Traditionally, the development of the aerial shoot has been defined by the ability of the plant to flower. In most cases, the transition from the non-flowering, juvenile to the adult phase, in which flowering is possible, cannot be determined with certainty until the plant actually produces flowers. Changes in the vegetative morphology often occur concomitantly with the transition to reproductive maturity and have been used as indicators of this transition; yet, conclusive proof of such correlation is lacking. In order to investigate the relationship between the changes in vegetative and reproductive development, we examined the expression of traits indicative of each phase in early and late flowering lines of the inbred A632. The early flowering line A632E was derived by Don Shaver by introgressing genes from Gaspe Flint. Our preliminary observations suggest that the early flowering phenotype of this line is conditioned by no more than two semidominant genes. The late flowering line carried the dominant mutation Lfy1, also obtained from Don Shaver. In maize, the presence of epicuticular wax and epidermal hairs is specific to the juvenile and adult vegetative phases, respectively, while the total number of leaves produced by the plant correlates with flowering time. Table 1 shows that both lines modify the total leaf number without affecting the expression of the vegetative markers. This suggests that the transition to reproductive maturity and changes in the vegetative development proceed independently of each other.
Table 1. Effect of Lfy1 and early flowering on placement of vegetative
and reproductive organs in A632.
|Genotype||Last leaf with wax||First leaf with hairs||Ear node||No. of husk leaves||No. of leaves between ear and tassel||Total number of leaves|
|+/+, +/+||10.0±0.1||4.7±0.2||14.0±0.2||9. 8±0.7||6.1±0.0||20.0±0.2|
|Lfy1/+, E/+||9. 6±0.3||4.9±0.3||14.1±0.5||11.9±0.8||13.8±1.0||27.7±1.3|
|+/+, E/+||9. 3±0.3||4.9±0.2||12.6±0.4||9.6±0.9||5.8±0.3||18.3±0.5|
|Lfy1/+, E/E||9. 7±0.2||5.4±0.3||12.1±0.3||10.6±0.5||10.5±0.5||22.5±0.6|
|+/+, E/E||9. 5±0.3||5.3±0.3||10.8±0.4||9.5±0.7||5. 3±0.2||16.1±0.4|
Additionally, we were interested in the effect of these mutations on the relative positioning of the reproductive organs. Most inbreds produce five to six leaves between the ear and the tassel, regardless of the number of leaves generated by the plant overall. This constancy has led to the idea that the placement of the reproductive organs relative to each other is coordinately regulated (W.C. Galinat, in Corn and Corn Improvement, p. 22, 1988). Such developmental regulation could involve one organ determining the position of the other. Alternatively, coordination could be accomplished by one reproductive stimulus determining the placement of both organs. In the latter scenario the reproductive stimulus would cause the plant to produce two independent shoots, one terminating in a female inflorescence (the ear), the other one in a male inflorescence (the tassel). The number of leaves on both shoots would thus be correlated as long as there is no change in an organ-specific sensitivity to such a stimulus.
To test these possibilities we studied the placement of the ear and the tassel in the lines described above. Our results confirm previously published findings (D. L. Shaver, Proc. Ann. Corn Sorghum Res. Conf. 38:161-180). In A632E the ear node and the total leaf number is reduced without a significant reduction in the number of husk leaves and only a small but significant decrease in the distance between the ear and the tassel. This is consistent with a role of the early flowering factor in A632E in a general reproductive program, to which all floral organs respond equally. In contrast, the principal effect of Lfy1 is to increase the number of leaves between the ear and the tassel. The effect on the position of the ear and on the number of husk leaves is small but significant. Thus, in Lfy plants all lateral buds develop normally, whereas the apical meristem remains vegetative longer than in wildtype. This makes it unlikely that the ear position is determined by the tassel. Plants carrying both the Lfy1 and early flowering traits display an additive phenotype, suggesting that these genes act in different developmental pathways.
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