Isolation and expression of two pectate lyase genes during fruit ripening of banana (Musa acuminata)

Abstract

Two cDNAs, designated MWPL1 and MWPL2, encoding putative pectate lyases (Pel; EC 4.2.2.2), which catalyze the cleavage by β-elimination of α-1 → 4-linked galacturonosyl residues of pectins found mostly in middle lamella and primary cell wall in plants, were isolated from ripening fruit of banana (Musa acuminata) and their expressions in fruit during ripening and in response to ethylene were investigated. MWPL1 and MWPL2 encode a single polypeptide of 407 and 454 amino acid residues, respectively. The two cDNAs shared an overall identity of 75% in both nucleotide and deduced amino acid sequences. Sequence comparison of MWPL1 and other plant Pels revealed the homology ranging from 76% with zinnia to 48% with ragweed. Southern analysis indicated that MWPL1 might be present as a single copy gene, and there might be up to two copies of MWPL2 in the banana genome. The two cDNAs were expressed differentially and/or spatially in various banana organs, with female flower and fruit tissues showing accumulation of the MWPL2 transcript, which was not detected in root, pseudostem, leaf, male flower and ovary, whereas the MWPL1 transcript was not detectable in all organs tested. In fruit tissue during ripening, although transcripts of both members were not detectable in unripe preclimacteric fruits, they began to accumulate as ripening progressed and the level remained high thereafter in overripe fruits. However, the magnitude of transcript accumulation differed between the two Pel members, with substantially more abundant MWPL2 than MWPL1 in ripening fruit. Similar differential transcript accumulation was also observed between peel and pulp, where the former was markedly higher than the latter. Expression of both Pel members was also affected by exogenous ethylene, whose presence at 5-100 ppm stimulated accumulation of MWPL1 and MWPL2 transcripts in preclimacteric fruit, suggesting that ethylene may play an important regulatory role in regulating Pel expression during fruit ripening of the banana.