Spatial pattern of catalase (Cat2) gene activation in scutella during postgerminative development in maize

The scutellum of maize is a fully differentiated, nondividing, diploid embryonic tissue. Two distinct structural genes (Cat1 and Cat2) encoding the enzyme catalase (CAT) are differentially expressed in this tissue during postgerminative development. As development proceeds, the expression of Cat1 diminishes, while that of Cat2 is enhanced, leading to the disappearance of the CAT-1 protein and the gradual accumulation of the CAT-2 protein. The present investigation was undertaken to determine whether all scutellar cells may be genetically programmed to activate expression of Cat2 synchronously or whether there is an asynchronous spatial gradient of Cat2 activation. By using immunofluorescence microscopy and anti-CAT-2 IgG, we have found that a gradient of Cat2 activation occurs within the scutellar cell mass during postgerminative development. The gradient of Cat2 activation occurs from the outer perimeter of the tissue inward toward the embryonic axis. To determine a potential site of origin for any putative "triggering signal" for Cat2 activation, we demonstrated that Cat2 is expressed in the single layer of aleurone cells prior to its expression in any other tissue during kernel development. To our knowledge, this is the first observation of a gradient-type spatial pattern of a eukaryote gene activation occurring in a stable, virtually nondividing tissue such as the maize scutellum. The significance of these results with respect to developmental gene regulation is discussed.