Root Gravitropic Response Research Articles

The axr4 auxin-resistant mutants of Arabidopsis thaliana define a gene important for root gravitropism and lateral root initiation.

To understand the molecular mechanism of auxin action, mutants of Arabidopsis thaliana with altered responses to auxin have been identified and characterized. Here the isolation of two auxin-resistant mutants that define a new locus involved in auxin response, named AXR4, is reported. The axr4 mutations are recessive and map near the ch1 mutation on chromosome 1. Mutant plants are specifically resistant to auxin and defective in root gravitropism. Double mutants between axr4 and the recessive auxin-resistant mutants axr1-3 and aux1-7 were characterized to ascertain possible genetic interactions between the mutations. The roots of the axr4 axr1-3 double mutant plants are less sensitive to auxin, respond more slowly to gravity, and form fewer lateral roots than either parental single mutant. These results suggest that the two mutations have additive or even synergistic effects. The AXR1 and AXR4 gene products may therefore act in separate pathways of auxin response or perhaps perform partially redundant functions in a single pathway. The axr4 aux1-7 double mutant has the same sensitivity to auxin as the aux1-7 mutant but forms far fewer lateral roots than either parental single mutant. The aux1-7 mutation thus appears to be epistatic to axr4 with respect to auxin-resistant root elongation, whereas in lateral root formation, the effects of the two mutations are additive. The complexity of the genetic interactions indicated by these results may reflect differences in the mechanism of auxin action during root elongation and the formation of lateral roots. The AXR4 gene product, along with those of the AXR1 and AUX1 genes, is important for normal auxin sensitivity, gravitropic response in roots and lateral root formation.

Calcium-dependent asymmetric movement of 3H-indole-3-acetic acid across gravistimulated isolated root caps of maize

There is evidence that the cap is the initial site of lateral auxin redistribution during the gravitropic response of roots. We tested this further by comparing asymmetric auxin redistribution across the tips of gravistimulated intact roots, decapped roots, isolated root caps and isolated apical sections taken from decapped roots. Gravistimulation caused asymmetric (downward) auxin movement across the tips of intact roots and isolated root caps but not across the tips of decapped roots or across isolated apical root segments. Naphthylphthalamic acid and pyrenoylbenzoic acid, inhibitors of polar auxin transport, inhibited asymmetric auxin redistribution across gravistimulated isolated root caps and across the tips of gravistimulated intact roots. For intact roots there was a positive correlation between the extent of inhibition of assymmetric auxin redistribution by polar auxin transport inhibitors and the extent of inhibition of asymmetric calcium chelating agent, ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid, also caused parallel inhibition of asymmetric auxin redistribution and gravitropic curvature and this effect was reversed by subsequent treatment with calcium. The results support the hypothesis that the cap is a site of early development of auxin asymmetry in gravistimulated roots and that calcium plays an important role in the development of lateral auxin redistribution.

Phytotropin‐induced root phototropism in maize

Phytotropins, even those not absorbing in the visible region of the spectrum, can induce a phototropic response in maize (Zea mays L. cv. PX‐75) roots when illuminated unilaterally with white light. The most active phytotropin, 2‐(1‐pyrenoyl) benzoic acid (PBA) can elicit a full response at 10 μM, while the other active molecules, 2‐carboxyphenyl‐3‐phenylpropane‐1,3‐dione (CPD), 2‐carboxyphenyl‐3‐phenyl‐1,2‐pyrazole (CPP), 1‐N‐naphthylphthalamic acid (NPA) and erythrosin elicit a full response at 100 μM. The less active phytotropins BBA and fluorescein give a reduced response. It is suggested that the observed effect cannot be explained solely on the basis of auxin transport inhibition. There is a photoreceptor in the extension zone of the root, which may be associated in some way with the receptor for NPA. The results are consistent with the proposal that the phototropic process may form part of the root gravitropic response mechanism.

The gravitropic response of roots and the shaping of the root system in cereal plants

The spatial distribution of seminal and nodal roots in cereals varies according to species and genotype. This is due to the plagiogravitropic growth of the axes. The plagiogravitropic response of a nodal root depends on its diameter and the position of the internode from which it emerges. The response also changes in the course of root growth and the stage of shoot development. The growth angle of the axes is, therefore, programmed and controlled by genetic, physiological and environmental factors. With respect to the latter, the seminal roots of wheat are sensitive to the water environment which may modify the growth angle as a result of the effect of hormonal regulators such as abscisic acid (ABA). Concerning the gravitropic response of nodal roots in rice, physiological controls may operate through assimilates coming from the shoot. It is concluded that the gravitropic response of axes, which determines the shape of the root system, is primarily controlled by genetic factors but is also influenced by environmental conditions.

Genetic analyses on flower development and root gravitropism of Arabidopsis thaliana.

We have analysed genetic systems of flower development and of root gravitropism of higher plant using Arabidopsis thaliana, which has several features suited for molecular genetic studies. Through isolation and analyses of many mutants which carry genetic defect(s) in the processes of floral development or of root gravitropic response, sets of genes involved in those processes are identified.