Identification and characterization of genes differentially expressed between two coffee cultivars 'Tall Mokka' and 'Typica'

Coffee (Coffea arabica L.) is an economically important crop of Hawai´i. 'Tall Mokka' (MA2) and 'Typica' (KO34) are two coffee cultivars renowned for their high quality beans. 'Tall Mokka' differs from 'Typica' in having a bushier appearance and smaller aerial organs. Differential gene expression analysis of the two cultivars, using a potato microarray as a cross species platform coupled with qRT-PCR, identified higher expression of a prolyl oligopeptidase (POP) gene in the shoot tips of 'Tall Mokka'. POP, a ubiquitous gene found in many forms of life, encodes for an enzyme in animals known to cleave small proteins (≤30aa) such as neuro-peptides and it interacts with the IP3 pathway in Dictyostelium. Studies of POP in plants are rare, therefore the functions of it in plants are not known. Cloning and sequencing POP from 'Tall Mokka' and 'Typica' revealed three variants of the gene arbitrarily named CaPOP1, CaPOP2 and CaPOP3. CaPOP3 is a product of recombination between the other two genes. CaPOP1 differs from CaPOP2 in having two deletions in the promoter region and seventeen mismatches in the coding region thought to be responsible for three amino acid differences at the protein level. Ectopic expression of CaPOP1::CaPOP1 in transgenic Arabidopsis yielded a bushy phenotype having significantly more secondary branches than the wild type. The AtPOP promoter directed stronger GUS expression in the guard cells of the leaf epidermis, at the tip of the developing leaf than in the middle of the leaf blade, and in the vascular tissues of the root. Fusion of GFP at the N-terminal of CaPOP1 failed to reveal any subcellular localization due to the lack of fluorescence under UV despite the expression and translation of CaPOP1. Expression of CaPOP1 under the 35S promoter did not affect Arabidopsis tolerance to salinity and osmotic stress. Seeds from these transformed plants lost viability earlier than the controls. Overall, this study has added new information regarding POP and suggested directions for future studies on POP in plants.