Brassinosteroids promote root growth in Arabidopsis

Abstract

Although brassinosteroids (BRs) are known to regulate shoot growth, their role in the regulation of root growth is less clear. We show that low concentrations of BRs such as 24-epicastasterone and 24-epibrassinolide promote root elongation in Arabidopsis wild-type plants up to 50% and in BR-deficient mutants such as dwf1-6 (cbb1) and cbb3 (which is allelic to cpd) up to 150%. The growth-stimulating effect of exogenous BRs is not reduced by the auxin transport inhibitor 2,3,5-triidobenzoic acid. BR-deficient mutants show normal gravitropism, and 2,3,5-triidobenzoic acid or higher concentrations of 2,4-dichlorophenoxyacetic acid and naphtaleneacetic acid inhibit root growth in the mutants to the same extent as in wild-type plants. Simultaneous administration of 24-epibrassinolide and 2,4-dichlorophenoxyacetic acid results in largely additive effects. Exogenous gibberellins do not promote root elongation in the BR-deficient mutants, and the sensitivity to the ethylene precursor 1-aminocyclopropane-1-carboxylic acid is not altered. Thus, the root growth-stimulating effect of BRs appears to be largely independent of auxin and gibberellin action. Furthermore, we analyzed BR interactions with other phytohormones on the gene expression level. Only a limited set of auxin- and ethylene-related genes showed altered expression levels. Genes related to other phytohormones barely showed changes, providing further evidence for an autonomous stimulatory effect of BR on root growth.

Figure 1.

Low levels of exogenous BRs stimulate root growth. Plants were grown on vertically oriented plates in the presence of EBL (top) and ECS (bottom), respectively. Root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants. Error bars = ±se.

Figure 2.

BR-deficient plants display normal respones to 2,4-D (top) and NAA (bottom). Plants were grown on vertically oriented plates, and root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants. Error bars = ±se.

Figure 3.

Effects of 100 μm 2,3,5-triiodobenzoic acid (TIBA) in the presence and absence of 10 nm ECS. Plants were grown on vertically oriented plates, and root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants. Error bars = ±se.

Figure 4.

The growth-stimulating effect of ECS is not diminished by 2,3,5-triiodobenzoic acid (TIBA). CPD-antisense plants were grown on vertically oriented plates, and root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants grown in parallel. Error bars = ±se. Similar results were obtained for wild-type, dwf1-6, and cbb3 plants (data not shown).

Figure 5.

EBL and 2,4-D effects on root growth are additive. Plants were grown on vertically oriented plates, and root length was measured 20 ± 1 d after sowing. Inhibitory effects (top) and stimulatory effects (bottom) are shown for wild-type and cbb3 plants, respectively. Relative root lengths are given as the percentage of root length of mock-treated wild-type and cbb3 plants, respectively. Error bars = ±se.

Figure 6.

GA does not induce root elongation in BR-deficient plants. Plants were grown on vertically oriented plates, and root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants. Error bars = ±se.

Figure 7.

BR-deficient plants exhibit normal responses to the ethylene precursor ACC. Plants were grown on vertically oriented plates and root length was measured 20 ± 1 d after sowing. Relative root lengths are given as the percentage of root length of untreated wild-type plants. Error bars = ±se.