Enigmatic transcription factor.
Glandular trichomes on tomato leaves produce a variety of volatile mono- and sesquiterpenes that play an important role in defense against pathogens and insect pests. The key players in this process are terpene synthases, whose function has been extensively studied. Relatively little is known, however, about the transcriptional regulation of these enzymes in glandular trichomes. Using existing transcriptome data, IPB scientists have now selected several transcription factors overexpressed in Solanum lycopersicum trichomes and dimmed their gene activity in planta by virus-induced gene silencing. As a result, they found a transcription factor of the scarecrow-like (SCL) subfamily whose partial absence in the tomato plant led to a significant decrease in both expressed terpene synthases and produced volatile terpenes. Meanwhile, overexpression of this transcription factor, designated SlSCL3, had the opposite effect: biosynthesis of volatile terpenes was strongly activated and even the head of the glandular trichomes swelled.
So far, so good. However, a complete knock-out of the SlSCL3 gene by CRISPR-Cas showed a more complicated picture. The homozygous progeny of the Slscl3 knockout line produced about 40 % less volatile terpenes, while the heterozygous F1 plants - similar to the overexpression lines - synthesized more terpenes than the wild type plants. In addition, the Halle scientists found further enigmatic features. They failed to observe direct protein-DNA binding or interaction with previously known regulators for the SlSCL3 transcription factor. The levels of the endogenous SlSCL3 mRNA also behaved unexpectedly: it was decreased in the overexpression lines but increased in both the heterozygous and homozygous knockout mutants. SlSCL3 regulates its own transcription, presumably through negative feedback, the researchers concluded. Still, many questions remain unanswered and additional interactors need to be found before this complex regulatory scenario can be better understood.
Elucidating this regulatory network is of great relevance because wild tomatoes such as Solanum habrochaites naturally produce more volatile terpenes than the cultivated tomato Solanum lycopersicum, whose ability to defend itself against insect pests has been partially lost during cultivation. A better understanding of the differences in metabolism between these two tomato species may be used to restore the resistance of cultivated tomato to pathogens and predators.
