COI1, a jasmonate receptor, is involved in ethylene-induced inhibition of Arabidopsis root growth in the light

Abstract

Plant response to stress is orchestrated by hormone signalling pathways including those activated by jasmonates (JAs) and by ethylene, both of which stunt root growth. COI1 is a JA receptor and is required for the known responses to this hormone. It was observed that the coi1 mutant, which is largely unresponsive to growth inhibition by JAs, was also partially unresponsive to growth inhibition by ethylene and by its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), in the light but not in the dark. Although COI1 was required for this response to ACC, other components of the JA signal perception pathway were not. Mutants selected for insensitivity to ethylene, including etr1, ein2, and ein3, showed greater ACC-induced root growth inhibition in the light than in the dark. However, the double mutants etr1;coi1, ein2;coi1, and ein3;coi1, and coi1 seedlings treated with silver ions to block the ethylene receptors showed almost complete unresponsiveness to ACC-induced root growth inhibition in the light. The light requirement for the COI1-mediated growth inhibition by ACC was for long photoperiods, and the ACC response was not abolished by mutations in the known photoreceptors. The complementation assay indicated that SCF complex assembly was not required for COI1 function in the ACC response, in contrast to the JA response. It is concluded that COI1 is required for the light-dependent, JA-independent, root growth inhibition by ethylene.

Fig. 1.

Effect of various root growth inhibitors on coi1-16. Seedlings were grown for 7 d in continuous light (CL) on Murashige and Skoog (MS) agar supplemented with either methyl jasmonate (MeJA), 1-aminocyclopropane-1-carboxylic acid (ACC), epi-brassinolide (EBR), salicylic acid (SA), or the cytokinin 6-benzylaminopurine (BAP) at the indicated concentration. Root lengths of treated seedlings compared with that of control seedlings are expressed as the mean of relative root growth (%, n ≥23). Bars indicate the standard error (SE). A significant difference (P <0.001) compared with the wild-type is indicated with asterisks.

Fig. 2.

Phenotypes of wild-type, coi1-16 (A), etr1-1, and etr1-1;coi1-16 (B) seedlings grown for 7 d in continuous light (CL) on Murashige and Skoog (MS) agar with (right panel) or without (left panel) 4 μM ACC. The scale bar indicates 5 mm.

Fig. 3.

Effect of MeJA and ACC on growth of wild-type, coi1-16, etr1-1, and etr1-1;coi1-16 seedlings. (A) Root growth of the wild-type, coi1-16, etr1-1, and etr1-1;coi1-16 in the light and in the dark. Seedlings were grown for 7 d in continuous darkness (CD) or continuous light (CL) on Murashige and Skoog (MS) agar supplemented with 50 μM MeJA or 4 μM ACC. Root lengths of treated seedlings compared with that of control seedlings are expressed as mean of relative root growth (%, n ≥12 except 4 for etr1-1 on MeJA in CL and none in CD). Bars indicate the standard error (SE). A significant difference (P <0.01 or P <0.05) compared with the wild-type is indicated with double or single asterisks, respectively. n.d. indicates no data. (B) Images of roots following ACC treatments. Seedlings were grown for 7 d in CL on MS agar supplemented or not with 4 μM ACC and observed at ×5 magnification under a brightfield microscope attached to a CCD camera. The scale bar indicates 200 μm.

Fig. 4.

Ethylene-responsive gene expression in the wild-type and coi1-16. Seedlings were grown for 7 d in continuous light (CL) on Murashige and Skoog (MS) agar supplemented or not with 4 μM ACC. Ten to 15 seedlings were harvested per sample and three biological replicates were tested for gene expression of ETR2 and ERS2. The average expression ratios of ACC-treated samples to controls are expressed in a log2 scale. Bars indicate the standard error (SE).

Fig. 5.

Complementation of coi1-16 with various COI1 constructs. (A) Diagrams of COI1 and the different constructs used for complementation studies. An asterisk indicates a substitution of Trp44 to alanine. (B) Root growth of the wild-type, coi1-16, and coi1-16 with each construct in the light and in the dark. Seedlings were grown for 7 d in continuous darkness (CD) or continuous light (CL) on Murashige and Skoog (MS) agar supplemented with 50 μM MeJA or 4 μM ACC. Root lengths of treated seedlings compared with that of control seedlings are expressed as the mean of relative root growth (%, n ≥11). Bars indicate the standard error (SE). A significant difference (P <0.05) compared with the wild-type is indicated with asterisks.

Fig. 6.

Model pathway. ACC/ethylene-induced root growth inhibition is mediated by the COI1 pathway in a light-dependent and JA-independent manner, in parallel with the conventional ethylene pathway. However, the COI1 pathway is not involved in ACC-induced root hair formation. It is also suggested that JA-induced COI1-mediated germination inhibition is antagonised by the ETR1 pathway.