The Physiology of Flowering Plants - KHAM PHA MOI

226 VEGETATIVE DEVELOPMENTFig: 9:4 Expansins cause leafprimordia to develop in the tobaccoshoot apical meristem and lead to areverse phyllotaxy. Transgenictobacco (Nicotiana tabacum), plantscontaining a tetracycline-inducibleexpansin gene were producedallowing localized induction ofexpansin gene expression. Scanningelectron micrograph (SEM) of anapex from transgenic plants inwhich a small pellet of lanolincontaining tetracycline (A) or buffer(B) was placed at position I2 on themeristem (m) between the leafprimordia P1 and P2. Localinduction of expansin led to thedevelopment of a bulge after 72 h(A) which eventually developed intoa normal leaf, but reversed thephyllotaxy (C). Initially leaves (P3, P2,P1)wereformedinananticlockwisedirection. Leaves formed subsequenttoI’2(I’3–I’7)havebeenformedinaclockwise direction. Control plantstreated with buffer developed a bulgeat position I1 as normal (B) andphyllotaxy was unaffected (notshown). Bar = 150 mm. Modified fromPien et al. (2001). # 2001 NationalAcademy of Sciences, USA.to develop would normally be placed at position I 1 . However, Fleminget al. (1997) showed that if a small bead coated with expansin is placedat position I 2 , an organ primordium is generated there and developmentat position I 1 is inhibited. This suggests that the release oftension at specific sites within the SAM is an important factor regulatingthe spatial development of organ primordia. In these earlyexperiments, the organ primordia were green and possessed trichomes,but did not generate a vascular system. However, only theoutermost layer of cells within the meristem was exposed to theexpansin protein. The technique was developed further by creatingtransgenic tobacco plants which contained an expansin gene linked toa tetracycline-inducible promoter (Pien et al. 2001). When small beadsof lanolin containing tetracycline were placed on the tobacco SAM atposition I 2 (Fig. 9.4A), expansin gene expression was induced in alocalized manner but within several cell layers of the meristem. Anorgan primordium developed within 72 hours of this treatment anddeveloped into a morphologically normal leaf. Control plants, treatedin the same way but with the tetracycline replaced by buffer (Fig. 9.4B)showed organ development at I 1 as normal. Remarkably, generatinga new organ in this way reversed the phyllotactic spiral of organsgenerated subsequently, (Fig. 9.4C) showing that organ developmentis regulated in a position-dependent manner.After initiation the primordia must grow rapidly, expandingseveral thousand-fold to become a complete organ. Beneath the leafprimordium the procambium forms. This divides and differentiates toform the primary phloem and xylem which will connect theACI2P1P2I’4I’7mI1P3P1BI’3P2I’5I’2P2mI’6P1I1