--Colasanti, JThe meaning of �vegetatively totipotent�: Maize plants that are homozygous for the grassy tillers mutation (gt1) exhibit a proliferation of small, grass-like shoots from the base of the culm. These tillers seem to form in lieu of normal tillers, although, occasionally, tillers of normal size form as well. I am studying the gt1 mutation because there is a tentative connection between the indeterminate1 mutant (id1) and the gt1 mutant. Mutant id1/id1 plants flower extremely late (or not at all) and they often produce tassels and ears that revert to vegetative growth; i.e., plantlets emerge from within the spikelets of the tassels and the ears form as branches. In addition to the late flowering phenotype, descriptions of the id1 mutant invariably include the following statement: "id1 is vegetatively totipotent with gt1 and factors for perennialism to produce a form of perennialism in maize" (Mutants of Maize, pg. 252, 1997). The origins of this statement can be traced back to studies by D. Shaver (J. Heredity 58:270-273, 1967). In this report he describes a double mutant of id1 and gt1, and then introduces a recessive factor for perennialism (pe1) from teosinte to create a form of perennial maize. The use of the word �perennial� in this instance means that the plants do flower eventually, but growth continues from basal branches indefinitely, under favorable environmental conditions. The key to this perennial behavior is the ability of branches to remain in a state of vegetative growth and continue propagating a vegetative meristem that is not consumed by inflorescence formation and, therefore, will make more shoots.
These traits are notable in the id1gt1 double mutant, which exhibits a sort of synthetic perennialism. Figure 1 shows two nodes (arrows) of an id1gt1 double mutant plant (with leaves removed). The part of the plant shown here is about 10 nodes from the ground. Whereas in normal plants prop roots form on the first or second nodes closest to the ground, in id1 mutants prop root formation expands to the upper nodes of the plant, often reaching to just a few nodes below the tassel. In the double mutant shown in Figure 1, the gt1 phenotype of grassy tillers is observed in every node that forms prop roots, thus the �tufts of grass� formation at each node. In addition, the number of small tillers increases compared to single gt1 mutants. If these �tufts� are allowed to contact the soil, the adventitious prop roots grow out and the small tillers develop into a new, somewhat bushy, plant.
Mapping gt1: The gt1 gene is located on chromosome 1, according to current maps, but its exact location (or even chromosome arm) is unknown. Since id1 is located on the long arm of chromosome 1, very near to bz2, the location of gt1 relative to id1 was tested. An F1 plant carrying mutant alleles of id1 and gt1 in repulsion was selfed and the resulting progeny scored for id1 and gt1 single mutants and id1gt1 double mutants (see Table). The original gt1 mutant allele was obtained from Ben Burr (Brookhaven Laboratory) and the id1 mutant allele, id1-m1, mutant was isolated by transposon tagging (Colasanti J. and Sundaresan V., MNL 65:5, 1995).
As shown in the table, the frequency of id1gt1 double mutants (~ 3%) is somewhat lower than expected for two genes that are not linked (6.25%). This would suggest that gt1 is on the long arm of chromosome 1, about 30 cM from id1. However, the population of plants examined here is too small to make a definite conclusion. Further, it does not tell us whether gt1 is proximal (which would put it very near the centromere) or distal to id1. Experiments are in progress to refine the map position of gt1.
Preliminary tagging experiment: An initial experiment to isolate the gt1 gene by Mu transposon tagging was attempted. Homozygous gt1 mutant plants were crossed as pollen parents to Mu active plants, and the F1 progeny were screened the next summer in Davis CA. From a total of about 35,000 F1 seeds planted, no plants with a clearly identifiable grassy tiller phenotype were found. The number screened might be too low to guarantee a tagged allele, but this initial experiment did reveal one problem with this screen. Specifically, the F1 plants had a large number of normal large tillers that made it difficult to identify the small tillers of the gt1 mutants.
To get around this problem, the gt1 allele was introgressed into a Mo17 inbred line that shows very little tillering in the field. (The Mu lines are already in a low-tillering background). However, and perhaps not surprisingly, the more the gt1 mutation was introgressed into the Mo17 background (after 5 backcrosses), the more difficult it became to score the grassy tiller phenotype. If nothing else, this finding suggests that the grassy tillers of gt1 mutants are a variation of normal tillers and are subject to the same developmental controls. It might be possible to proceed with this tagging by using one of the less introgressed lines.
A connection between grassy tillers and silky tassels: By putting gt1 in an id1 mutant background, the developmental abnormalities caused by the loss of gt1 function were amplified and more obvious. That is, in the double mutant it is clear that the grassy tillers emerging from each node are the result of uncontrolled proliferation of meristems at the base of each shoot (Fig. 1). In the gt1 single mutation, this tiller upon tiller proliferation is present but is not as conspicuous.
One other characteristic that seems to be associated with the gt1 mutant allele is the presence of a silky tassel phenotype. I have noticed that nearly all gt1 homozygous plants are associated with silks emerging from the tassels. These silky tassels resemble the tassels of tillers, which often undergo feminization. Of course it is possible that the silky tassel trait is caused by another mutation that is simply linked to gt1. However it is interesting to note that, in the Mo17 introgressions described above, the increased difficulty of identifying the grassy tiller phenotype was accompanied by a parallel reduction of silk formation in gt1 tassels. Only one gt1 mutant allele has been available so far; therefore, characterization of other gt1 alleles could clarify the relationship between the formation of grassy tillers and silky tassels.
If both traits are in fact the result of a lesion in the same gene, it might give clues about how the gt1 gene functions. The profusion of grassy shoots and the growth of silks from the tassel could be traits associated with unrestrained proliferation, as is evident of gt1 in the id1 background. Is it possible that the gt1 mutation is a manifestation of reduced apical dominance? This could explain the excess proliferation of small tillers at the base of the culm. Further, tassel feminization might indicate that the central stalk of the plant is developing characteristics of an axillary tiller.
TABLE
id1 Gt1+/Id1+gt1
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| Total # plants | Normal | id1/id1 | gt1/gt1 | id1/id1 gt1/gt1 |
| 188 | 93 | 48 | 41 | 6 |
| Expected (unlinked) | 106 | 35 | 35 | 12 |
| Expected (very closely linked) | 94 | 47 | 47 | 0 |
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