More studies on tassel plants of maize
--Ting, YC; Nguyen, DQ

In the winter of 1996, seven plants propagated through cloning were transplanted to the greenhouse. Under natural light illumination, and within a temperature range of 60 F (night) to 80 F (day), all of them grew into adult plants. Tillers and ear shoots were as many as when they grew in the open field. Five months later, only one plant produced plantlets on its tassel. This plant had delayed senescence in growth, and the leaves were thicker and dark green. In other words, the whole plants appeared more vigorous than all the rest of its sibs. The pistillate inflorescences were well developed and fertile. Each ear bore more than 100 kernels upon backcrossing to the hybrid F1s.

One of five plants in the selfed progeny of the tassel plant also produced plantlets, but light green in plant and leaf color. The plant had reduced vigor and the plantlets sprouted out only on the central spike of the tassel.< p> Since the same plantlets reappeared in the selfed progeny it is tenable to say that this characteristic is genetically controlled and under suitable conditions it would be transmitted to the next generation, even though the frequency of transmission was very low, 20 percent expression. None of the plants from the cross-fertilized progeny, a total of seven, initiated any plantlets on their tassels. These plants had on average three tillers, both staminate and pistillate florets were fertile, each ear bearing more than 50 kernels. The inflorescences of three plants in the crossed progeny were well developed and subsequently they were collected and fixed for meiotic observations. It was found that they were all tetraploid. Pachytene chromosomes were well-spread and stained intensely. Likewise, 12 plants of the backcrossed progeny, 95-clone x 95 tp x 4n perennial maize, survived transplanting in the field. Six of the plants were of short stature and they had reduced vigor and barren stalks. The stalks had only a few antheses. No pollen sheds were observed. They seemed completely infertile. The other six plants of the same progeny grew into normal height. Both male and female flowers were highly fertile, but having a small number of ears per plant. None of the above plants regenerated any tassel plants in the tassels. The segregation ratio between short and tall plants was 6:6 or 1:1.

Seven plants (plantlets), propagated asexually (cloning) in the greenhouse, were also transplanted to the field. When they were about 3-inches in height, three of them converted into pistillate inflorescences with well developed silks. In consequence of this, all of them were pollinated with haploid pollen. None of them set any seeds. Then, these small plants (plantlets) soon senesced. The other four plantlets of the same pedigree grew normally into adult plants. Their male and female flowers were fertile. On average, each plant had three to four tillers. However, none of their tassels grew out plantlets.

In summary, the characteristic growth and their transmission of the tassel plants suggest the presence of a mutation. The expression of this mutation is regulated by, perhaps, a gene similar to det-2 in Arabidopsis. When this gene exists as a homozygote, it causes reduced fertility and aborted anthers. The senescence of chloroplasts, leaves and flowers is accordingly delayed. In addition, this mutation also blocks the biosynthesis of a hormone, brassinosteroid, which is required for development and light-regulated gene expression. In the last summer, among a total of 25 progeny plants of tp derived from cross-fertilization, backcrossed, as well as cloning propagated, none of them regenerated into tassel plants (plantlets) on the tassels. It was postulated that in the summer, the natural light intensity was high. Thus, it might turn on the mutation (det-2 like), inhibiting the production of the hormone, brassinosteroid. The hormone-deficiency plants were unable to respond to light signals to call for tp expression. It is known that brassinosteroid may play an important role in regulation of gene expression of plants by responding to light signals. In the winter, the same progeny plants grown in the greenhouse without any artificial lights, synthesized adequate amounts of brassinosteroid and demonstrated the capacity to sprout out plantlets on the tassels. On average the daily light intensity during the winter in the greenhouse is much lower than that in the field during the summer. However, this explanation of light regulation of the expression of the mutant, tp, of perennial maize is tentative. The exact answer to the question of how tp is expressed has yet to be elucidated in the future.

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