The Physiology of Flowering Plants - KHAM PHA MOI

ORGAN FORMATION 225in the clv1 mutant encodes a protein (CLV1) which acts to reduce thenumber of cells in the meristem, whilst STM and WUS act to increasethe number of meristematic cells. These genes interact antagonisticallyand, together with many other genes, act to maintain the size andstructure of the SAM (Fig. 9.3C–D) (Clark et al. 1996).The expression of these genes within the SAM has been investigated,as have the proteins which they encode (Fig. 9.3B). CLV1 isprimarily expressed in the L 3 layer of the CZ and encodes a leucinerichrepeat (LRR) receptor kinase. Another gene, CLAVATA3, whoseloss also leads to meristem enlargement, is primarily expressed inlayers L 1 and L 2 of the CZ. The protein encoded by CLV3 is small, andmost probably secreted from the cell. It is currently hypothesizedthat CLV3 may act as a short-range signal detected by CLV1. Theinteraction between CLV3 and CLV1 represses the expression ofWUS, reducing the size of the meristem. However, as WUS is requiredfor the expression of CLV3, a feedback loop is set up which maintainsthe meristem at the correct size. STM is expressed throughout themeristem and is thought to maintain the meristematic cells in anindeterminate state, as well as playing a role in the regulation ofCLV3 expression. Although many of the details remain to be resolved,it is clear that the SAM is controlled by a network of interacting genesand signals passed over short distances. Hence when a cell is displaced,from one position within the SAM to another, it will receive anew set of signals and develop in a different manner.Box 9.1A detailed description of gene,mutant and protein nomenclatureis given in the Appendix.9.3 Organ formationAs the cells in the PZ divide they are pushed away from the apex ofthe meristem and form organ primordia. The signals which regulatethis process are not well understood but, again, networks of interactinggenes are likely to be important. It is possible that one of thevery earliest steps in primordium formation occurs in response toalterations in cell wall properties which facilitate cell expansion.A number of lines of evidence support this:(1) Irradiation of the SAM with gamma rays inhibits cell divisionsbut a bulge, indicating cell expansion, still appears on the flanks of theSAM in the position where the next primordium would have developed.(2) Plants with mutations in cell division still initiate organ primordianormally.(3) Application of the cell wall loosening expansins (Section 8.4,Fleming et al. 1997) can induce the formation of leaf primordia intomato (Lycopersicon esculentum). Localized expression of expansinscan reverse the phyllotaxy in tobacco (Nicotiana tabacum).The outermost layer L 1 of the SAM is thought to be under tension,restraining the layers below which are under compression. Within theSAM, organs are generated in a phyllotactic spiral (Fig. 9.1C). Organprimordia P 1 and P 2 have developed most recently and the next organ