Pycnospore germination fluid of Mycosphaerella pinodes, a fungus pathogenic on pea, contains both an elicitor and a suppressor of the accumulation of pisatin, a major phytoalexin of pea. Transcription of the genes encoding key enzymes in the biosynthesis of pisatin, namely PAL (a gene encoding phenylalanine ammonia-lyase) and CHS (a gene encoding chalcone synthase), was shown to be activated upon the treatment of pea epicotyl tissues with the fungal elicitor and suppressed upon treatment with the fungal suppressor. To investigate the mechanisms underlying activation and suppression of plant defense genes by signal molecules secreted by a fungal pathogen and other stresses, such as ultraviolet (UV) light, we constructed chimeric genes composed of the 5'-flanking regions of two members of the PSPAL family (the genes encoding phenylalanine ammonia-lyase in Pisum sativum) fused to a bacterial gene for chloramphenicol acetyltransferase. Then, the ciy-regulatory elements necessary for elicitor-mediated activation and suppressor- mediated suppression were examined in pea protoplasts. Functional analysis of 5' nested deletions of PSPALl and PSPAL2 suggested that an enhancer-like element is located in the TATA-distal region (-2,196 to -406) in PSPALL2 A cis-acting element(s) responsible for elicitor-mediated activation was found in the TATA-proximal region (-340 to -95 in PSPALl; -406 to -158 in PSPALl), in which the consensus sequence motifs known as box 1, box 2 and box 4 [Yamada et al. (1992) Plant CellPhysiol. 33: 715, Lois et al. (1989)EMBO J. 8: 1641] were present in close proximity. Furthermore, both promoters were activated by UV light but were partially suppressed in response to the fungal suppressor.
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