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<p> </p> <p>In the last few years, many genes have been identified that are involved in DZ differentiation and in the associated biochemical and physiological changes prior to cell separation. Genes have been isolated through several different approaches. The identification of mutants both through classical mutagenesis and reverse genetics has led to the molecular cloning of genes that direct DZ cell differentiation (<a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C25">Liljegren<em> et al</em>., 2000</a><em>a</em>; <a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C38">Rajani and Sundaresan, 2001</a>). An alternative strategy with valuable results has been the identification of genes specifically expressed in the DZ. Given the anatomical and physiological specificity of this tissue, genes that are specifically expressed in these cells and not in the valves are likely to be involved in dehiscence. In <em>Brassica</em>, DZ genes have been identified mainly by isolating mRNAs expressed in the valve margins (<a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C6">Coupe<em> et al</em>., 1993</a>, 1994; <a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C36">Petersen<em> et al</em>., 1996</a>; <a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C50">Whitelaw<em> et al</em>., 1999</a>). This direct isolation is more difficult in <em>Arabidopsis</em> due to the smaller size of the silique, but the identification of <em>Arabidopsis</em> sequences related to <em>Brassica</em> DZ genes has revealed that sequence homology often correlates with expression pattern similarity (<a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C21">Jenkins<em> et al</em>., 1999</a>; <a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C42">Sander<em> et al</em>., 2001</a>). In the last few years, different laboratories have generated collections of <em>Arabidopsis</em> enhancer or gene trap lines. These experiments are based on the random insertion of reporter genes in the <em>Arabidopsis</em> genome, so that when inserted in the proximity of an enhancer or within a gene, the expression of the reporter is driven by the neighbouring regulatory sequences (<a href="http://jxb.oxfordjournals.org/cgi/content/full/53/377/2031?ijkey=67854b68c61211f3d1de4e849ef3462a9112006d#ERF082C48">Sundaresan<em> et al</em>., 1995</a>). Several of these lines showing reporter expression in the valve margins have allowed the cloning of new DZ-related genes and the identification of different cell identities within the valve margin. </p> <p>A variety of activities are required for pod shattering to occur. First, early regulators of cell differentiation must act to mediate cell fate specification. Once the different cell types are determined, then several enzymatic activities must work to accomplish the associated processes, such as changes in cell wall composition, lignification, and disintegration of the middle lamella in the separation layer. This programme of differentiation and the downstream enzymatic activities must be tightly regulated; therefore the signalling mechanisms are important to ensure a perfect co-ordination of events. </p>
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