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The HD-ZIP IV transcription factor Tril regulates fruit spine density through dose effects in cucumber.

Trichomes and fruit spines are important traits that directly affect the appearance quality and commercial value of cucumber. Tril (Trichome-less), an HD-Zip IV transcription factor, plays a crucial role in the initiation of trichomes and fruit spines, but little is known about their exact regulatory mechanisms. In this study, tissue expression pattern analysis indicated that Tril is expressed and functions in the early stage of organ initiation and development. Expression of the Tril gene under the control of its own promoter (the TrilPro::Tril-3*flag fragment) could partly rescue the mutant phenotypes of tril, csgl3 (cucumber glabrous 3, an allelic mutant of tril) and fs1 (few spines 1, a fragment substitution in the Tril promoter region), providing further evidence that Tril is responsible for the initiation of trichomes and fruit spines. In dense spine-type lines, fs1-type lines and different background transgenic lines containing the TrilPro::Tril-3*flag foreign fragment, fruit spine number increased along with the increase in Tril expression, indicating that Tril has a gene dosage effect on fruit spine density in cucumber. The Numerous Spines (NS) gene is a negative regulatory factor of fruit spine density. Characterization of the molecular and genetic interaction between Tril and NS/ns demonstrated that Tril functions upstream of the negative regulator NS with respect to fruit spine initiation. Overall, our results uncover a novel mechanism governing the effect of the Tril gene in regulating fruit spine development and provide a reference for future work on the quality breeding of cucumber.