Promoter analyses and transcriptional profiling of eggplant polyphenol oxidase 1 gene (SmePPO1) reveal differential response to exogenous methyl jasmonate and salicylic acid.

The transcriptional regulation of multigenic eggplant (Solanum melongena) polyphenol oxidase genes
(SmePPO) is orchestrated by their corresponding promoters which mediate developmentally regulated
expression in response to myriad biotic and abiotic factors. However, information on structural features
of SmePPO promoters and modulation of their expression by plant defense signals are lacking. In the
present study, SmePPOPROMOTERs were cloned by genome walking, and their transcription start sites
(TSS) were determined by RLM-RACE. Extensive sequence analyses revealed the presence of evolutionarily
conserved and over-represented putative cis-acting elements involved in light-regulated transcription,
biosynthetic pathways (phenylpropanoid/flavonoid), hormone signaling (abscisic acid, gibberellic acid,
jasmonate and salicylate), elicitor and stress responses (cold/dehydration responses), sugar metabolism
and plant defense signaling (W-BOX/WRKY) that are common to SmePPOPROMOTER1 and 2. The TSS for
SmePPO genes are located 9–15 bp upstream of ATG with variable lengths of 5� untranslated regions.
Transcriptional profiling of SmePPOs in eggplant seedlings has indicated differential response to methyl
jasmonate (MeJA) or salicylic acid (SA) treatment. In planta, while MeJA elicited expression of all the six
SmePPOs, SA was only able to induce the expression of SmePPO4–6. Interestingly, in dual treatment, SA
considerably repressed the MeJA-induced expression of SmePPOs. Functional dissection of SmePPOPROMOTER1
by deletion analyses using Agrobacterium-mediated transient expression in tobacco leaves has
shown that MeJA enhances the SmePPOPROMOTER1-ˇ-glucuronidase (GUS) expression in vivo, while SA
does not. Histochemical and quantitative GUS assays have also indicated the negative effect of SA on
MeJA-induced expression of SmePPOPROMOTER1. By combining in silico analyses, transcriptional profiling
and expression of SmePPOPROMOTER1–GUS fusions, the role of SA on the modulation of MeJA-induced
SmePPO1 expression has been elucidated. It is concluded that similar to the coding regions of multigenic
SmePPOs, the regulatory elements are also evolutionarily conserved and fall into two distinct sub-classes
based on their responses to MeJA and SA.