Both light-dependent protochlorophyllide oxidoreductase A and protochlorophyllide oxidoreductase B are down-regulated in the slender mutant of barley

The gibberellin-insensitive overgrowth mutant of barley, slender, exhibits altered expression of a number of nuclear genes in comparison with the wild type. There is a particularly marked reduction in slender seedlings of tra nscript encoding protochlorophyllide oxidoreductase (POR), the enzyme which catalyses the penultimate and only light-requiring step in chlorophyll bio synthesis. The expression of the two barley genes encoding light-dependent POR, PORA and PORB was investigated. Expression of both genes was found to be reduced in slender seedlings relative to the wild type, in both etiolate d and light-grown leaf tissue; this was most marked in the zone of rapid ce ll extension. Western blot analysis showed that POR protein was also less a bundant in etiolated and in light-grown slender than in the equivalent wild -type leaf tissue, although the effect was less pronounced than at the tran script level. Protochlorophyllide content in etiolated slender seedlings wa s reduced in comparison with wild-type seedlings, though chlorophyll conten t in light-grown leaf blades was unaffected. The reduction in POR expressio n in slender barley may reflect a novel response to the constitutive activa tion of gibberellin signalling in this mutant. Despite the consequences of the mutation for POR gene expression, slender seedlings develop apparently normal chloroplasts in the light, and etioplasts with well-defined prolamel lar bodies when grown in continuous darkness. This suggests that the POR co ntent of wild-type barley seedlings is well in excess of the minimum requir ed for normal plastid development.