Plant defense multigene families: II Evolution of coding sequence and differential expression of PR10 genes in Pisum

While it is not possible to directly the observe evolution of multigene families, the best alternative is to compare orthologous family members among several closely-related species with varying degrees of reproductive isolation. Using RT-PCR we show that in pea (Pisum sativum) each member of the pathogenesis-related PR10 family has a distinct pattern of expression in response to the fungus Fusarium solani, and in treatment with salicylic acid, chitosan and abcisic acid. Sequencing reveals that PR10.1, PR10.2 and PR10.3 exist in P. humile, P. elatius and P. fulvum, except that no PR10.2 orthologue was identified in P. elatius. PR10.1, PR10.2 and PR10.3 appear to have diverged from a single gene in the common Pisum ancestor. For the recently diverged PR10.1 and PR10.2, the timing of fungal-induced expression differs greatly among species. For example, PR10.1 was strongly induced in P. sativum by F. solani within 8 hours postinoculation (h.p.i.), whereas little PR10.1 expression was seen in pea's closest relative, P. humile, and in the more distantly-related P. elatius. In P. fulvum, expression did not peak until 48 h.p.i. Expression of the more ancient PR10.4 and PR10.5 genes is more tightly conserved among Pisum species. These data indicate that expression, as well as sequence, can evolve rapidly. We hypothesize that changes in differential expression of multigene family members could provide a source of phenotypic diversity in populations, which may be of particular importance to plant/pathogen coevolution.