The Physiology of Flowering Plants - KHAM PHA MOI

Specific leaf area (cm 2 g –1 )262 PHOTOMORPHOGENESISFig: 10:11 Transgenic tobaccoplants (Nicotiana tabacum) whichover-express oat phytochrome Aare less responsive to far-red lightand crowding than wild-typeplants. Filled circle = wild typeplants; open circle = transgenicplants. Plants were grown at anirradiance of 160 mmol m 2 s 1 andwere exposed to increasingamounts of far-red light from oneside. Wild-type plants exhibited acharacteristic shade-avoidanceresponse with far-red light causingstem elongation (A) and an increasein specific leaf area (B). Theseresponses were much less marked,or abolished, in transgenic plantsover-expressing oat phytochrome A.A similar response was seen whenplants were grown in increasinglydense canopies (C). As the initialleaf area index (LAI i ) was increased,wild-type plants exhibited a shadeavoidanceresponse whilst phyAover-expressorswere shade tolerant.Note that the stature of the transgenicplants was less than that of the wildtype as increased phyA expressionresults in a dwarf phenotype. Datafrom Ballaré et al. (1994).# National Academy of Sciences,USA.Fig: 10:12 Shade tolerance intransgenic tobacco plants (Nicotianatabacum) expressing the oatPHYA cDNA. Plants were grown inwhite light (A) or white lightsupplemented with far-red light (B).When the oat PHYA cDNA wasexpressed at moderate levels (2.5-to 3-fold), the stature of the plantsgrown under white light was similarto that of the wild type. Only plantsexpressing the highest levels (5-fold) showed a reduced stature.However, when supplemented withfar-red light, plants expressing thePHYA gene showed a reverse shadeavoidanceresponse (i.e. theybecame shade-tolerant). (Robsonet al., 1996). Photograph byA. McCormac, courtesy of H. Smith.∆stem length (mm)∆stem length (mm)18161412108642076543210A0C10 20 30 40 50 60 70–0.8 –0.4 0.0 0.4log LAI iFR irradiance (µmol m –2 s –1 )22510 20 30 40 50 60 70or are grown in dense canopies. The internodes elongate and thespecific leaf area (the area of leaf per unit dry weight) increases; i.e.the leaves become thinner. These responses were abolished in transgenicplants which contained five times as much phyA as the wild type(Fig. 10.11). However, increasing the phyA content by this muchresulted in dwarf plants, presumably because they became hypersensitiveto far-red light. Smith and coworkers (Robson et al. 1996) foundthat transgenic tobacco plants which contained lower amounts of oatphyA developed normally under white light, but showed a reducedshade avoidance response (Fig. 10.12). This ability to manipulate plantAWhite lightBB0White light + FR350325300275250WT 2.5x 3x 5x WT 2.5x 3x 5x