45].A complete understanding of how organ identity genes modifythe spatial and temporal expression of other genes, however,will require understanding how transcriptional control overlapswith additional levels of regulation. One of these is chromatinaccessibility, which might determine which genes are availablefor regulation by organ identity genes. One example of overlappingcontrol by organ identity genes and chromatin modificationis NAP. In addition to being activated by AP3/PI [15], NAP expressionalso requires methylation of lysine 4 of histone 3, mediatedby the Trithorax homologues ATX1 and ATX2 [46]. Transcriptionalcontrol can also be filtered through downstream control of RNAprocessing and stability—a regulatory layer that has just begun tobe studied at the global level in flowers [47]. In the long term,all this information will be relevant to simulate and predict thebehaviour of the gene expression networks that underpin floralorgan development, and the first steps in this direction have alreadybeen taken [48]. Quantitative analysis and modelling should alsoreveal how gene expression is translated into localised changes incell behaviour and how these add up to produce organs with differentshapes and functions. The ultimate goal is to understandhow evolutionary changes in gene networks resulted in a dazzlingvariety of flowers built around a conserved core of regulatorygenes.
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