The Regulation of Compound Leaf Development in Tomato

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Sinha9983063The purpose of this research is to understand how compound leaves differ from simple leaves and to elucidate the developmental steps that give rise to this complex leaf form. The starting point of this analysis is the finding that Class 1 homeobox genes like KNOTTED1 (KNOX1 genes) are expressed only in the shoot apical meristem (SAM) and unexpanded shoots, and not in the incipient leaf primordium (P0), of simple leaf producing apices in both dicots and monocots. Over-expression of these genes is sufficient to cause leaf lobing and ectopic meristem formation. However, in compound leaf producing SAMs in tomato, the KNOX1 genes (like LeT6 and TKN1) are expressed both in the SAM and in emerging and developing leaf primordia. This suggests that KNOX1 genes may provide a degree of indeterminacy to the leaf primordia in tomato, thereby leading to an extended stage of morphogenesis and a more complex leaf form. This project will identify the proteins that interact with LeT6 to provide DNA binding specificity and organ specific gene regulation, and the downstream targets of LeT6 regulation. Expression analyses in the tomato leaf mutations suggest that trans-acting factors at the LeT6 promoter, targets of LeT6, and other genes that may act as LeT6 partners in transcriptional regulation would lead to the identification of other key genes in compound leaf development. Preliminary data suggests that LeT6 may regulate the expression of the PHANTASTICA homolog in tomato, which in turn may regulate other KNOX1 genes. The following questions will be addressed. What are the downstream targets of the KNOX1 genes like LeT6? Do the KNOX1 genes act in a complex with other transcription factors (TFs)? How are the levels of KNOX1 genes like LeT6 regulated in tomato shoot apices? What role does PHAN play in the regulation of KNOX1 gene expression in tomato shoot meristems and leaves? Are the expression patterns of PHAN and KNOX1 genes complementary in tomato apices (as shown for simple leaf producing apices)? Does PHAN expression in tomato predict sites of leaflet initiation (as suggested for the simple leaved Antirrhinum and Arabidopsis)? A careful study of KNOX1 genes like LeT6 and TKN1, PHAN, and their regulatory relationships will allow an understanding of the developmental steps that give rise to the complex leaf form in compound leaves.