ASCORBATE SYSTEM IN PLANT DEVELOPMENT

By using lycorine, a specific inhibitor of ascorbate biosynthesis, it was possible to demonstrate that plant cells consume a high quantity o f ascorbate (AA). The in vivo metabolic reactions utilizing ascorbate are the elimination of H2O2 by ascorbate peroxidase and the hydroxylat ion of proline residues present in the polypeptide chains by means of peptidyl-proline hydroxylase. Ascorbate acts in the cell metabolism as an electron donor, and consequently ascorbate free radical (AFR) is c ontinuously produced. AFR can be reconverted to AA by means of AFR red uctase or can undergo spontaneous disproportion, thus generating dehyd roascorbic acid (DHA). During cell division and cell expansion ascorba te consumption is more or less the same; however, the AA/DHA ratio is 6-10 during cell division and 1-3 during cell expansion. This ratio de pends essentially on the different AFR reductase activity in these cel ls. In meristematic cells AFR reductase is very high, and consequently a large amount of AFR is reduced to AA and a small amount of AFR unde rgoes disproportionation; in expanding cells the AFR reductase activit y is lower, and therefore AFR is massively disproportionated, thus gen erating a large quantity of DHA. Since the transition from cell divisi on to cell expansion is marked by a large drop of AFR reductase activi ty in the ER, it is suggested here that AFR formed in this compartment may be involved in the enlargement of the ER membranes and provacuole acidification. DHA is a toxic compound for the cell metabolism and as such the cell has various strategies to counteract its effects: (i) m eristematic cells, having an elevated AFR reductase, prevent large DHA production, limiting the quantity of AFR undergoing disproportionatio n. (ii) Expanding cells, which contain a lower AFR reductase, are, how ever, provided with a developed vacuolar system and segregate the toxi c DHA in the vacuole. (iii) Chloroplast strategy against DHA toxicity is efficient DHA reduction to AA using GSH as electron donor. This str ategy is usually poorly utilized by the surrounding cytoplasm. DHA red uction does play an important role at one point in the life of the pla nt, that is, during the early stage of seed germination. The dry seed does not store ascorbate, but contains DHA, and several DHA-reducing p roteins are detectable. In this condition, DHA reduction is necessary to form a limited AA pool in the seed for the metabolic requirements o f the beginning of germination. After 30-40 h ascorbate ex novo synthe sis starts, DHA reduction declines until a single isoform remains, as is typical in the roots, stem, and leaves of seedlings. Finally, DHA r ecycling also appears to be important under adverse environmental cond itions and ascorbate deficiency.