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A multidisciplinary approach to elucidate glandular trichome differentiation in tomato

Subject Area Plant Cell and Developmental Biology
Plant Biochemistry and Biophysics
Plant Genetics and Genomics
Term from 2013 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 238047333
 
Final Report Year 2018

Final Report Abstract

Glandular trichomes are epidermal differentiations that protect the plant against a variety of aggressors through the chemicals that they produce and store, or secrete. In cultivated tomato (Solanum lycopersicum), a major horticultural crop, there are several types of glandular trichomes, of which type VI is the most abundant on the leaf surface. Volatiles terpenes are major products of the type VI trichomes and they are stored in an extracellular storage cavity located between the four glandular cells. In a related wild species, Solanum habrochaites, the overall architecture of type VI trichomes is the same, but the four glandular cells form a round shape and the storage cavity is much larger, representing around 65% of the total volume of the trichome head. This contributes to the much greater production capacity of type VI trichomes in S. habrochaites, and thereby to their increased resistance against herbivore pests. In this project, we investigated the reasons for these differences by a number of approaches. The first consisted in a thorough description of the development and anatomical features of type VI trichomes in both species. This showed that the sequence of development is very similar in both species and that the differences become visible when the trichomes enlarge. Immunostainings revealed the presence of specific cell wall components in distinct areas of the trichome glandular head. One interesting observation is the presence of an autofluorescent compound that is present in the early stages but disappears as the trichome matures. Chemical analysis indicates this is a derivative of riboflavin, but its function remains unclear. This autofluorescence was key in establishing a method to isolate trichomes at different development stages using fluorescence-activated cell sorting. This turned out to take a very long time to establish (around 1.5 years). One consequence was that it was difficult to make significant progress with the characterization of candidate genes within the time frame of the project. This is now under way using virus-induced gene silencing and transgenic approaches, including CRISPR/Cas9 gene editing. In parallel, a back-cross population of 116 individuals was generated with S. habrochaites LA1777 and S. lycopersicum Wva106, using the latter as the recurrent parent. A score was designed to reflect the shape of the type VI trichomes, which correlates with greater accumulation of terpenes. This was used to identify two major QTLs in the population, respectively on chromosome 1 and 7, each covering several hundred genes. Due to restricted recombination in these regions, the intervals could not be significantly reduced even in advanced populations. Nonetheless, using trichome transcriptome data available in the lab, a list of candidate genes was established. Further characterization of these by VIGS and gene editing is also in progress. In conclusion, this project allowed us to gain deeper insights in the development and physiology of tomato type VI trichomes. These results contribute to a long-term effort to introduce complex insect-resistance traits from related wild germplasm into cultivated tomato.

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