Phenylpropanoids Accumulation in Eggplant Fruit: Characterization of Biosynthetic Genes and Regulation by a MYB Transcription Factor.

Teresa Docimo,Giuseppe L Rotino,Giuseppe Mennella,Laura Toppino,Marina Tucci,Giorgia Batelli,Gianluca Francese,Alessandra Ruggiero,Laura Bassolino,Monica De Palma

doi:10.3389/fpls.2015.01233

Abstract

Phenylpropanoids are major secondary metabolites in eggplant (Solanum melongena) fruits. Chlorogenic acid (CGA) accounts for 70–90% of total phenolics in flesh tissues, while anthocyanins are mainly present in the fruit skin. As a contribution to the understanding of the peculiar accumulation of these health-promoting metabolites in eggplant, we report on metabolite abundance, regulation of CGA and anthocyanin biosynthesis, and characterization of candidate CGA biosynthetic genes in S. melongena. Higher contents of CGA, Delphinidin 3-rutinoside, and rutin were found in eggplant fruits compared to other tissues, associated to an elevated transcript abundance of structural genes such as PAL, HQT, DFR, and ANS, suggesting that active in situ biosynthesis contributes to anthocyanin and CGA accumulation in fruit tissues. Putative orthologs of the two CGA biosynthetic genes PAL and HQT, as well as a variant of a MYB1 transcription factor showing identity with group six MYBs, were isolated from an Occidental S. melongena traditional variety and demonstrated to differ from published sequences from Asiatic varieties. In silico analysis of the isolated SmPAL1, SmHQT1, SmANS, and SmMyb1 promoters revealed the presence of several Myb regulatory elements for the biosynthetic genes and unique elements for the TF, suggesting its involvement in other physiological roles beside phenylpropanoid biosynthesis regulation. Transient overexpression in Nicotiana benthamiana leaves of SmMyb1 and of a C-terminal SmMyb1 truncated form (SmMyb1Δ9) resulted in anthocyanin accumulation only of SmMyb1 agro-infiltrated leaves. A yeast two-hybrid assay confirmed the interaction of both SmMyb1 and SmMyb1Δ9 with an anthocyanin-related potato bHLH1 TF. Interestingly, a doubled amount of CGA was detected in both SmMyb1 and SmMyb1Δ9 agro-infiltrated leaves, thus suggesting that the N-terminal region of SmMyb1 is sufficient to activate its synthesis. These data suggest that a deletion of the C-terminal region of SmMyb1 does not limit its capability to regulate CGA accumulation, but impairs anthocyanin biosynthesis. To our knowledge, this is the first study reporting a functional elucidation of the role of the C-term conserved domain in MYB activator proteins.

Highlights

Eggplant, known as brinjal, is a berry-producing vegetable belonging to the large Solanaceae family and, to other popular and important Solanaceous crop such as tomato, potato, and pepper, is cultivated across all continents
The patterns of accumulation of the major phenylpropanoid metabolites of S. melongena, namely the anthocyanin D3R, the phenolic acid Chlorogenic acid (CGA) and the flavonoid rutin were investigated in several tissues and organs of the eggplant cultivar “Lunga Napoletana,” namely two leaf stages, stems, flowers, roots, and fruit skin and flesh
The flavonoid rutin was detected only in leaves at both stages, stems and fruit skin and its amount in the green tissues ranged from 1 μg in stems and young leaves to 3 μg/100 mg dw in mature leaves, while about a three times higher content was detected in the fruit skin

Summary

Introduction

Known as brinjal, is a berry-producing vegetable belonging to the large Solanaceae family and, to other popular and important Solanaceous crop such as tomato, potato, and pepper, is cultivated across all continents. Opposite to the other widely cultivated Solanaceae, tomato, potato, and pepper, which are native of the New World (Fukuoka et al, 2010; Albert and Chang, 2014; Hirakawa et al, 2014), eggplant has a phylogenetic uniqueness, due to its exclusive Old World origin. In the Solanaceae family, eggplant is the second most consumed fruit crop after tomato. Eggplant has been used in traditional medicine; its tissue extracts have been considered useful for the treatment of asthma, bronchitis, cholera, and dysuria, beneficial in lowering blood cholesterol and showed antimutagenic properties (Khan, 1979; Hinata, 1986; Kalloo, 1993; Collonnier et al, 2001; Kashyap et al, 2003)

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