Phosphorylation of histone proteins during cell cycle & genomic analyses of the maize shoot apical meristem
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This dissertation contains two distinct sections. The first section characterizes the phosphorylation of the maize centromeric histone H3 variant (CENH3) at Ser50. Phosphorylation of histone H3 occurs at Ser10/28 and Thr3/11, and marks chromosome arms or pericentromeres during cell division. Using molecular and cytological methods, we find that maize CENH3 is efficiently phosphorylated at Ser50 in a prophase-telophase pattern that mirrors known H3 phosphorylation events. These data extend the proposed “histone code” to centromeres. We propose that the primary role of histone H3 phosphorylation is to demarcate distinct chromosomal domains (i.e. centromere, pericentromere, chromosome arm) so that chromosomes can align and segregate properly. CENH3 phosphorylation may function as a component of the anaphase checkpoint, and/or mediate kinetochore maturation. The second section describes the meristem-specific transcript profiling analyses of two maize leaf developmental mutants, narrow sheath1 (ns1) and ragged seedling2 (rgd2). ns mutant plants harbor mutations in the duplicate genes ns1 and ns2, and display narrow leaves due to the loss of a mediolateral domain. The ns genes encode duplicate WUSCHEL1-like homeobOX (WOX) transcription factors that function redundantly and non-cell autonomously to direct recruitment of leaf founder cell-initials in a lateral domain of the shoot apex. Our analyses indicated that genes predicted to be involved in hormonal transport and signaling, signal transduction, and growth are especially implicated during NS-mediated leaf founder cell recruitment and mediolateral axis specification. Moreover, potentially conserved WOX gene functions during the regulation of two component response pathways and of jasmonate-induced gene expression are identified. RGD2 is required to coordinate leaf lateral expansion and dorsiventral patterning. rgd2-R mutant plants display narrow to radial leaves that fail to expand laterally, with no reduction of adaxial or abaxial identity. This peculiar phenotype enables the use of the rgd2-R mutant as a genetic tool to re-examine a widely accepted model for dorsiventral and mediolateral patterning in plants. Transcriptome analyses revealed that genes predicted to be involved in transcription regulation, chromatin remodeling, signal transduction, and proteolysis/protein fate are especially misexpressed in the rgd2-R mutant apex. Our data suggest that RGD2 may function through either the ARP (ASYMMETRIC LEAVES 1/ROUGH SHEATH2/PHANTASTICA) pathway or via a previously undescribed proteolytic pathway to coordinate dorsiventral patterning and mediolateral growth during maize leaf development.