Getting out of a hairy situation: Increased trichome density improves pest resilience in tomato

Abstract

The plant epidermis serves as a crucial interface between the aerial parts of the plant and the external environment. It regulates interactions with the surroundings and provides protection against abiotic stress, pests and pathogens. Trichomes—small, often hair-like appendages—play an important role in these protective functions. They are classified as either glandular or non-glandular based on their ability to produce, store and secrete specialized metabolites, and can be further divided into eight distinct types based on morphology (Khan et al., 2021). Glandular trichomes produce biologically and commercially important chemicals such as essential oils, cannabinoids, alkaloids and terpenes, many of which have roles in defence and deter pathogens and herbivores. Understanding how glandular trichomes are formed and patterned is thus not only a fundamental question in plant developmental biology but also holds promise for agricultural and pharmaceutical applications (Huchelmann et al., 2017).

Glandular trichomes are multicellular and arise from epidermal cells that receive developmental cues from surrounding tissues. These cells then undergo a series of tightly regulated divisions and differentiation events to form the mature trichome, consisting of a base, a stalk and a glandular head. Since Arabidopsis thaliana does not produce glandular trichomes, researchers have turned to other model systems to study their development. In tomato (Solanum lycopersicum), the homeodomain-Leu zipper IV (HD-ZIP) transcription factor Woolly (Wo) regulates the formation of Type I trichomes (Wu et al., 2024), which are characterized by a multicellular stalk and a single glandular head cell, while the bHLH transcription factor SlMYC1 controls the formation of Type VI trichomes (Xu et al., 2018), which feature four glandular head cells. Despite the discovery of these factors, the regulatory networks guiding glandular trichome development remain incompletely understood and additional components are yet to be identified.

Baowen Huang and Zhenguo Li from Chongqing University have extensive experience in tomato breeding and improvement, while Julien Pirrello, based at the Université de Toulouse, studies transcriptional regulation in tomato. The groups have a longstanding partnership, spanning more than 20 years and involving multiple student exchanges between the laboratories. The highlighted study represents one output of this partnership: Shi et al. investigated genes selectively expressed in glandular trichomes to identify new regulators of trichome development. They found that the GAI, RGA and SCR (GRAS) family transcription factor SlGRAS9 was highly expressed in Type VI trichomes but showed low expression in non-glandular trichomes and surrounding leaf tissue. Across a range of tomato cultivars, SlGRAS9 transcript levels were inversely correlated with Type VI trichome density, suggesting a possible repressive role for SlGRAS9 in trichome formation.

To test this hypothesis, Shi et al. generated Slgras9 knock-out mutants using CRISPR/Cas9 technology. These mutants exhibited a marked increase in Type VI trichome density on all aerial organs, with only minor effects on other trichome types (Figure 1a). Given that Type VI glandular trichomes are major sites of terpenoid biosynthesis (McDowell et al., 2011), the team measured metabolite levels and found that Slgras9 mutants had elevated concentrations of several mono- and sesquiterpenes in leaves, petals and fruits. These metabolic changes coincided with higher transcript levels of terpene synthase genes.

How does SlGRAS9 affect glandular trichome development? Previously generated DNA affinity purification sequencing (DAP-seq) data showed that SlGRAS9 binds to the promoter of the known regulator of glandular trichome formation, SlMYC1 (Shi et al., 2024). The authors further confirmed this finding using EMSA and yeast-1-hybrid assays. Furthermore, luciferase reporter assays suggested that SlGRAS9 represses SlMYC1 expression, consistent with the elevated levels of SlMYC1 transcript observed in Slgras9 mutants.

To determine whether SlMYC1 functions downstream of SlGRAS9, the researchers generated SlMYC1 overexpression and CRISPR knock-out lines. SlMYC1 overexpression lines phenocopied Slgras9 mutants, showing increased trichome density and elevated terpenoid levels when compared with the wild type. Slmyc1 mutants, on the other hand, displayed a markedly reduced density of Type VI trichomes but an increased density of Type VII glandular trichomes as well as nearly undetectable terpenoid levels. The trichome phenotype of the Slgras9 Slmyc1 double mutant was indistinguishable from that of the Slmyc1 single mutant, showing that the Slmyc1 mutation is epistatic to Slgras9 and that SlGRAS9 acts through SlMYC1 to regulate Type VI trichome development.

Glandular trichomes have a well-documented role in promoting resistance to pests such as spider mites and aphids (Glas et al., 2012), which cause damage through both direct consumption and disease transmission. When challenged with these pests, Slgras9 mutants and SlMYC1 overexpression lines displayed fewer chlorotic lesions and harboured reduced numbers of mites and aphids than wild-type plants (Figure 1b). These findings underscore the defensive role of glandular trichomes and of the SlGRAS9-SlMYC1 module in deterring herbivorous pests.

In summary, Shi et al. firmly established the SlGRAS9-SlMYC1 transcription factor module as a regulator of glandular trichome development in tomato. The exact site of its action remains uncertain. As a negative regulator of trichome density, SlGRAS9 may act non-cell-autonomously, potentially moving from trichomes into adjacent epidermal cells to suppress trichome precursor identity. Similar intercellular mobility has been observed for other trichome-related transcription factors in Arabidopsis (Khan et al., 2021). While the precise mechanisms remain to be fully understood, SlGRAS9 and SlMYC1 emerge as promising targets for crop improvement: manipulating the expression of either gene can boost pest resistance by increasing trichome density and enhancing the production of defensive secondary metabolites. This work lays a strong foundation for future breeding and gene-editing strategies aimed at strengthening plant defences against the growing threat of pests and pathogens.