The Physiology of Flowering Plants - KHAM PHA MOI

280 REPRODUCTIVE DEVELOPMENTFig: 11:4 Multiple pathwaysregulate the transition fromvegetative to reproductive growth.In Arabidopsis thaliana bothenvironmental and internal signalsinteract. In this simplified diagram,many interactions betweensignalling pathways have beenomitted for clarity.AUTONOMOUSFRIFLCVEGETATIVECRY1, PHYBPHOTOPERIODCOCLOCKGIVRN2VERNALIZATIONCRY2, PHYAREPRESSIONEMFGIBBERELLINREPRODUCTIVEFig: 11:5 The emf 2–3 mutant ofArabidopsis, 17 days after sowing.The seedling possesses twocotyledons, c, but a flower bud, fb,has been produced instead of thefirst true leaves. Scale bar = 1.3 mm.From Yang et al. (1995). Withpermission from Elsevier.early flowering as the repressive signal is lost. Vernalization isthought to repress the expression of FLC, allowing flowering to proceed.A number of genes regulate this vernalization response forminga regulatory network. One such gene, FRIGIDA (FRI), is found invernalization-sensitive ecotypes and promotes the expression ofFLC, thus repressing flowering. Other genes, including VRN1 andVRN2, are required for the perception of the vernalizing temperaturesand lead to a decrease in FLC expression. Most ecotypes which are notvernalization-sensitive, and hence flower early, have lesions in theFRI gene which disrupt its function, although a few have lesions inthe FLC gene. The role of DNA methylation in this signal transductionpathway, as discussed in Section 11.3.5, remains to be resolved, andvernalization can trigger flowering in a FLC-independent manner.Models which seek to explain the photoperiodic induction of floweringby long days must also incorporate the role of the circadian clock. Itis thought that this circadian clock consists of a series of proteins whichregulate each others’ expression forming an oscillating system. Thiscentral circadian oscillator is synchronized with the photoperiod bythe action of two photoreceptors, phy B and cry 1, which perceive highirradiancered and blue light respectively (Chapter 10). The output ofthe circadian clock controls the expression of many other genes including,but not limited to, those which regulate the photoperiodic inductionof flowering. Mutations in these output genes cause late floweringunder long days (LD) but have little effect on flowering time under shortdays (SD). Such mutations include the co (constans) andgi (gigantea),