Functional Analysis of PAL2 Gene Promoter in Arabidopsis Thaliana (L.) Heynh. During Plant Development Exposed to Biotic and Abiotic Stresses

Phenylalanine ammonia-lyase (PAL; E.C.4.3.1.5) enzyme is essential for plant normal growth, development and adaptation to different environmental stresses. In Arabidopsis thaliana, the PAL enzyme is encoded by four gene isoforms which are designated as PAL1 (AT2G37040), PAL2 (AT3G53260), PAL3 (AT5G04230), and PAL4 (AT3G10340) respectively. PAL1 and PAL2 genes are closely related to each other phylogenetically and functionally. PAL1 promoter is involved in plant development and also plant response under induction of a myriad of stresses. However, functional analysis on the PAL2 promoter of A. thaliana has not been carried out. The PAL2 promoter activities were investigated by fusing 1.8-kb 5’ upstream of the translation start site with a β-glucuronidase (GUS) gene in transgenic A. thaliana. The PAL2 promoter activities were associated with the structural development of the plant and its organs and avirulent Pseudomonas syringae pv. tomato JL1065 induced the promoter response in an organ-specific manner. In the context of plant development, the PAL2 promoter was active from the germination of young seedling to the reproductive stage, particularly in the rosette leaf, root, and inflorescence stem which are the major structural organs supporting the floral organs particularly bud, flower, and silique. The rosette leaf, root and stem are considered as the major structural organs as they provide the mechanical strength to support vertical position of the whole plant. The PAL2 promoter activities in both rosette leaf and root were roughly 3-fold, and stem 2-fold higher than the floral organs and silique. The PAL2 promoter activities displayed decreasing trend in the aerial organs with position further from the rosette leaves. In the context of plant adaptation, PAL2 promoter activities was induced in the distal root with roughly 2-fold increase after 4-day post-inoculation with avirulent JL1065 in the aerial organs, suggesting PAL2 promoter was involved in induced defence system. During moderate water deficit stress mediated by sodium chloride and polyethylene glycol solution for short-term period, PAL2 promoter activities were not significantly induced for water stress-responsiveness. These findings imply that PAL2 promoter maybe regulated transcriptionally for the normal plant structural development, and plant adaptation to avirulent Pseudomonas infection.

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