Involvement of peroxidases in the Medicago truncatula-Rhizobium meliloti symbiosis

Abstract

The symbiotic interaction between legume plants and bacteria of the family Rhizobiaceae leads to the production of nitrogen fixing nodules on the plant. Nodule production is an expensive process and hence is highly regulated by the plant. In Medicago truncatula, only about 5% of the bacterial infections proceed to form fully functional nodules, while the remaining 95% become arrested at a very early stage in the root cortex. The goal of this work was to study the early interactions between the legume and the bacterium and establish a role for peroxidases in the role of infection arrest. Peroxidases are known to play a role in defense responses and disease resistance. In the M. truncatula-Rhizobium meliloti symbiosis, a peroxidase gene, Mtrip1, is induced in the host cells early in response to Rhizobium signals (Cook et al. 1995; Peng et al., 1996). The timing, localization and manner of induction of rip1 indicate that it may play a role in infection establishment or arrest. Native peroxidase gels were used to identify the putative Mtrip1 protein. Two peroxidase bands accumulate in plant roots with timing that correlates well with Mtrip1 gene expression. One of these bands also seems to be quantitatively greater in M. truncatula mutants that show an infection arrest phenotype. In situ localization showed that accumulation of hydrogen peroxide and peroxidases is mainly in the root tips and in the root hair and epidermal region. Hydrogen peroxide accumulates in response to inoculation in wild type and in infection arrest mutants. These results are consistent with the hypothesis that internal hydrogen peroxide is one of the main inducers of peroxidase accumulation in the roots. An apparently greater accumulation of hydrogen peroxide occurs in the infection arrest mutants spk, pdl, and lin than occurs in wild type M. truncatula. This factor, and subsequent higher levels of peroxidase activity, could contribute to the infection arrest phenotype in these mutants.