Ascorbate system in plant development

Abstract

By using lycorine, a specific inhibitor of ascorbate biosynthesis, it was possible to demonstrate that plant cells consume a high quantity of ascorbate (AA). The in vivo metabolic reactions utilizing ascorbate are the elimination of H2O2 by ascorbate peroxidase and the hydroxylation 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 continuously produced. AFR can be reconverted to AA by means of AFR reductase or can undergo spontaneous disproportion, thus generating dehydroascorbic acid (DHA). During cell division and cell expansion ascorbate 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 depends essentially on the different AFR reductase activity in these cells. 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 undergoes disproportionation; in expanding cells the AFR reductase activity is lower, and therefore AFR is massively disproportionated, thus generating a large quantity of DHA. Since the transition from cell division to cell expansion is marked by a large drop of AFR reductase activity 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) meristematic cells, having an elevated AFR reductase, prevent large DHA production, limiting the quantity of AFR undergoing disproportionation (ii) Expanding cells, which contain a lower AFR reductase, are, however, provided with a developed vacuolar system and segregate the toxic DHA in the vacuole. (iii) Chloroplast strategy against DHA toxicity is efficient DHA reduction to AA using GSH as electron donor. This strategy is usually poorly utilized by the surrounding cytoplasm. DHA reduction does play an important role at one point in the life of the plant, that is, during the early stage of seed germination. The dry seed does not store ascorbate, but contains DHA, and several DHA-reducing proteins are detectable. In this condition, DHA reduction is necessary to form a limited AA pool in the seed for the metabolic requirements of the beginning of germination. After 30-40 h ascorbate ex novo synthesis starts, DHA reduction declines until a single isoform remains, as is typical in the roots, stem, and leaves of seedlings.(ABSTRACT TRUNCATED AT 400 WORDS)

Quiescent center formation in maize roots is associated with an auxin-regulated oxidizing environment

“…But more dramatically, the levels of oxidized form (DHA) are 1000× greater in the QC than in the adjacent PM (Table 2). High levels of DHA in the QC are particularly noteworthy, since DHA accumulation is generally considered a negative event for cell metabolism (Arrigoni, 1994). Thus, in rapidly dividing cells of the PM, not only is AA higher than in the QC, but DHA is 1000× lower.…”

Auxin Metabolism in the Root Apical Meristem

“…9). While it is widely known that AAO is present in most if not all higher plants, its regulation and biological function are not clearly defined (Arrigoni, 1994;Có rdoba and González-Reyes, 1994;Smirnoff, 1996). That ascorbate is a substrate in vivo for AAO is highly probable (Avigliano and Finazzi-Agro, 1997).…”

Probing the location of the substrate binding site of ascorbate oxidase near type 1 copper: an investigation through spectroscopic, inhibition and docking studies

“…During activity, the blue copper oxidases couple the four-electron reduction of dioxygen to water with one-electron oxidation of four molecules of the substrate. For AO, the fastest and possibly physiological substrate is l-ascorbate (Kroneck, Armstrong, Merkle, & Marchesini, 1982;Avigliano & Finazzo-Agrò, 1997), which in the common plant sources of the enzyme is involved in cell processes associated with plant growth, protection and development (Arrigoni, 1994;Cordoba & Gonzales-Reyes, 1994;Wheeler, Jones, & Smirnoff, 1998). The complex mechanism through which the enzyme catalysis proceeds is far from being understood (Solomon et al, 1996;Zaitseva et al, 1996;Casella, Monzani, Santagostini, De Gioia, Gullotti, Fantucci, Beringhelli, & Marchesini, 1999).…”