.This is amply demonstrated in the examples described inthe following section.