The two non-functional myrosinase genes TGG3 and TGG6 in Arabidopsis are expressed predominantly in pollen

Myrosinases are a family of β-thioglucoside glucohydrolases (TGGs) that catalyze hydrolysis of glucosinolates to deter pest insects and pathogens. Arabidopsis has six myrosinase genes, of which TGG1, TGG2, TGG4 and TGG5 have been reported as functional, and TGG3 as a pseudogene, while TGG6 has received little attention. Both genomic and cDNA of TGG6 were cloned from five ecotypes and sequenced in this research. TGG6 had the same gene structure as TGG4 and TGG5, but contained several frameshift mutations and should be non-functional. Reverse transcription polymerase chain reaction indicated that TGG6 was specifically expressed in flowers. Promoter-driven GUS expression showed that TGG6 was solely expressed in mature pollen grains when flowers initiated blossom. TGG6 expression was not detected in other cell types of anthers and other organs. The TGG3 promoter-driven GUS expression was similarly predominantly expressed in mature pollen, but the expression of TGG3 in pollen was initiated before flower bud blossoming, earlier than of TGG6, and there was weak expression in petals and pollen sac cells for a short period.