Chalcone synthase and phenylalanine ammonia-lyase mRNA levels following exposure of sorghum seedlings to three fungal pathogens

Oligonucleotide primers made complementary to conserved sequences in phenylalanine ammonialyase (PAL) and chalcone synthase (CHS) genes previously cloned from other species were used to amplify segments of the corresponding genes from sorghum. Greater than 70% base sequence identity with homologous genes confirmed that a 535 bp clone (PAL1-1) and a 620 bp clone (CHS2G) were derived from the coding regions of PAL and CHS respectively. When used to probe genomic digests, the two clones detected 3 and 5 EcoRI fragments, respectively. One fragment for each probe was polymorphic in the parents of a mapping population, permitting a locus for each gene to be located on a sorghum RFLP linkage map. RNA gel blot analyses were performed on extracts from control seedlings and from seedlings challenged with either of two fungal sorghum pathogens or a pathogen of maize that elicits a hypersensitive response in sorghum. Although low levels of PAL and CHS mRNA transcripts were present constitutively in the controls, both rapidly accumulated to high levels following inoculation with the maize pathogen, Bipolaris maydis, and decreasing amounts of mRNA were apparent by 120 h postinoculation. Since challenge with Sporisorium reilianum was by needle inoculation, an additional control of mock inoculations was included for this pathogen. However, no differences in the amount of mRNA for either gene were detected over the sampling period, whether samples from uninoculated, mock inoculated or spore-inoculated seedlings were tested. Differences in the timing and level of mRNA accumulation were detected in seedlings after inoculation with Peronosclerospora sorghi. While seedlings of both resistant and susceptible cultivars accumulated higher levels of PAL and CHS mRNA than uninoculated controls, the accumulation of mRNA in resistant cultivars was higher and longer lasting than that in susceptible cultivars.