Developmental mutants are a useful tool for dissecting development and understanding the genetic program underlying this process. We focused our attention on seed development by analyzing recessive mutants with defective seed morphology acting as lethals at the seed or seedling stage. The survey of these mutants is aimed at establishing their involvement in embryo development and, possibly, the embryogenic phase affected.
The preliminary data gathered so far are presented in the table below. The mutants analyzed, 13 in total, appear heterogeneous. They all show more or less drastic alterations of the endosperm, while the embryo is either normal but reduced in size, or arrested at some early developmental stage, or not recognizable at the time of observation. This heterogeneity is similar to that reported by Sheridan and Neuffer (J. Hered. 73:318-329, 1982) in other defective kernel mutants.
Some of these mutants have also been cultured as immature embryos, starting 20 days after pollination up to 50 days, on mineral and enriched RM media. A detailed presentation of the results will be given elsewhere. Here only mutants 7 1, 27 1 and 48 1 are considered. They all germinate and yield normal looking seedlings when cultured as immature embryos on both mineral and enriched media, while as mature embryos they exhibit a significantly lower germination capacity and reduced (7 1 and 48 1) or suppressed growth (27 1).
They can be further differentiated on the basis of their growth rate as excised shoot tips. In fact, after seven days of culture in liquid media, the length of seedlings is 80 mm for mutant 7 1, 21 mm for 27 1, and 8 mm for 48 1, respectively (control seedling length = 106 mm).
The recovery of normal seedlings by culturing immature embryos is the result expected from phase-specific lethal mutants, since lethality occurring in advanced stages of embryogenesis is bypassed by inducing precocious germination.
An unexpected result is germination on filter paper, even though incomplete, of mutants originally isolated as absolute lethals when planted in sand benches. Germination in these mutants varies from 24% for 55 1 to 93% for G33; seedlings are ostensibly retarded in their growth compared to normal siblings. At the time these observations were made (20 days after germination), part of the seedlings are necrotic, while the remaining are still alive but likely to die.
It might well be that this behavior is the result of hormone imbalance. It is hoped that further histological characterization of these mutants and an analysis of their metabolism requirements will allow an understanding of the basis of their lethality, as well as the role played by both embryo and endosperm in seed development.
S. Faccio Dolfini, G. Gavazzi and G. Todesco
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