The evolutionary fate of Δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1) genes in allotetraploid Brassica napus

Although the regulatory and the functional aspects of proline accumulation in plant stress tolerance have been well-documented since it was discovered more than half a century ago, the evolution of proline metabolic genes in polyploidy species and their diploidy progenitors is not understood so far. Here, we have studied the retention and the expression regulation of 1-pyrroline-5-carboxylate synthetase 1 (P5CS1) genes in allotetraploid, Brassica napus, in comparison with its diploids progenitors, B. rapa and B. oleracea under salt stress. Our results indicated that all six P5CS1 genes from two diploid progenitors were retained. Sequence analysis revealed that all Brassica P5CS1 genes had a highly conserved exon-intron structure, similar to P5CS1 in Arabidopsis. In addition, two homeologous genes with different origin, BnaA.P5CS1.a and BnaC.P5CS1.d exhibited biased expression in different organs in B. napus, implying possible sub-functionalization of P5CS1 in B. napus. All six P5CS1 genes were induced in response to salt stress in B. napus, which rendered gene dose advantage for the level of proline accumulation. These results indicate that in B. napus, P5CS1 gene loss has not occurred during polyploidization, which suggest conserved functionality and evolutionary benefit of proline accumulation for plant adaptation to environmental stresses.