Abstract
Eggplant produces a variety of hydroxycinnamic acid amides (HCAAs) that have an important role in plant development and adaptation to environmental changes. In this study, we identified and characterized a spermidine hydroxycinnamoyl transferase (SHT) from eggplant (Solanum melongena) and its wild relative S. richardii, designated as SmSHT and SrSHT, respectively. SmSHT was abundant in flowers and fruits, whereas the level of SrSHT was remarkably low in all tissues. Heat-shock/drought treatment stimulated the expression of SmSHT in both leaves and fruits, indicating its involvement in plant stress response. Both SHT polypeptides had extremely high identity with just five amino-acid substitutions. Recombinant SmSHT catalyzed the synthesis of mono-, bi- and tri- acylated polyamines. Using caffeoyl-CoA as the acyl donor, SmSHT preferred spermidine as the acyl acceptor. When spermidine was the acyl acceptor, the donor preference order for SmSHT was caffeoyl-CoA>feruloyl-CoA>ρ-coumaroyl-CoA. SrSHT exhibited the same substrate specificity as SmSHT, yet exhibited significantly higher catalytic activity than SmSHT. For example, under caffeoyl-CoA and spermidine, Kcat of SrSHT was 37.3% higher than SmSHT. Molecular modeling suggests that five amino-acid substitutions in SrSHT result in four alterations in their predicted 3D structures. In particular, the conserved Lys402 adjacent to the DFGWG motif, and Cys200 in the crossover loop in SmSHT were replaced by Glu and Ser in SrSHT. These substitutions may contribute to the enhanced activity in SrSHT. Our study provides a platform to generate HCAA rich fruits for eggplant and other solanaceous crops.
Highlights
Hydroxycinnamic acid amides (HCAAs)[1] have a critical role in plant growth and developmental processes, including cell division, cytomorphogenesis, flowering, cell wall cross-linking, tuberization and stress responses.[2,3,4,5,6] They are beneficial to human health since these compounds have anticarcinogenic, antihypertensive, antimicrobial and other potentially therapeutic activities.[7,8,9,10,11]HCAAs are derived from the phenylpropanoid pathway (Figure 1)
For HCAAs biosynthesis, the ultimate and committed step is the condensation of hydroxycinnamoyl-CoA thioesters with polyamines such as spermidine (Spd), spermine (Spm) and putrescine (Put) catalyzed by hydroxycinnamoyl transferases.[15]
Eggplant fruit is highly rich in HCAAs.[21,22,23,24]
Summary
Hydroxycinnamic acid amides (HCAAs)[1] have a critical role in plant growth and developmental processes, including cell division, cytomorphogenesis, flowering, cell wall cross-linking, tuberization and stress responses.[2,3,4,5,6] They are beneficial to human health since these compounds have anticarcinogenic, antihypertensive, antimicrobial and other potentially therapeutic activities.[7,8,9,10,11]. Spermidine hydroxycinnamoyl transferase (SHT) belongs to the acyl-CoA-dependent BAHD super family The members of this family are responsible for biosynthesis or modification of diverse metabolites such as alkaloids, terpenoids and phenolics.[16] They possess a conserved catalytic HXXXD domain and a functionally unknown DFGWG motif.[16] Several SHTs have been isolated and characterized in plants.[5] An Arabidopsis thaliana SHT (AtSHT) was shown to catalyze the formation of mono-, di- and tri- acylatedSpd.[17] Some other SHTs in Arabidopsis regulate the accumulation of disinapoyl-Spd and dicoumaroyl-Spd in seeds.[4] A tobacco SHT (NaDH29) catalyzes the synthesis of hydroxycinnamoyl spermidine (HCSpd).[18] all these enzymes prefer Spd as acyl donor substrate, they display distinct catalytic specificities and activities. The structural features, substrate specificity and catalytic activities of both SHTs were analyzed
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