Tp1 is a dominant homeotic mutation whose most conspicuous phenotype is the production of leaves or leaf-like structures in the ear and the tassel. This phenotype could result from a defect in supra-cellular factor(s) that initiate or suppress leaf development, or a defect in the ability to respond to such factor(s). Mutations that affect these two systems can be differentiated by examining their expression in genetic mosaics. Mutations that affect supra-cellular (i.e., diffusible) factors are likely to be non-cell-autonomous, whereas those that affect the ability of cells to respond to such factors are likely to be expressed in a cell-autonomous fashion.

Seeds of the genotype + Tp1/o5-1241 + were irradiated (1 Krad, 250 KV, 15 mA, 2 mm Al) 24 hours after imbibition, grown to maturity, and then screened for sectors expressing the pale yellow phenotype of o5-1241. The o5 locus is located about 10 map units proximal to Tp1. Thus, cells expressing o5-1241 have a high probability of being hemizygous for the wild type allele of Tp1 because terminal deletions that result in the loss of o5+ will also remove Tp1. Although some sectors may result from interstitial deletions that remove o5+ but not Tp1, such deletions require 2 chromosome breaks and are therefore much less frequent than terminal deletions, which arise from single breaks.

Seventeen plants with o5-1241 tassel sectors, 9 plants with ear sectors and 15 plants with tiller sectors were found in a total of 1660 plants. Sectors only encompassed about 1/20 to 1/10 of the circumference of the tassel, but generally occupied a much larger fraction, if not the entire circumference, of an ear or tiller. All but 2 of the tassel sectors extended from the tassel into the vegetative part of the shoot. Tiller and ear sectors generally encompassed all the nodes in these structures.

The feature of primary interest in this experiment was the phenotype of sectors in the tassel and ear, in particular the presence or absence of spathes subtending spikelets. With one exception, all of the sectors in either of these inflorescences had a Tp1 phenotype indistinguishable from that of surrounding tissue. In the exceptional case, spikelets within the sector were normal in appearance (i.e., lacked spathes). However, because other non-sectored regions of the tassel also had a normal phenotype, it is unclear whether to attribute the phenotype of this o5-1241 sector to its Tp1+ genotype, or to the poor expression of Tp1 in surrounding tissue.

Twenty-two of the sectors we observed were located in the L1 layer of the meristem and 15 were located in the L2 layer. The location of the remaining 4 sectors was not determined. Although cells from one of these lineages may replace cells from the other lineage, this usually only occurs during leaf development, not during the growth of the shoot meristem. Thus most of the structures containing o5-1241 sectors also possessed an epidermis or some sub-epidermal tissue that still carried Tp1. In 2 cases, however, spathes arose in regions of the shoot in which o5-1241 cells in the L1 had displaced L2 cells so that all the tissue in these spathes was genetically o5-1241.

These results strongly suggest that Tp1 is not expressed in a cell-autonomous fashion because wild type cells in proximity to Tp1 cells can be induced to form mutant structures. What is unclear is the distance over which this induction operates. The fact that Tp1 cells in the epidermis are capable of recruiting Tp1+ cells in the sub-epidermis, and that Tp1 cells in one half of a spathe can recruit Tp1+ cells to form the other half demonstrates that this effect extends to at least the boundaries of an organ. The 2 cases involving larger sectors of o5-1241 tissue suggest that Tp1 can operate over even greater distances, but we have not observed enough of these sectors to be confident about this conclusion.

Scott Poethig
 
 

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