Correlations among six hormone-induced transcription factors and the alcohol acyltransferase gene in apple

Alcohol acyltransferase (AAT) plays an important role in ester biogenesis in ripening fruit. In apple, MdAAT2 is up-regulated in response to treatment with defense-related hormones. We cloned a novel MdAAT2 promoter via thermal asymmetric interlaced PCR (TAIL-PCR). Sequence analysis indicated the presence of salicylic acid (SA)- and methyl jasmonate (MeJA)-responsive elements in the promoter region. To examine further the operational mechanism for transcription factors (TFs) in regulating expression of the MdAAT2 promoter, we isolated and investigated six potential stress-induced TFs — MdMYB1, MdMYB2, MdMYB6, MdERF1, MdERF2, and WRKY. These were monitored during various stages of fruit development as well as under several hormonal treatments. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that all of these TF genes, as well as MdAAT2 , were detectable in the stigmas. Levels of expression by MdMYB1, MdMYB2 , and MdERF1 were significantly correlated with that by MdAAT2 during fruit ontogeny. Moreover, transcription of three TFs ( MdMYB1, MdMYB6 , and MdERF1 ) was significantly correlated with that of MdAAT2 upon exposure to ethephon, SA, or MeJA. Sequence analysis demonstrated that CAAT, CCAAT elements, and several MYB transcription factor binding sites (CNGTTmotif, AACCA, and CTAACCA elements) existed in the MdAAT2 promoter region. Using that analysis, we performed modeling of MdMYB1 and MdMYB6 based on PROTEINDNA complex1 and complex3 (PDB Accession Numbers 1H88 and 1H8A). There, similarities were found with the protein complex of CAAT or CCAAT consensus sequences. Thus, we propose that two TFs — MdMYB1 and MdMYB6 — are directly or indirectly involved in the activation of MdAAT2 expression in apple fruit.