Identification and expression studies of a PNP from Opuntia ficus-indica

Natriuretic plant peptides (PNPs) are biomolecules with hormonal function that lead to homeostasis of solutes and water in plant cells and are involved in the opening and closing of stomata. In this study, an Opuntia ficus-indica PNP gene was identified and was characterized. It was called OfPNPA and its sequence was deposited in the NCBI database with the accession number KF649980. OfPNPA gene has a similarity greater than 90% with other PNPs sequences. The amino acid sequence obtained by virtual translation presented a double-psi β barrel structural domain (DPBB), two conserved cysteines and the KVVD and LSRDAFRVI motifs. The phylogenetic analysis showed two branches, in one was grouped OfPNPA with other PNPs and with endoglucanases, and in the other branch with expansins proteins. Opuntia ficus-indica has only one copy of PNP gene. In situ hybridization analysis revealed that PNP gene is expressed in stomatal guard cells. The Real-Time quantitative reverse transcriptional PCR analysis showed that PNP gene expression from ex-vitro grown plants was maximum when the analyzed samples were taken in the dark, while it was minimum in the samples taken during the day. The expression pattern changed in plants grown in vitro. However, in both cases the PNP expression was higher when stomata must be open. In addition, the immunolocation technique revealed the presence of PNPs receptors in O. ficus-indica stomata cells. This is the first report of PNPs in cacti and contribute to understand the role of PNPs in stomatal opening related to acid metabolism of crassulaceae.