The histone acetyltransferase GCN5 and associated transcriptional coactivator ADA2b are required to couple endoreduplication and trichome branching. Mutation of ADA2b also disrupts the relationship between ploidy and leaf cell size. Dynamic chromatin structure has been established as a general mechanism by which gene function is temporally and spatially regulated, but specific chromatin modifier function is less well understood. To address this question, we have investigated the role of the histone acetyltransferase GCN5 and the associated coactivator ADA2b in developmental events in Arabidopsis thaliana. Arabidopsis plants with T-DNA insertions in GCN5 (also known as HAG1) or ADA2b (also known as PROPORZ1) display pleiotropic phenotypes including dwarfism and floral defects affecting fertility. We undertook a detailed characterization of gcn5 and ada2b phenotypic effects in rosette leaves and trichomes to establish a role for epigenetic control in these developmental processes. ADA2b and GCN5 play specific roles in leaf tissue, affecting cell growth and division in rosette leaves often in complex and even opposite directions. Leaves of gcn5 plants display overall reduced ploidy levels, while ada2b-1 leaves show increased ploidy. Endoreduplication leading to increased ploidy is also known to contribute to normal trichome morphogenesis. We demonstrate that gcn5 and ada2b mutants display alterations in the number and patterning of trichome branches, with ada2b-1 and gcn5-1 trichomes being significantly less branched, while gcn5-6 trichomes show increased branching. Elongation of the trichome stalk and branches also vary in different mutant backgrounds, with stalk length having an inverse relationship with branch number. Taken together, our data indicate that, in Arabidopsis, leaves and trichomes ADA2b and GCN5 are required to couple nuclear content with cell growth and morphogenesis.

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While genome sequencing projects have helped us uncover what information is encoded in DNA of many species, one critical challenge that remains is to understand how gene function is controlled. We investigate the roles of chromatin‐modifying factors in regulating developmental processes in Arabidopsis thaliana. Our current work explores how the histone acetyltransferase GCN5 and the associated transcriptional adaptor ADA2b act to define which leaf cells are selected to become trichomes, single‐celled protrusions implicated in defense mechanisms. In a loss‐of‐function ada2b mutant background, trichome number on rosette leaves is decreased while in gcn5 mutant backgrounds the number of trichomes tends to increase. Using qRT‐PCR analysis, we have established a role for GCN5 in impacting the expression of key players in the trichome initiation pathway including the paralogs Repressor of GA (RGA) and GA Insensitive (GAI). Gibberellic acid (GA) signaling stimulates trichome initiation by blocking the DELLA repressors RGA and GAI, which in turn block a transcription factor cascade that controls trichome development. This approach has the potential to identify novel connections between GA, a critical plant hormone, and histone acetylation in plant growth. It also provides an example of how chromatin modifiers and transcription factors interact to drive developmental processes.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.