Morphological structures and associated chemical compounds serve important functions during the life cycle of plants, including deterrence of herbivores. Terpenoids represent a major class of compounds involved in defending plants from herbivores. Their differential accumulation in specific tissues and cell types is at present not well understood. In cultivated tomato (Solanum lycopersicum), terpenoids and flavonoids accumulate in type VI glandular trichomes that are found on most above ground plant organs. Due to our ability to isolate them, these glandular trichomes of tomato are an attractive model system that can help us better understand how plants regulate the biosynthesis of terpenoids. The mutations in the odorless-2 (od-2) and the anthocyanin free (af) genes produce defective type VI glandular trichomes that fail to accumulate flavonoids and volatile terpenoids. Accordingly, these mutants provide useful genetic tools to study the relationship between the development and metabolic capacity of glandular trichomes.In this work, I determined the molecular basis of the od-2 mutation and showed that the wild-type Od-2 gene encodes an enzyme (designated LCB1A for long chain base 1A) involved in the biosynthesis of long-chain base precursors of sphingolipids. A functional LCB1A gene is necessary for type VI trichomes to produce high levels of volatile terpenoids. Transgenic expression of the wild-type LCB1A cDNA from the Cauliflower Mosaic Virus 35S promoter complemented both the leaf developmental and sphingolipid content phenotypes of the leaflets of the mutant but failed to rescue the terpenoid deficiency. Conversely, expression of LCB1A from a trichome-specific promoter restored terpenoid production in type VI trichomes of od-2 but did not complement the foliar sphingolipid profile or other leaf developmental defects. These results provide new insights into the tissue- and cell type-specific function of sphingolipid biosynthesis in tomato and establish an essential role for sphingolipids in trichome-borne specialized metabolism.In a similar fashion, I used the chalcone isomerase 1 (CHI1)-deficient af mutant to study the tissue and cell-type specific role of flavonoids. Specifically, I constructed transgenic lines in which CHI1 was expressed in the af mutant background from either the 35S or a trichome-specific promoter. Detailed biochemical and physiological characterization of these lines showed that CHI1 activity in type VI glandular trichomes, but not in other cell types of the leaf, is required for abundant production of volatile terpenoids. Expression of CHI1 in type VI trichomes was also required to restore resistance of af leaves to feeding by the two-spotted spider mite Tetranychus urticae. In summary, the results described in this thesis advance knowledge of how various metabolic pathways, including sphingolipid and flavonoid biosynthesis, operate to promote normal development and function of glandular trichomes.
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