Langridge, J. and R. D. Brock
A spontaneous, single-gene mutant of the tomato has been shown to be unable to synthesize thiamin. The specific reaction lost is either the methylation of position 2 or the activation of position 5 of the pyrimidine ring.
Plants homozygous for the mutation may survive for several months under good growing conditions, but they grow extremely slowly. The cotyledons are normal in size, shape, and colour, presumably because of the diffusion of metabolites into the seed from the heterozygous maternal parent. The few leaves formed by the mutant are small and at first pale yellow in colour. Later the leaves may become light green, but they soon lose their chlorophyll and turn white except at the main veins which remain green. After the chlorophyll is lost, the leaves wither and die. Frequently the main shoot apex also dies and short secondary shoots develop. Plants after about four months growth are only three to six inches in height, very chlorotic but still alive. There is some variation in phenotype, depending on the time of sowing, but experiments showed that this variation was not correlated with temperature or with light intensity during growth. After mutant plants, which received three treatments with thiamin (2 mg per 100 ml water) per week, had been grown for about four months, they were almost indistinguishable from normal. The plants were nearly normal in size, normal in chlorophyll formation, flowering and seed production. We suggest the symbol tl (thiaminless) for this mutant. Aust. J. Biol. Sci. 1961. (In press).