ABSTRACT
Terpenoid aldehyde and naphthofuran phytoalexins formed by different Gossypium spp. vary in amounts of methylation of the C-3 hydroxyl group and methoxylation of the C-7 position of the naphthyl ring system (based on a ring numbering system with the aldehyde group at C-1). Both of these structural variations apparently arise from alternative metabolic routes originating with desoxyhemigossypol (desHG). Methylation of the C-3 hydroxyl of desHG gives rise to desoxyhemigossypol-3-methyl ether (desMHG) which subsequently oxidizes and reacts with other molecules to form as many as eight terpenoid aldehyde derivatives. Oxidation at the C-7 position of desHG followed by methylation of this hydroxyl group and further oxidation of the ring structure gives rise to 7-methoxyhemigossypol (trivialname raimondal). The occurrence of 3-methylation and 7-methoxylation was surveyed among Gossypium species. Large proportions (>50%) of the terpenoid aldehydes were methylated at C-3 in G. barbadense, G. tomentosum, G. sturtianum, and G. longicalvx. Raimondal was a prominent compound only in G. raimondii. The genetic control of these structural variations was determined by making crosses among various species and synthetic interspecific tetraploids and hexaploids. Methylation of the C-3 hydroxyl was controlled by a single recessive gene in G. barbadense, G. tomentosum, and G. longicalvx, but was controlled by a single dominant gene in G. sturtianum. Raimondal production was controlled by a single dominant gene. The C-3 methylating genes of G. barbadense and G. sturtianum have been backcrossed five times into G. hirsutum lines, and give 23-55% and 57-62% methylation of the total terpenoid aldehydes, respectively, in sampled individual progeny. The raimondal gene also has been transferred for five backcrosses and gives 6-48% raimondal in the total terpenoid aldehydes in individual progeny. Selections are being made for homozygous lines that produce consistent amounts of methylated and methoxylated compounds. These may be useful as new sources of pest resistance.
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