The Physiology of Flowering Plants - KHAM PHA MOI

290 REPRODUCTIVE DEVELOPMENTFig: 11:12 The role of SEPALLATAgenes in floral development.Inactivation of the SEPALLATA1, 2and 3 genes in Arabidopsis converts awild-type flower (A) into a flower inwhich all organs develop as sepals(B). If transgenic plants areproduced (C) which constitutivelyexpress B-class floral-developmentgenes (PISTILLATA and APETALA3)and SEPALLATA3, which is necessaryfor B gene function, the cotyledons,C, are unaffected by thetransformation but the true leavesdevelop as petaloid organs. A fewstamens, S, and a terminal flower,TF, are produced. Scale bar = 1 mm.(D) Constitutive expression ofB-class genes, SEP3 and the C-classgene AGAMOUS results in thedevelopment of staminoid organs.Both flowers, F, and cauline leaves,CL, are affected. Scale bar = 0.5 mm.(E) Current models of flowerdevelopment propose thatmultimeric complexes of homeoticproteins include SEPALLATA as acomponent. (B) courtesy of S. Pelazand M. Yanofsky, (C) & (D) fromHonma & Goto (2001). # NaturePublishing Group. (E) adapted fromTheißen (2001).A B C D2CSCE?AP1 AP1AP1? AP3 PIAP1 SEPAP3/PISEP3 4TF1demonstrated in a landmark paper by Honma and Goto (2001). Thefirst true leaves of transgenic Arabidopsis plants constitutively expressingPI, AP3 and SEP3 were transformed into petaloid organs, whilst thecauline leaves of transgenic plants expressing PI, AP3, SEP3 and AG weretransformed into staminoid organs (Fig. 11.12). SEP3 was shown to bean important component required for the interaction between the PI,AP3 and AG proteins, and activity of the multimeric complex. Thispaper demonstrates that it is possible to convert vegetative tissue intoreproductive tissue by constitutively expressing homeotic genes.It is important to remember that the ‘ABC’ model is just that – amodel. Not all species will necessarily behave in the same way, andother factors must control the formation of the whorls and theexpression of the individual activities. However, the success of thismodel, and its longevity in what is a rapidly developing field, implythat it is essentially correct. As our understanding of flower developmentgrows, these models become increasingly sophisticated. Forexample, Theiben (2001) proposed a ‘quartet’ model of interactionsbetween homeotic proteins which seeks to integrate the observedmutations, expression patterns and interactions between theseproteins (Fig 11.12).PISEPAGAP3AGAGSEPSepal Petal Stamen CarpelFSEPAGCLCL11.7.2 Helical flowersSo far our discussion of flower patterning has been restricted toflowers in which relatively few organs are formed in a circular arrangementwithin whorls. However, in many species, believed primitive,