An overlooked function of trichomes in defense against parasitic plants

Trichomes have rarely been considered as components of pre-attachment resistance mechanisms against parasitic plants, and their potential role in host defense has scarcely been addressed in reviews of trichome function. In this study, we investigated the interaction between tomato plants ( Solanum lycopersicum ‘Minibel’) and the parasitic plant Cuscuta campestris . Analyses focused on trichome distribution and the structural modifications they undergo at various stages of infestation and host resistance. Parasite–host contact may occur through the direct attachment of Cuscuta seedlings following germination (primary parasitism), or via lateral branches originating from previously infested hosts (secondary parasitism). Primary parasitism on tomato stems was inefficient, seemingly due primarily to the long type I and II trichomes hindering parasite attachment to the host surface. Only ~20% of Cuscuta seedlings successfully overcame this physical barrier and developed haustoria. These long trichomes also appeared to contribute to resistance during secondary parasitism, as they seemingly impede penetration of the haustoria by bending under the pressure exerted by the dodder stem. As infestation progressed, marked changes were also observed in type VI glandular trichomes. In the vicinity of the parasite, the glandular heads of trichomes often turned brown, exhibited significant autofluorescence and sometimes ruptured, which is consistent with the release of secondary metabolites. During the formation of an epidermal and subepidermal defensive barrier, the heads of additional type VI trichomes darkened, lost turgor and acquired a flattened four-lobed appearance. Together, our findings highlight the importance of trichomes in the defense of tomato plants against dodder and expand our understanding of pre-attachment resistance mechanisms.