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

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 temperatur e, changes in the abundance of PAL mRNA and PAL activity were examined in f lavedo tissue of the chilling-sensitive Fortune cultivar (Citrus clementina Hort, es Tanaka x Citrus reticulata, Blanco). A cDNA library was construct ed 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 full-length cDNA clones (FPAL 1 and FPAL2) were isolated, and the deduced amino acid sequences showed a 7 5-85% similarity with PAL genes from other plant species, A comparative ana lysis of the changes in PAL activity and P.-IL mRNA levels was conducted in fruits stored at chilling (2 degrees C) and non-chilling (12 degrees C) te mperatures. Northern blot analyses, using both FPAL1 and FPAL2 cDNAs as pro bes, recognised a single mRNA that accumulated in fruits exposed to 2 degre es C prior to the appearance of physical chilling symptoms and the accompan ying 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 P AL 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 accum ulation of PAL transcript occurred in the leaves and roots of citrus plants exposed to a low temperature in the absence of any detectable chilling-ind uced damage.