Molecular evolution and transcriptional regulation of the oilseed rape proline dehydrogenase genes suggest distinct roles of proline catabolism during development

Six BnaProDH1 and two BnaProDH2 genes were identified in Brassica napus genome. The BnaProDH1 genes are mainly expressed in pollen and roots' organs while BnaProDH2 gene expression is associated with leaf vascular tissues at senescence. Proline dehydrogenase (ProDH) catalyzes the first step in the catabolism of proline. The ProDH gene family in oilseed rape (Brassica napus) was characterized and compared to other Brassicaceae ProDH sequences to establish the phylogenetic relationships between genes. Six BnaProDH1 genes and two BnaProDH2 genes were identified in the B. napus genome. Expression of the three paralogous pairs of BnaProDH1 genes and the two homoeologous BnaProDH2 genes was measured by real-time quantitative RT-PCR in plants at vegetative and reproductive stages. The BnaProDH2 genes are specifically expressed in vasculature in an age-dependent manner, while BnaProDH1 genes are strongly expressed in pollen grains and roots. Compared to the abundant expression of BnaProDH1, the overall expression of BnaProDH2 is low except in roots and senescent leaves. The BnaProDH1 paralogs showed different levels of expression with BnaA&C.ProDH1.a the most strongly expressed and BnaA&C.ProDH1.c the least. The promoters of two BnaProDH1 and two BnaProDH2 genes were fused with uidA reporter gene (GUS) to characterize organ and tissue expression profiles in transformed B. napus plants. The transformants with promoters from different genes showed contrasting patterns of GUS activity, which corresponded to the spatial expression of their respective transcripts. ProDHs probably have non-redundant functions in different organs and at different phenological stages. In terms of molecular evolution, all BnaProDH sequences appear to have undergone strong purifying selection and some copies are becoming subfunctionalized. This detailed description of oilseed rape ProDH genes provides new elements to investigate the function of proline metabolism in plant development.