Milan University

Identification of new gibberellin sensitive dwarf mutants --G. Todesco, E. Arreghini, D. Ceppi and G. Gavazzi Even though extensive biochemical and genetic work particularly with maize, has contributed to elucidating the sequence of events leading to GA1 (the only gibberellin active per se in the control of shoot elongation) many specific steps are still unresolved. Since the unraveling of the pathway was greatly facilitated by a series of GA mutants lacking either all or specific combinations of endogenous GA, we started a search for mutants with a dwarf phenotype. The aim is to identify new mutants with genetic lesions in steps of the gibberellin pathway different from those already described (Fig. 1).

Figure 1. Genetic blocks in the pathway to GA1 identified in maize.

In maize five GA mutants have been identified, dwarf-1, dwarf-2, dwarf-3, dwarf-5 and anther ear-1, and for four of them the genetic block has been correlated to a specific metabolic step.

We induced seven mutants, all of them recessives, with a dwarf phenotype from the early seedling stage to maturity and resuming normal growth in response to exogenous GA (GA4 + GA7 10-5 M).

The segregation ratios obtained by selfing heterozygous plants together with the origin of each mutant and its provisional symbol are given in Table 1.

Table 1. Origin and segregation of recessive dwarf mutants.
Mutant symbol Origin
dwarf %
d*-2 EMS to seeds
d*-3 EMS to seeds
d*-4 Xrays to pollen
d*-6 EMS to pollen
d*-7 EMS to pollen
d*-8 Xrays to pollen
d*-9 spontaneous [A188xMu]

The seven mutants have been crossed with the known mutants d1, d2, d3, and d5 to find out their allelic relationship to these four defined gene mutants.

The results of this allelism test, accomplished so far for five of the seven mutants isolated, are reported in Table 2. The test was performed by mating each homozygous d* male mutant to at least three progeny plants of selfed +/d heterozygous parents or alternatively by pooling pollen of these progeny plants (at least three) to pollinate plants obtained by selfing +/d heterozygous parents. An average of 10 ears was tested for each specific d/d* genetic combination by germinating a sample of 30 seeds and scoring for dwarf seedlings.

Table 2. Allelism test of five new GA mutants with d1, d2, d3 and d5. The + and - symbols stand for lack of complementation and complementation respectively.

The test discloses allelism of d*-2 with d1 and d*-7 with d3 while d*-3, d*-4 and d*-6 show complementation with all four d mutants. These results suggest that d*-3, d*-4 and d*-6 represent mutational events affecting one or more genes not identifiable with the four d mutants. When crossed inter se, d*-3 with d*-6, d*-3 with d*-4 and d*-4 with d*-6 all the pairwise combinations show complementation, a result expected if the three mutants belong to different genes (results not shown).

However, evidence of nonallelism based on complementation tests could be unreliable for at least two reasons:

(i) the reduced transmission of the d* mutant vs. + as observed in selfing +/d* heterozygotes (see Table 1) might account for the apparent complementation. However, this seems an unlikely explanation since an average test of 10 ears for each combination should be sufficient to recover the appropriate heterozygous combinations to test for allelism.

(ii) molecular evidence is available of situations where complementation takes place even among mutants belonging to the same gene.

More data will be obtained to confirm these preliminary results and to detect the role played by these new gene mutants in GA metabolism.

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