Involvement of phenylalanine ammonia-lyase in the response of Fortune mandarin fruits to cold temperature

Abstract

L-phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) is generally recognised as a marker of environmental stress in different plant tissues. To investigate the involvement of PAL in the response of citrus fruits to cold temperature, changes in the abundance of PAL mRNA and PAL activity were examined in flavedo tissue of the chilling-sensitive Fortune cultivar (Citrus elementina Hort. ex Tanaka × Citrus reticulata, Blanco). A cDNA library was constructed from flavedo tissue of chilling-stressed fruits and screened with a 660 bp PAL probe, obtained by polymerase chain reaction using oligonucleotides derived from conserved sequence regions. Two fulllength cDNA clones (FPAL1 and FPAL2) were isolated, and the deduced amino acid sequences showed a 75-85% similarity with PAL genes from other plant species. A comparative analysis of the changes in PAL activity and PAL mRNA levels was conducted in fruits stored at chilling (2°C) and non-chilling (12°C) temperatures. Northern blot analyses, using both FPAL1 and FPAL2 cDNAs as probes, recognised a single mRNA that accumulated in fruits exposed to 2°C prior to the appearance of physical chilling symptoms and the accompanying increase in PAL activity. Once symptoms were obvious, accumulation of PAL transcript and PAL activity were restricted to the tissue in and around the necrotic regions. However, exposure to a low non-chilling temperature produced an early, moderate and transient increase in PAL mRNA levels and PAL activity that declined after 1 day. This transient induction of both PAL gene expression and activity could be part of a rapid adaptive response of the tissue to low temperatures. Interestingly, a rapid and sustained accumulation of PAL transcript occurred in the leaves and roots of citrus plants exposed to a low temperature in the absence of any detectable chilling-induced damage.