We are studying plant growth and development using various molecular genetic techniques using Arabidopsis thaliana as the model plant. The major research areas include:
Control of biological surface curvature: Leaf surface is usually flat by coordinated growth, perturbation of which can introduce surface curvature that can be negative with saddle-shaped leaf or positive with cup-like shape. Little is known about the molecular basis that underlies leaf flatness, primarily due to the paucity of mutants with altered surface curvature. To identify novel genes involved in this process, we have performed a forward genetic screen in Arabidopsis and isolated a mutant called tarani (tni) with enlarged, cup-shaped leaf. Morphometric and gene expression analyses showed that the positive curvature of tni leaf is linked to excess cell proliferation and growth at the centre compare to the margin. Excess cell proliferation in tni mutant is also found in other tissues such as embryo, shoot and root meristems. Map-based cloning has identified TNI as ubiquitin protease required to convert poly-ubiquitin chains into monomers.
Divergent polarity of leaf growth: Leaves of model plants show basipetal growth wherein the proximal part grows more due to sustained cell proliferation. Deviation from such stereotypic growth has not been identified so far. By a survey and morphometric analysis of the leaf growth allometry in 75 eudicot species, we show that the polarity of leaf growth along the proximo-distal axis is divergent, and classify these growth patterns into four distinct classes of allometry: positive, negative, mixed and isometric. Expression gradient of the microRNA miR396 and its target GROWTH REGULATING FACTORs (GRFs) strongly correlate with growth allometry. Alteration of the endogenous pattern of miR396 expression in transgenic Arabidopsis leaves show that miR396 can partially modify the spatial pattern of cell differentiation.
Identification of CIN/TCP4 targets: Mutations in the CINCINNATA (CIN)-like TCP genes in Antirrhinum and in Arabidopsis result in crinkly leaves due to excess growth towards leaf margin. Even though it is known that the CIN-homologs code for TCP transcription factors that accelerate cell maturation, their functional basis has remained unclear. We have compared the global transcription profile of wild type and the cin mutant of Antirrhinum to identify CIN targets. We have cloned and studied the direct targets using RNA in situ hybridization, DNA-protein interaction, chromatin- immunoprecipitation and reporter gene analysis. Many of the genes involved in the auxin and cytokinin signaling pathways showed altered expression in the cin mutant. Five CIN-like TCP genes in Arabidopsis – namely TCP2, 3, 4, 10 and 24 - are post-transcriptionally regulated by the micro RNA miR319 and redundantly regulate leaf morphogenesis. To identify their direct targets, we have generated a chemically-inducible version of TCP4 that is resistant to miR319 and expressed it under its endogenous promoter in the jaw-D mutant, where all the miR319-regulated TCP transcripts are maintained at basal level due to the over-expression of miR319. Comparison of transcriptomes of the induced plants with that of the un-induced control identified links between TCP4 function and multiple hormone response pathways. We are currently studying these target genes in more detail.
Regulation of leaf margin architecture: Leaf margins can be smooth, serrated or loabed, and is a major contributor to leaf form and its natural diversity. We have isolated a novel Arabidopsis mutant called tooth (tth) with highly serrated leaf margin compared to the wild type. TOOTH codes for the microRNA miR160 that degrades transcripts of three auxin-related transcription factors ARF10, 16 and 17.
Control of trichome development: Trichomes are hair-like epidermal cells that project from the surface of leafs, petioles, stems, and sepals. Trichomes on Arabidopsis leaf surface are branched structures, the number of which branching varies from two to four. Our recent work shows that TCP4 and TNI regulate trichome density and branching by modulating the transcription of several trichome patterning genes. We are currently studying these genes in greater detail using molecular genetic approach.