Arabidopsis root growth movements and their symmetry: progress and problems arising from recent work

Abstract

Over the last fifteen years, an increasing number of plant scientists have become interested in the Arabidopsis root growth pattern, that is produced on the surface of an agar plate, inclined from the vertical. In this situation, the roots wave intensely and slant preferentially towards one side, showing torsions in the epidermal cell files alternately right-and left handed. In addition, the pattern switches to the formation of large or strict coils when the plate is set horizontally. After this finding, different hypotheses were advanced attempting to explain the forces that shape these patterns. These basically appear to be gravitropism, circumnutation and negative thigmotropism. With regard to the symmetry, the coils and the slanting in the wild-type are essentially right-handed, but mutants were also reported which show a left-handed symmetry, while some do not show a regular growth pattern at all. This review article discusses the earlier as well as the most recent findings on the topic, and investigates the possibility of describing the different mechanisms shaping the root growth patterns via unifying hypothesis.

Keywords: auxin; circumnutation; gravitropism; root pattern; root waving; symmetry.

Figure 1

(A) right-handed and (B) left-handed twining plant. In (A) Ipomea purpurea Roth. (Campanulaceae), in (B) Polygonum baldschuanicum Regel. (Polygonaceae).

Figure 2

(A) Seedlings of Arbidopsis thaliana growing on an inclined hard-agar (1.5%) petri dish. (B) Schematic representation of the slanting produced to the right-hand in the wild type and to the left-hand in some mutants. (C) Darwin's drawing of Phaseolus multiflorus Lam. roots growing on an inclined and smoked glass plate. (D) Spurny's drawing of pea roots growing in the shape of right-handed helices.

Figure 3

(A and B) Arabidopsis seedlings intensely waving, coiling and slanting to the right-hand from the wild-type ecotype Wassilewskija. (C) right-handed (clockwise) coil from the wild-type showing a strong left-handed torsion. (D) Left-handed (counterclockwise) coil from the mutant spr1 showing right-handed torsion. (E) particular of a left-handed torsion in a right-handed coil. (F) a segment of waving primary root from the wild-type showing the alternate left-handed and right-handed torsion.

Figure 4

Arabidopsis plants after a 6-day run on the Random Positioning Machine (three dimensional clinostat). Note the large right-handed (clockwise) coils made on the machine by the roots in the ecotypes Colombia and Wassilewskija. (The pictures are taken from Fig. 1 of the Piconese et al. published by Oxford Univ. Press).

Figure 5

Mutants of the root growth pattern in Arabidopsis. (A) spr1-1, left-handed mutant. (B) clg1, coiling mutant. (C) eir1/pin2, agravitropic and auxin transport mutant, lacking a gene for the auxin efflux. (D) axr2, agravitropic and auxin mutant, lacking the gene for the protein IAA7.