An Arabidopsis mutant defective in jasmonate response is allelic to the auxin-signaling mutant axr1

Abstract

A screen for Arabidopsis mutants that were insensitive to methyl jasmonate (MeJA) in an assay for seedling root growth yielded only alleles of previously isolated mutants jar1 and coi1, with one exception. Mapping of the locus and morphological characterization of the new mutant suggested it might be allelic to axr1, which had not previously been reported to show resistance to MeJA. The F(1) from a cross of the new mutant with axr1-3 did not show complementation, confirming that these are the same genes. The new allele is called axr1-24. In addition to MeJA and indole-3-acetic acid (IAA), axr1-24 had decreased sensitivity to 1-aminocyclopropane-1-carboxylic acid, 6-benzylamino-purine, epi-brassinolide, and abscisic acid. Both axr1-24 and the previously characterized axr1-3 allele were shown to be susceptible to the opportunistic pathogen Pythium irregulare, a trait found in other jasmonate response mutants, including jar1-1. The double mutant jar1-1/axr1-3 was more resistant to inhibition of root growth by MeJA and was more susceptible to P. irregulare infection than either single mutant, suggesting these genes might act in independent response pathways. In contrast, resistance to IAA in the double mutant was not different from axr1-3. Northern-blot analysis showed that IAA induced the jasmonate-responsive lipoxygenase 2, AOS, and AtVSP gene transcripts and induction was strongly impaired in axr1-3. However, transcript induction by MeJA was only minimally affected in axr1-3. This study demonstrates that in addition to auxin signaling, the AXR1 locus is involved in MeJA response, providing a mechanistic link between jasmonate and auxin-signaling pathways.

Figure 1

Mapping of the MeJA resistance locus. Molecular markers used are indicated above the chromosome I interval that is depicted. Numbers below denote the number of recombinants between the mutant locus and the respective marker. The relative position of AXR1 is shown.

Figure 2

Dose response curve for root growth inhibition on IAA and MeJA. Wild-type (wt) and mutant (jar1-1, axr1-24, and axr1-3) seedling root length was measured after 10 d growth at 21 C. Error bars indicate sd (n = 20). A, Growth on IAA. B, Growth on MeJA.

Figure 3

Insensitivity of mutants to hormones. Root length determined as in Figure 2. A, Inhibition of root growth by the ethylene precursor ACC. B, Inhibition of root growth by BR. C, Seed germination on 0.5 μm ABA.

Figure 4

Infection of mutant and wild-type seedlings with P. irregulare. One hundred and fifty-three seedlings were used per genotype. The percentage of plants with symptoms at the times indicated is shown for each genotype. Wild type did not show symptoms during the duration of the experiment.

Figure 5

Transcript level for LOX2, AOS, AtVSP, and IAA1 in light-grown wild-type (WT) and axr1-24 seedlings. Numbers above lanes indicate time (h) after treatment was initiated. Hybridization probe for each row of samples is indicated to the left. A, Total RNA isolated from volatile MeJA-treated seedlings. B, RNA isolated after spraying with 1 μm IAA.