A BAHD acyltransferase is expressed in the tapetum of Arabidopsis anthers and is involved in the synthesis of hydroxycinnamoyl spermidines

BAHD acyltransferases catalyze the acylation of many plant secondary metabolites. We characterized the function of At2g19070, a member of the BAHD gene family of Arabidopsis thaliana. The acyltransferase gene was shown to be specifically expressed in anther tapetum cells in the early stages of flower development. The impact of gene repression was studied in RNAi plants and in a knockout (KO) mutant line. Immunoblotting with a specific antiserum raised against the recombinant protein was used to evaluate the accumulation of At2g19070 gene product in flowers of various Arabidopsis genotypes including the KO and RNAi lines, the male sterile mutant ms1 and transformants overexpressing the acyltransferase gene. Metabolic profiling of flower bud tissues from these genetic backgrounds demonstrated a positive correlation between the accumulation of acyltransferase protein and the quantities of metabolites that were putatively identified by tandem mass spectrometry as N-1,N-5,N-10-trihydroxyferuloyl spermidine and N-1,N-5-dihydroxyferuloyl-N-10-sinapoyl spermidine. These products, deposited in pollen coat, can be readily extracted by pollen wash and were shown to be responsible for pollen autofluorescence. The activity of the recombinant enzyme produced in bacteria was assayed with various hydroxycinnamoyl-CoA esters and polyamines as donor and acceptor substrates, respectively. Feruloyl-CoA and spermidine proved the best substrates, and the enzyme has therefore been named spermidine hydroxycinnamoyl transferase (SHT). A methyltransferase gene (At1g67990) which co-regulated with SHT during flower development, was shown to be involved in the O-methylation of spermidine conjugates by analyzing the consequences of its repression in RNAi plants and by characterizing the methylation activity of the recombinant enzyme.