Expression profiles of key phenylpropanoid genes during Vanilla planifolia pod development reveal a positive correlation between PAL gene expression and vanillin biosynthesis

Abstract :

In Vanilla planifolia pods, development of flavor precursors is dependent on the phenylpropanoidpathway. The distinctive vanilla aroma is produced by numerous phenolic compounds of which vanillinis the most important. Because of the economic importance of vanilla, vanillin biosynthetic pathwayshave been extensively studied but agreement has not yet been reached on the processes leading to itsaccumulation. In order to explore the transcriptional control exerted on these pathways, five key phenylpropanoidgenes expressed during pod development were identified and their mRNA accumulationprofiles were evaluated during pod development and maturation using quantitative real-time PCR. As aprerequisite for expression analysis using qRT-PCR, five potential reference genes were tested, and twogenes encoding Actin and EF1 were shown to be the most stable reference genes for accurate normalizationduring pod development. For the first time, genes encoding a phenylalanine ammonia-lyase(VpPAL1) and a cinnamate 4-hydroxylase (VpC4H1) were identified in vanilla pods and studied duringmaturation. Among phenylpropanoid genes, differential regulation was observed from 3 to 8 monthsafter pollination. VpPAL1 was gradually up-regulated, reaching the maximum expression level atmaturity. In contrast, genes encoding 4HBS, C4H, OMT2 and OMT3 did not show significant increase inexpression levels after the fourth month post-pollination. Expression profiling of these key phenylpropanoidgenes is also discussed in light of accumulation patterns for key phenolic compounds.Interestingly, VpPAL1 gene expression was shown to be positively correlated to maturation and vanillinaccumulation.