in the homozygous b1 b2 double mutant both beta-hydroxylase activities are eliminated, but beta-ring-hydroxylated xanthophylls (L, V, and N) still are synthesized, albeit at lower levels than in ecotype Wassilewskija or in either single mutant. Xanthophylls N and V are reduced by 90 and 65%, respectively, and there is a 30% increase in xanthophyll L relative to ecotype Wassilewskija. As with the single beta-hydroxylase mutants, beta-carotene is unchanged relative to the wild type and beta-cryptoxanthin does not accumulate
mis-expressing of the individual hydroxylases can impact carotenoids metabolic flux, with the effect of AtB1 over-expression on beta-carotene metabolism being the most pronounced
the homozygous b1/b2 double mutant does not show obvious growth defects and though beta-carotene-derived xanthophylls are reduced to 20% of wild type levels (much more than either b1 or b2 single mutant), the full complement of alpha- and beta-carotene-derived xanthophylls is still synthesized, albeit beta-carotene-derived xanthophylls are produced at lower levels in the double mutant
the beta-carotene hydroxylase gene DSM2 confers drought and oxidative stress resistance by increasing xanthophylls and abscisic acid synthesis in rice. DSM2 contributes to photosynthetic efficiency and NPQ capacity in rice