The Physiology of Flowering Plants - KHAM PHA MOI

UNRAVELLING PHOTOMORPHOGENESIS 261A phyB-deficient plant will exhibit a constitutive shade-avoidanceresponse both as a seedling and a mature plant. Under white light thephyB mutant has elongated petioles (Fig. 10.10) but if exposed to a10-minute pulse of far-red light at the end of the day, a more extremeshade-avoidance response is triggered, indicating that other photoreceptorsare still responsive. If phyD is also mutated, the petiolesbecome further elongated under white light, whilst mutations inphyE cause the rosette habit to be lost. The role of these minorphytochromes is normally masked by phyB.Although mature plants of many species do not exhibit shadetolerance, it can be re-established using genetic engineering techniques.Although the phyA content of a mature plant is normallylow, an artificial gene can be made where a strong, unregulatedpromoter is fused to the coding region of the PHYA gene. When thisover-expression construct is introduced into plants they will now containmuch more phyA than normal. Vierstra and coworkers (Ballaré et al.1994) used this approach to produce shade-tolerant tobacco plants.Wild-type tobacco plants normally show a strong shade-avoidanceresponse when they are illuminated with far-red light from the side,phyAphyBphyAphyBphyDphyAphyBphyEFig: 10:10 Shade-avoidanceresponses in phytochromedeficientmutants of Arabidopsis.ThephyAphyB double mutant shows apartial shade-avoidance responsewhen grown under white light for60 days. The leaves are smaller thanin wild-type plants, have elongatedpetioles and are held more upright.This is due primarily to the absenceof phyB. If these plants are exposedto 10 minutes far-red light at theend of each day (EOD-FR), a strongresponse is still observed, indicatingthat other phytochromes can alsodetect this signal; the leaves areeven smaller and the internodesetiolate. This response is masked inplants containing PHYB and isapparent only in the mutant. Plantswhich also lack PHYD show etiolatedpetioles when grown under whitelight, whilst plants lacking PHYEshow internode extension underthese conditions. Scale bar = 5 cm.From Whitelam et al. (1998).White light+EOD FR