Analysis of new gibberellin sensitive dwarf mutants
--G. Gavazzi, G. Todesco, M. Galbiati and T. Helentjaris

In a previous note (MNL64:86) we presented data regarding the identification of several recessive dwarf mutants and preliminary results on their complementation pattern to the known d mutants, affecting a specific block in the gibberellin (GA) pathway. We have extended these results and completed the complementation test (Table 1).

Table 1. Complementation of new dwarf isolates (d*) with known dwarfs (d). Designations + and - stand for complementation or lack of complementation, as established, for each combination, on the basis of several crosses (from 10 to 25).
 

	d*-2	d*-3	d*-4	d*-6	d*-7	d*-8	d*-9
d1	-	+	+	+	+	+	+
d2	+	+	+	+	+	+	+
d3	+	+	+	+	-	+	+
d5	+	+	+	+	+	+	+

Of the seven new d* isolates, two appear allelic to known d mutants (d*-2 allelic to d1 and d*-7 allelic to d3), while the remaining five identify new genes affecting growth. They all appear responsive to subministration of exogenous GA, applied at a concentration of 10-5M, and they all look alike in terms of plant growth. However, d*-8 is lethal even after prolonged GA subministration, while d*-9 is distinguishable from the others for its reduced pigmentation in leaves and its more stunted growth. We don't yet know whether these five mutants represent different lesions on the pathway to GA, but the RFLP linkage analysis together with the results of inter se complementation seem to indicate that these mutants represent five different genes involved in GA metabolism. Information on linkage and inter se complementation so far obtained is given in Table 2.

Two comments seem pertinent here: 1) The RFLP mapping of d*-3 gave unexpected results. To find segregating polymorphism the mutant was outcrossed to inbred T232 and the F1 selfed. Segregation of the dwarf phenotype in the F2 was surprisingly low, five mutants out of 100 seedlings scored, a segregation expected as a result of duplicate factors. Indeed the results seem to support this hypothesis, since the two regions on 1S and 4L, where positive correlation was found between the mutant and polymorphism, are known to share duplicated genes. 2) Lethality associated with d*-8 is also unexpected since previous work (Phinney, in The Biosynthesis and Metabolism of Plant Hormones, 1984) has proved that genetic lesions in the GA pathway do not cause lethality. The possibility that d*-8 is affecting a very early step in the biosynthesis seems unlikely because such a mutation should be associated with pleiotropic effects that are not observed in homozygous d*-8 seedlings.

Table 2. Genetic information about the new dwarf mutants.
 

Dwarf isolates	Genetic information
d*-2	allelic to d1
d*-3	RFLP mapping: 1S and 4L close to the centromere
d*-4	complementing to the other d* isolates
d*-6	RFLP mapping on 5S
d*-7	allelic to d3
d*-8	complementing to d*-2, d*-3, d*-4, d*-9; lethal at about 15 days after germination
d*-9	complementing to d*-3, d*-4, d*-6, d*-8; TB-A mapping on chromosome 6L

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