  The defense response in plants challenged with pathogens is characterized by
the activation of a diverse set of genes. Many of the same genes are induced in
the defense responses of a wide range of plant species. How plant defense gene
families evolve may therefore provide an important clue to our understanding of
how disease resistance evolves. Because studies usually focus on a single host
species, little data are available regarding changes in defense gene expression
patterns as species diverge. The expression of defense-induced genes PR10,
chitinase and chalcone synthase was assayed in four pea species (Pisum sativum,
P. humile, P. elatius and P. fulvum) and two Lathyrus species (L. sativus and
L. tingitanus) which exhibited a range of infection phenotypes with Fusarium
solani . In P. sativum, resistance was accompanied by a strong induction of
defense genes at 8 hr. post-inoculation. Weaker induction was seen in
susceptible interactions in wild species. Divergence in the timing of PR10
expression was most striking between P. sativum and its closest realtive, P.
humile. Two members of this multigene family, designated PR10.1 and PR10.2, are
strongly-expressed in response to Fusarium, while the PR10.3 gene is more
weakly expressed, among Pisum species. The rapidity with which PR10 expression
evolves raises the question, is divergence of defense gene expression a part of
the phenotypic diversity underlying plant/pathogen coevolution?
