Chlorophyll formation and photosynthetic activity in rice mutants with alterations in hydrogenation of the chlorophyll alcohol side chain

The nuclear gene mutants M134 and M249 that accumulate chlorophyllides a and b esterified with incompletely reduced alcohol, such as geranylgeraniol, dihydrogeranylgeraniol, and tetrahydrogeranylgeraniol, were obtained from rice plants (Oryza sativa L.) by treatment of fertilized egg cells with N-methyl-N-nitrosourea. Chlorophyll formation and photosynthetic activity were measured in both greening and fully developed leaves of the mutants and wild-type plants. On illumination of etiolated seedlings, geranylgeranyl- chlorophyll a was the only esterified pigment detectable during the early phase of greening in the leaves of wild-type, M134, and M249. Upon prolonged illumination, M134 showed preferential accumulation of geranylgeranyl- chlorophylls a and b, and M249 accumulated all six intermediates presumed to be involved in the three-step reduction of the chlorophyll alcohol side chain from geranylgeraniol to phytol. The time courses of chlorophyll formation suggest that genetic blocking occurred in reduction of the side chain from geranylgeraniol to dihydrogeranylgeraniol in M134 and the reduction from tetrahydrogeranylgeraniol to phytol in M249. When seedlings were grown under low light conditions, the photosynthetic activity measured as oxygen evolution in fully developed leaves showed no large difference between wild-type and mutant. However, on exposure of seedlings to high light, leaf chlorophyll content decreased gradually, and photosynthetic activity was lost in both mutants. These results suggest that photosynthesis occurs without reduction of the geranylgeraniol side chain to phytol, but the phytol moiety is required to prevent leaves from chlorosis under light-stressed conditions.