Gravitropism and starch statoliths in an Arabidopsis mutant

Abstract

The mutant TC 7 of Arabidopsis thaliana (L.) Heynh. has been reported to be starch-free and still exhibit root gravitropism (T. Caspar and B. G. Pickard 1989, Planta 177, 185-197). This is not consistent with the hypothesis that plastid starch has a statolith function in gravity perception. In the present study, initial light microscopy using the same mutant showed apparently starch-free statocytes. However, ultrastructural examination detected residues of amyloplast starch grains in addition to the starch-depleted amyloplasts. Applying a point-counting morphometric method, the starch grains in the individual amyloplasts in the mutant were generally found to occupy more than 20% and in a few cases up to 60% of the amyloplast area. In the wild type (WT) the starch occupied on average 98% of the amyloplast area and appeared as densely packed grains. The amyloplasts occupied 13.9% of the area of the statocyte in the mutant and 23.3% of the statocyte area in the WT. Sedimentation of starch-depleted amyloplasts in the mutant was not detected after 40 min of inversion while in the WT the amyloplasts sedimented at a speed of 6 micrometers h-1. The gravitropic reactivity and the curvature pattern were also examined in the WT and the mutant. The time-courses of root curvature in the WT and the mutant showed that when cultivated under standard conditions for 60 h in darkness, the curvatures were 83 degrees and 44 degrees, respectively, after 25 h of continuous stimulation in the horizontal position. The WT roots curved significantly more rapidly and with a more normal gravitropic pattern than those of the mutant. These results are discussed in relation to the results previously obtained with the mutant and with respect to the starch-statolith hypothesis.

Plastid Sedimentation Kinetics in Roots of Wild-Type and Starch-Deficient Mutants of Arabidopsis1

“…The plastid positioning in nonreoriented roots agrees with data from inverted studies of the TC7 starchless mutant (Caspar and Pickard, 1989) and qualitative observations of TC7 (Saether and Iversen, 1991) as well as data from the starch-deficient mutant of maize (Hertel et al, 1969). The similar results at the plastid level, along with studies of the kinetics of gravitropic curvature (Kiss et al, 1996), are particularly significant, because these two starchless mutants were isolated from different ecotypes and by different mutational methods (chemical and T-DNA insertion).…”

Sucrose Synthase Catalyzes the de novo Production of ADPglucose Linked to Starch Biosynthesis in Heterotrophic Tissues of Plants

“…Furthermore, the gluconeogenic process illustrated in Fig. 1a is also inconsistent with the presence of plants with null or severely reduced plastid PGM and AGPase activities accumulating almost normal or readily detectable amounts of starch (Saether and Iversen 1991, Harrison et al 1998, Tauberger et al 2000, Fernie et al 2002.…”

Auxin polar transport of etiolated Ageotropum pea epicotyls is not affected by gravistimulation: Relevance to automorphosis-like growth and development

“…In addition to a 3-D clinostat, single gene mutants of gravitropism showing abnormalities in gravitropic responses are valuable tools in studying the role of gravity in plant growth and development under 1g gravitational conditions on Earth (Olsen and Iversen, 1980a,b;Jaffe et al, 1985;Roberts, 1987;Scott, 1988;Kiss and Sack, 1990;Saether and Iversen, 1991;Stinemetz et al, 1996). The Ageotropum mutant, which is an X-ray induced mutant of Pisum sativum cv.…”