INFLUENCE OF CARBON SOURCE AND CO2-ENRICHMENT ON BIOCHEMICAL PARAMETERS ASSOCIATED WITH PHOTOMIXOTROPHIA IN MAIZE CALLUS-CULTURES

To learn about the biochemical processes underlying the induction of p hotomixotrophia in maize cell culture, maize calli were cultured in me dium containing either glucose or starch as the carbon source. The eff ect of a CO2-enriched atmosphere on different parameters was tested. L evels of chlorophyll and CO2 fixing enzymes were measured to assess th e greening process concomitant to histological observations of chlorop last development. Both starch and glucose promoted higher chlorophyll accumulation in callus cultured under light than sucrose. Histological analysis of green callus grown on glucose-containing medium revealed the formation of poorly developed chloroplasts containing starch grain s, whereas in starch medium a large number of elongated chloroplasts c ontaining thylakoids were observed. Exposure of these calli to a CO2-e nriched atmosphere enhanced the plastid differentiation process up to mature chloroplasts with grana and intergranal thylakoids. Western-blo t analysis demonstrated the presence of CO2-fixing enzymes, Rubisco (E C 4.1.1.39) and PEP carboxylase (EC 4.1.1.31), as well as Rubisco acti vase in greening callus. Rubisco and PEP carboxylase activities showed large values when starch was me carbon source in the medium. Results of histological analysis and a/b chlorophyll ratios indicated that the chloroplasts formed were of the C-3-type. PEP carboxylase kinetic pro perties were also consistent with a C-3-type enzyme involved in anaple rotic functions. It is concluded that under the experimental condition s tested, starch plus CO2-enriched atmosphere are the best carbon sour ce for inducing and supporting photomixotrophia in maize cultures, as indicated by several biochemical parameters.