The Physiology of Flowering Plants - KHAM PHA MOI

278 REPRODUCTIVE DEVELOPMENTre-rooted. However, once the plant has attained a certain size, the SAMbecomes committed to forming reproductive tissues and its fate isnow determined. If the top of the plant is removed and re-rootednow, only a few vegetative nodes will be produced prior to flowering.The signals which cause this reprogramming of SAM development arenot known but include transmissible signals produced in the leaves.In tobacco the SAM switches to reproductive development duringthe vegetative growth period of the plant. In maize (Zea mays) thesituation is somewhat different in that the fate of the cells within theSAM is thought to be determined already during embryogenesis.During maize embryogenesis the SAM is formed together with 4–5embryonic vegetative leaves. As the plant develops it will producebetween 16 and 22 nodes (depending upon the variety) before a tasselis produced. The tassel consists of hundreds of closely packed nodesbearing clusters of flowers. It is possible to map the fate of differentregions of the SAM, at a broad level, whilst still within the embryo,although the fate of individual cells is not fixed (Fig. 11.3). It ispossible to make a maize plant produce twice as many vegetativenodes by removing the shoot apices and growing them in culture fora time. This extra vegetative growth results from cells in the upperregion of the SAM, which would normally develop as the base of thetassel, now developing as vegetative nodes. There has been no extraproduction of nodes, rather the fate of a few existing nodes has beenaltered.Fig: 11:3 A fate map of the maize(Zea mays) shoot meristem at themature embryo stage. (A) Adiagram of the maize shootmeristem. At this stage leaf 6 is justabout to be initiated and leaf 5 is asmall primordium. There are noclearly defined boundaries withinthe shoot, hence the domainsindicated are only approximate.Nodes to which cells in each domaintypically contribute are indicated.From McDaniel & Poethig (1988).(B) A mature maize plant showingleaves and tassels. Image from How aCorn Plant Develops (Special Report48, Iowa State University of Scienceand Technology), reproduced withpermission.A514-1812-1510-137-9upper tasseltassel branches17-tassel base6B