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 transcript encoding protochlorophyllide oxidoreductase (POR), the enzyme which catalyses the penultimate and only light-requiring step in chlorophyll biosynthesis. 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 etiolated and light-grown leaf tissue; this was most marked in the zone of rapid cell extension. Western blot analysis showed that POR protein was also less abundant in etiolated and in light-grown slender than in the equivalent wild-type leaf tissue, although the effect was less pronounced than at the transcript level. Protochlorophyllide content in etiolated slender seedlings was reduced in comparison with wild-type seedlings, though chlorophyll content in light-grown leaf blades was unaffected. The reduction in POR expression in slender barley may reflect a novel response to the constitutive activation 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 prolamellar bodies when grown in continuous darkness. This suggests that the POR content of wild-type barley seedlings is well in excess of the minimum required for normal plastid development.