The Arabidopsis MAP kinase kinase 7: A crosstalk point between auxin signaling and defense responses?

Plant-pathogen interaction induces a complex host response that coordinates various signaling pathways through multiple signal molecules. Besides the well-documented signal molecules salicylic acid (SA), ethylene and jasmonic acid, auxin is emerging as an important player in this response. We recently characterized an Arabidopsis activation-tagged mutant, bud1, in which the expression of the MAP kinase kinase 7 (AtMKK7) gene is increased. The bud1 mutant plants accumulate elevated levels of SA and display constitutive pathogenesis-related (PR) gene expression and enhanced resistance to pathogens. Additionally, increased expression of AtMKK7 in the bud1 mutant causes deficiency in polar auxin transport, indicating that AtMKK7 negatively regulates auxin signaling. Based on these results, we hypothesized that AtMKK7 may serve as a crosstalk point between auxin signaling and defense responses. Here we show that increased expression of AtMKK7 in bud1 results in a significant reduction in free auxin (indole-3-acetic acid) levels in the mutant plants. We propose three possible mechanisms to explain how AtMKK7 coordinates the growth hormone auxin and the defense signal molecule SA in the bud1 mutant plants. We suggest that AtMKK7 may play a role in cell death and propose that AtMPK3 and AtMPK6 may function downstream of AtMKK7.