Opaque-6 allelic to pro1 mutant

The pro1 and o6 mutants have been extensively studied since the first reports (Ma and Nelson, Cereal Chem. 52:412-419, 1975; Gavazzi, Racchi and Tonelli, Theor. Appl. Genet. 46:339-345, 1975). Both mutations affect storage protein synthesis in the endosperm and lead to seedling lethality. Both share a number of phenotypic traits like a dull and collapsed endosperm morphology, stunted seedling growth, striation of the first two leaves and seedling lethality.

Previous studies had shown that the o6 mutation is associated with significant reduction in zein content (observed also in pro1 mutants) as well as with suppression of an endosperm soluble protein, named b32 (Soave et al., Cell 27:403-410, 1981), while pro1 mutant seedlings recover if supplemented with L or D proline and other amino acids are without effect (Racchi et al., Plant Sci. Letts. 13:357-364, 1978; Tonelli, Plant and Cell Physiol. 26:1205-1210, 1985).

The common features shared by the two mutants suggested to us to test the response of o6 seedlings to proline supplementation. The positive result obtained prompted us to test the allelism between pro1 and o6. Plants grown from non mutant seeds obtained upon selfing +/o6 and +/pro1 plants were crossed inter se.

Complementation was then ascertained on the basis of different criteria such as endosperm and leaf morphology, seedling growth and lethality. The o6/pro1 heterozygous combination promoted collapsed endosperm morphology, striation of leaves and conditional seedling lethality, thus indicating functional allelism of the two mutants.

Out of 40 crosses, 24 gave ears with a 3 to 1 segregation for normal and collapsed endosperm seeds yielding mutant seedlings. These results are those expected if pro1 and o6 are functionally allelic.

Other monogenic recessive mutants resembling pro1 in endosperm phenotype have been tested for allelism against pro1; however, none of them (ed-4-v, ed-47-1, 73-1, 118-1 and E-487-c) showed functional allelism with pro1.

Chiara Tonelli, Giuseppe Gavazzi, Lucia Manzocchi, Natale Di Fonzo and Carlo Soave

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