Graduate Thesis Or Dissertation
 

Characterization of the LeIAA family of auxin-regulated genes in tomato (Lycopericon esculentum, Mill.)

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  • The plant hormone auxin regulates many aspects of plant growth and development. At the cellular level, auxin can stimulate cell division, cell elongation, and cell differentiation. Little is known about the molecular mechanisms that mediate auxin action at this level, although changes in gene expression have been implicated in each of them. An important step in elucidating the signal transduction steps involved in these responses is the identification of intermediates that respond to auxin treatment within a physiologically relevant range, and can be functionally linked to specific response(s). The Aux/IAA family of auxin-regulated genes, which has been identified in several species, may constitute such a group of signalling intermediates. This dissertation reports on the isolation and characterization of eleven tomato genes, LeIAA1 - 11, which are homologous to these Aux/IAA genes. Expression characteristics in response to exogenously-applied auxin were determined in segments of etiolated hypocotyls. The LeIAA genes fell into three kinetic classes and displayed differential responsiveness to the treatment. Comparison of auxin-induced expression in wild-type and the auxin-insensitive diageotropica (dgt) mutant revealed that induction of only a subset of genes was affected by the mutation while the other genes retained wild-type response levels in the mutant. This finding indicates that at least two signal transduction chains lead to auxin-induced gene expression, and the dgt gene product acts on only one of them. The expression pattern of nine LeIAA genes was determined in a wide variety of tissue samples in order to link individual LeIAA genes with specific physiological responses. To that end, a qualitative multiplex RT-PCR method was developed that allowed quick assessment of relative expression levels in a large number of small RNA samples. It was found that LeIAA10 expression was highest in tissues undergoing rapid growth, suggesting a role for this gene in the process of cell expansion. The expression of LeIAA1 1 was also variable during development, while LeIAA2 was strongly down-regulated after light treatment. These experiments demonstrated organ-preferential expression of LeIAA genes under the control of environmental factors and depending on developmental stages. The LeIAA genes may thus act as integrators of several other signals with the auxin stimulus.
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