Bx1 benzoxazin1


Function Maize gene for first step in biosynthesis of benzoxazin, which aids in resistance to insect pests, pathogenic fungi and bacteria.
First report Hamilton 1964, as a mutant sensitive to the herbicide atrazine, and lacking benzoxazinoids.
Molecular characterization reveals that the BX1 protein is a homologue to the alpha-subunit of tryptophan synthase. The reference mutant allele has a deletion of about 900 bp, located at the 5'-terminus and comprising sequence upstream of the transcription start site and the first exon. Additional alleles are given by a Mu transposon insertion in the fourth exon and a Ds transposon insertion in the maize inbred line W22 genetic background. Gene sequence diversity analysis has been performed for 281 inbred lines of maize, and the results suggest that bx1 is responsible for much of the natural variation in DIMBOA synthesis. Genetic variation in benzoxazinoid content influences maize resistance to several insect pests.

Map">Genetic map">Map location

AB chromosome translocation analyses place on short arm of chromosome 4. Gene bx1 is 2490 bp from bx2 ; between umc123 and agrc94 on 4S. Mapping probes: SSR p-umc1022 ; Overgo PCO06449.

[Phenotypes]

Mutants are viable, but may be distinguished from normal plants by FeCl3 staining: plants able to synthesize benzoxinoids have pale blue color when crushed and treated with FeCl3 solutions. Mutations in the bx1 gene reduce the resistance to first generation European corn borer that is conferred by benzoxazinoids. Bx1 mutant maize deposited less callose in response to chitosan elicitation than isogenic wildtype plants. Genetic mapping using recombinant inbred lines derived from maize inbred lines B73 and Mo17 showed that a 3.9 kb cis-regulatory element that is located approximately 140 kb upstream of Bx1 causes higher 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one accumulation in Mo17 than in B73 seedlings. This genetic variation is also associated with higher corn leaf aphid reproduction on B73 compared to Mo17 maize seedlings. Relative to maize inbred line W22, Bx1::Ds mutant maize plants are more sensitive to corn leaf aphids and beet armyworms . Highly localized induction of benzoxazinoid accumulation in response to Egyptian cotton leafworm feeding is abolished in a maize bx1 mutant.

Gene Product

Catalyzes the first step in the synthesis of DIMBOA, forming indole from indole-3-glycerol phosphate. The enzyme is called indole-3-glycerol phosphate lyase, chloroplast, EC 4.1.2.8 and is located in the chloroplast. The X-ray structure of BX1 protein has been resolved and compared with bacterial TSA. Three homologs of the BX1 protein occur in maize. One is encoded by the gene tsa1, tryptophan synthase alpha1, on chromosome 7, another by igl1, indole-3-glycerol phosphate lyase1.

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