Abstract
Glandular trichomes contribute to the high resistance of wild tomato species against insect pests not only thanks to the metabolites they produce but also because of morphological and developmental features which support the high production of these defense compounds. In Solanum habrochaites, type VI trichomes have a distinct spherical shape and a large intercellular storage cavity where metabolites can accumulate and are released upon breaking off of the glandular cells. In contrast, the type VI trichomes of S. lycopersicum have a four-leaf clover shape corresponding to the four glandular cells and a small internal cavity with limited capacity for storage of compounds. To better characterize the genetic factors underlying these trichome morphological differences we created a back-cross population of 116 individuals between S. habrochaites LA1777 and S. lycopersicum var. cerasiforme WVa106. A trichome score that reflects the shape of the type VI trichomes allowing the quantification of this trait was designed. The scores were distributed normally across the population, which was mapped with a total of 192 markers. This resulted in the identification of six quantitative trait locus (QTLs) on chromosomes I, VII, VII, and XI. The QTL on chromosome I with the highest LOD score was confirmed and narrowed down to a 500 gene interval in an advanced population derived from one of the back-cross lines. Our results provide the foundation for the genetic dissection of type VI trichome morphology and the introgression of these trichome traits into cultivated tomato lines for increased insect resistance.Key Message: This work shows that the shape of type VI glandular trichomes in tomato is a genetically defined trait controlled by multiple QTLs with one on chromosome I being the major contributor.
Highlights
Trichomes are differentiations of the epidermis that occur at the surface of the aerial parts of many higher plants
The type VI trichomes of S. habrochaites have the shape of a ball, whereas in S. lycopersicum the four glandular cells can be clearly distinguished giving the head of the trichome the appearance of a four-leaf clover
For S. habrochaites this score approaches 1 (0.972 ± 0.004), reflecting its almost perfect circular shape, whereas the S. lycopersicum score is 0.780 ± 0.006. We calculated this score for a number of different tomato species and cultivars (Figure 2). This revealed that all three different S. habrochaites lines have the highest score, whereas cultivars of S. lycopersicum and S. pimpinellifolium have the lowest score
Summary
Trichomes are differentiations of the epidermis that occur at the surface of the aerial parts of many higher plants. They come in a great variety of shapes, they can be classified as either nonglandular or glandular. Glandular trichomes are present in about 30% of all vascular plants (Fahn, 2000). These structures accumulate large quantities of secondary metabolites involved in the protection against herbivores and pathogens (Glas et al, 2012; Tissier, 2012), and are the source of industrially important ingredients in the flavor, fragrance and pharmaceutical areas (Schilmiller et al, 2008).
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