Characterization of a new jasmonate signaling mutant in Arabidopsis

Jasmonic acid and its methyl ester form, MeJA, influence several important aspects of plant growth and development, including root and shoot growth, flower fertility, senescence, to protection of plants from biotic and abiotic stresses. A screen for methyl jasmonate (MeJA) insensitive root growth in Arabidopsis thaliana yielded alleles of the previously isolated mutant loci jar1 and coil, with one exception, a mutant locus called 54-1. The response of wild type and 54-1 primary root growth was tested on a range of concentrations of indole-3-acetic acid, 1-aminocyclopropane-1-carboxylic acid, 6-benzylamino-purine, epi-brassinolide, and absisic acid. The data showed that 54-1 is not only insensitive to MeJA, but also to the foregoing hormones, as well as certain other compounds that inhibit root elongation. Genomic mapping, morphological characterization, and non-complementation in a F1 cross of 54-1 by axr1 (auxin resistant 1), confirmed that 54-1 and axr1 are the same genes. The new axr1 allele, named axr1-24, was further shown to be susceptible to the opportunistic pathogen Pythium irregulare, a trait previously found in jasmonate response mutants. The jar1/axr1 double mutant was more resistant to inhibition of root growth by MeJA and more susceptible to P. irergulare infection than either single mutant, suggesting they define independent signaling pathways. Gene expression analysis showed that IAA induced the JA responsive genes and this induction was reduced in axr1. However, transcript induction by MeJA, was only minimally affected in axr1-24. A screen for suppressor of axr1-24 yielded seven plants harvested from independent parental group seed pools. The new mutations in these seven lines partially suppressed most aspects of the axr1 phenotype, including plant height, silique shape and size, fertility. Furthermore, a detailed characterization of one of these suppressor lines, 16-1, revealed that it partially overcomes the defects in jasmonate responses, including root growth, pathogen susceptibility, and possible male infertility. These results provide new information about auxin and jasmonate response of AXR1, and the possible interaction between these two important signaling pathways