b-Glucosidase activity in seedling tissues is reduced by exposure to light

--Carla Frova

b-glucosidase is an enzyme present in several seedling and adult plant tissues. In dark grown seedlings the strongest activity was found in coleoptiles and mesocotyls (Frova, C, unpublished). However, accidental exposure to light resulted in a reduction of the enzymatic activity in both tissues. Here the results of a first analysis of this phenomenon are reported.

Genetic variability of the response to light. Seeds of 10 inbred lines and 3 F1s were germinated for 24h in the dark and thereafter either kept in the dark or exposed to continuous white light. Five days after germination coleoptiles and mesocotyls were collected from the seedlings, extracted and analyzed spectrophotometrically (Esen and Cockmus, Biochem. Genet. 28:319-336, 1990) for b-glucosidase activity. Exposure to light resulted in reduction of enzymatic activity in all cases in both tissues, but pronounced differences between genotypes were found. In particular, variability in reduction was larger in coleoptiles (10-85%) than in mesocotyls (25-65%).

Among the genotypes tested B77 showed the strongest response to light: 85% reduction in the coleoptile and 55% in the mesocotyl. This line was therefore used for further analyses.

Effect of light pulses. In order to test i) whether the light inhibitory effect is transient or irreversible, and ii) what seedling developmental stages are most responsive to the treatment, seeds or seedlings were exposed to white light for 24h at different times during seedling development (0-24, 24-48, 48-72, 72-96, 96-120h after imbibition) and then put back in the dark until day 8. b-Glu activity was measured each day in each treatment. The data show that light is totally ineffective when given in the first 48 hours. Subsequent pulses, 48-72, 72-96, 96-120, drastically reduce b-glucosidase activity, which in no case is restored upon resumption of the dark condition. By day 7 the enzymatic activity declines to low levels also in the dark controls in both tissues tested.

Shorter exposures, 12h pulses, yield a similar effect, although enzymatic inhibition is somewhat weaker.

Effect of different light qualities. The seedlings were germinated in the dark and exposed for 24h, between 48 and 72h after germination, to white, red, far red and blue light. In all cases an inhibitory effect was detected. No significant differences between light qualities were observed. The reduction in b-glu activity was nearly 85% in coleoptiles and 55% in mesocotyls in each treatment. These data suggest the possible implication of more than one photoreceptor in the response.

Experiments to test the effect of different light intensities in both seedling and adult plant tissues are underway, and crosses for a genetic analysis of the phenomenon have been planned.

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