PsPMEP, a pollen-specific pectin methylesterase of pea (Pisum sativum L.)
Pectin methylesterases (PMEs) are a family of enzymes involved in plant reproductive processes such as pollen development and pollen tube growth. We have isolated and characterized PsPMEP, a pea (Pisum sativum L.) pollen-specific gene that encodes a protein with homology to PMEs. Sequence analysis showed that PsPMEP belongs to group 2 PMEs, which are characterized by the presence of a processable amino-terminal PME inhibitor domain followed by the catalytic PME domain. Moreover, PsPMEP contains several motifs highly conserved among PMEs with the essential amino acid residues involved in enzyme substrate binding and catalysis. Northern blot and in situ hybridization analyses showed that PsPMEP is expressed in pollen grains from 4 days before anthesis till anther dehiscence and in pollinated carpels. In the PsPMEP promoter region, we have identified several conserved cis-regulatory elements that have been associated with gene pollen-specific expression. Expression analysis of PsPMEP promoter fused to the uidA reporter gene in Arabidopsis thaliana plants showed a similar expression pattern when compared with pea, indicating that this promoter is also functional in a non-leguminous plant. GUS expression was detected in mature pollen grains, during pollen germination, during pollen tube elongation along the transmitting tract, and when the pollen tube reaches the embryo sac in the ovule.