The Physiology of Flowering Plants - KHAM PHA MOI

334 GROWTH MOVEMENTSFig: 12:11 Optical tweezers canbe used to move statoliths in Chararhizoids. Before the experiment thestatoliths are visible in the middle ofthe rhizoid. Switching on the lasercauses the statoliths to be displaced.From Leitz et al. (1995).# Springer-Verlag, 1995.exposure to low temperatures or treatment with gibberellins, whichcause the statoliths to be lost, lead to a loss of gravitropism.Responsiveness is restored when the cells are allowed to recovertheir starch. The lazy1 mutant of tomato lacks starch and is agravitropic.Treatment of this mutant with auxin restores both starchformation and gravitropism. The shoot gravitropism 1 (sgr1) mutant ofArabidopsis has agravitropic hypocotyls and these lack the endodermiscontaining sedimentable starch.At one time the role of statoliths in gravity perception was challengedas it was reported that mutants of Arabidopsis which lackedstarch, and hence lacked statoliths, were still able to show gravitropicresponses. However, when these experiments were repeated undernon-saturating gravitational conditions it was found that these plantswere in fact less responsive. A careful examination of intermediatestarchmutants of Arabidopsis (Kiss et al. 1996) demonstrated that therewas a good correlation between statolith density and graviresponsiveness,although the gravity-sensing mechanism appears to besomewhat ‘over-built’ in that the response of mutants with 50–60%starch is more similar to that of wild-type plants than to that ofstarchless mutants.Of course, moving statoliths directly would provide excellentevidence for, or against, their role in gravity perception, but suchexperiments are somewhat challenging technically! Kuznetsov andHasenstein (1996, 1997) exposed the root tips of Arabidopsis andbarley (Hordeum vulgare) coleoptiles to extremely strong magneticgradients which were sufficient to move the statoliths. In root tipsthis treatment led to the development of curvature in the direction ofstatolith displacement (as expected for a positively gravitropic organ)whilst the opposite was observed in the coleoptiles and hypocotyls(as expected for negatively gravitropic organs). In the characean algaChara, which contains statoliths composed of BaSO 4 , statoliths couldbe displaced using optical tweezers. This precise arrangement of laserbeams uses the tiny, but perceptible, forces generated by the highlyfocused light to hold and move objects within cells (Leitz et al. 1995).Such experiments confirmed the role of statoliths in gravity perceptionin Chara rhizoids (Fig. 12.11).10 µmTime (s)0423252632