Expression of the Pib rice-blast-resistance gene family is up-regulated byenvironmental conditions favouring infection and by chemical signals that trigger secondary plant defences

The rice blast resistance gene Pib is a member of the nucleotide binding si te (NBS) and leucine-rich repeat (LRR) class of plant disease resistance (R ) genes and belongs to a small gene family. We describe here the isolation and characterization of a Pib homologue (PibH8), and extensive investigatio n of the expression of the Pib gene family (Pib, PibH8, HPibH8-1, HPibH8-2) under various environmental and chemical treatments. PibH8 shows 42% ident ity and 60% similarity to Pib and, like Pib, has a duplication of the kinas e 1a, 2, and 3a motifs of the NBS region in the N-terminal half of the prot ein. Interestingly, genes of the Pib family exhibit a diurnal rhythm of exp ression. RNA gel blot analysis revealed that their expression was regulated dramatically by environmental signals, such as temperature, light and wate r availability. Their expression was also induced by chemical treatments, s uch as jasmonic acid, salicylic acid, ethylene and probenazole. Our finding s suggest that expression of the Pib gene family is up-regulated by environ mental conditions that would favour pathogen infection. This may reflect th e evolution of anticipatory control of R gene expression.