Shoot apical meristem form and function.
ABSTRACT The shoot apical meristem (SAM) generates above-ground aerial organs throughout the lifespan of higher plants. In order to fulfill this function, the meristem must maintain a balance between the self-renewal of a reservoir of central stem cells and organ initiation from peripheral cells. The activity of the pluripotent stem cell population in the SAM is dynamically controlled by complex, overlapping signaling networks that include the feedback regulation of meristem maintenance genes and the signaling of plant hormones. Organ initiation likewise requires the function of multifactor gene regulatory networks, as well as instructive cues from the plant hormone auxin and reciprocal signals from the shoot meristem. Floral meristems (FMs) are products of the reproductive SAM that sustains a transient stem cell reservoir for flower formation. Regulation of FM activity involves both feedback loops shared with the SAM and floral-specific factors. Recent studies have rapidly advanced our understanding of SAM function by adopting newly developed molecular and computational techniques. These advances are becoming integrated with data from traditional molecular genetics methodologies to develop a framework for understanding the central principles of SAM function.
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ABSTRACT: Auxin signaling through the SCFTIR1-Aux/IAA-ARF pathway is one of the best-studied plant hormone response pathways. Components of this pathway, from receptors through to transcription factors, have been identified and analyzed in detail. Although we understand elementary aspects of how the auxin signal is perceived and leads to a transcriptional response, many questions remain about the in vivo function of the pathway. Two crucial issues are the tissue-specificity of the response, i.e. how distinct cell types can interpret the same auxin signal differently, and the response to a signaling gradient, i.e. how a graded distribution of auxin can elicit distinct expression patterns along its range. Here, we speculate on how signaling through the canonical SCFTIR1-Aux/IAA-ARF pathway may achieve divergent responses.Physiologia Plantarum 11/2013; DOI:10.1111/ppl.12135
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ABSTRACT: Morphogenesis does not just require the correct expression of patterning genes; these genes must induce the precise mechanical changes necessary to produce a new form. Mechanical characterization of plant growth is not new; however, in recent years, new technologies and interdisciplinary collaborations have made it feasible in young tissues such as the shoot apex. Analysis of tissues where active growth and developmental patterning are taking place has revealed biologically significant variability in mechanical properties and has even suggested that mechanical changes in the tissue can feed back to direct morphogenesis. Here, an overview is given of the current understanding of the mechanical dynamics and its influence on cellular and developmental processes in the shoot apex. We are only starting to uncover the mechanical basis of morphogenesis, and many exciting questions remain to be answered.Journal of Experimental Botany 08/2013; DOI:10.1093/jxb/ert199
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ABSTRACT: Leaves are produced postembryonically at the flanks of the shoot apical meristem. Their initiation is induced by a positive feedback loop between auxin and its transporter PIN1. The expression and polarity of PIN1 in the shoot apical meristem is thought to be regulated primarily by auxin concentration and flow. The formation of an auxin maximum in the L1 layer of the meristem is the first sign of leaf initiation and is promptly followed by auxin flow into the inner tissues, formation of the midvein, and appearance of the primordium bulge. The ERECTA family genes (ERfs) encode leucine-rich repeat receptor-like kinases and in Arabidopsis this gene family consists of ERECTA (ER), ERECTA-LIKE 1 (ERL1) and ERL2. Here we show that ERfs regulate auxin transport during leaf initiation. The shoot apical meristem of the er erl1 erl2 triple mutant produces leaf primordia at a significantly reduced rate and with altered phyllotaxy. This phenotype is likely to be due to deficiencies in auxin transport in the shoot apex, as judged by altered expression of PIN1, the auxin reporter DR5rev::GFP, and the auxin inducible genes MP, IAA1, and IAA19. In er erl1 erl2 auxin presumably accumulates in the L1 layer of the meristem, unable to flow into the vasculature of a hypocotyl. Our data demonstrate that ERfs are essential for PIN1 expression in the forming midvein of future leaf primordia and in the vasculature of emerging leaves.Plant physiology 07/2013; DOI:10.1104/pp.113.218198