PaKRP, a cyclin-dependent kinase inhibitor from avocado, may facilitate exit from the cell cycle during fruit growth

Previous studies using 'Hass' avocado cultivar showed that its small-fruit (SF) phenotype is limited by cell number. To explore the molecular components affecting avocado cell production, we isolated four cDNAs encoding: an ICK/KRP protein, known to play cell cycle-regulating roles through modulation of CDK function; two CDK proteins and a D-type cyclin, and monitored their expression patterns, comparing NF (normal fruit) versus SF profiles. The accumulation of PaKRP gradually deceased during growth in both fruit populations. Despite these similarities, SF exhibited higher levels of PaKRP accumulation at early stages of growth. Moreover, in NF, augmented PaKRP expression coincided with a decrease in CDK and PaCYCD1 levels, whereas in SF, enhanced PaKPR expression was coupled with an earlier decline of CDK and PaCYCD1 levels. For both NF and SF, enhanced mesocarp PaKRP transcript accumulation, was associated with elevated abscisic acid (ABA) and ABA catabolites content. Nevertheless, the collective ABA levels, including catabolites, were substantially higher in SF tissues, as compared with NF tissues. Finally, additional expression analysis revealed that in cultured cells, PaKRP could be induced by ABA. Together, our data links PaKRP with exit from the fruit cell cycle and suggest a role for ABA in controlling its expression.