Biosynthesis of peroxidases: a role as interplay between primary and hormonal metabolisms

Hyperhydric, fully habituated (growth independent from exogenous auxin and cytokinin), fully heterotrophic (achlorophyllous, dependent on sucrose supply, nevertheless able to fix CO2 non-photosynthetically) and non-organogenic (complete loss of any organogenic totipotency) sugarbeet cells in culture were characterized by a very low peroxidase activity, among other deficient heme compounds. This deficiency resulted from the lack of synthesis of the precursor aminolevulinic acid (ALA) through the chloroplastic Beale pathway; the low productive unusual ALA synthesis through the mitochondrial Shemin pathway (commonly used by animals and fungi) functioned but further at a limited rate due to inhibition of ALA-dehydratase by benzoic derivatives, predominant among the phenolic acids of these cells (compared to normal ones). A thorough investigation of the metabolic and hormonal functioning of these neoplastic cells showed that the above deviation originated from a disturbed nitrogen metabolism that diverted glutamate from the Kreb's cycle into polyamine (over) synthesis, which had also as consequence a low ethylene production. The Kreb's cycle could be replenished by oxaloacetate and malate deriving from the anaplerotic fixation of CO2 onto phosphoenolpyruvate. A privileged pentose phosphate pathway (PPP) allowed the formation of substrates for a non-limited biosynthesis of (endogenous) auxins and cytokinins. The PPP, through NAD(P)H formation, enhanced nitrogen metabolism, but also, together with a putative H2O2 accumulation (originated from high superoxide dismutase activity and high polyamine oxidation combined with low catalase activity), favoured the alternative respiratory pathway. These results not only illustrate a novel view of integration of hormonal metabolisms with the C and N primary and secondary ones but also allow to consider the biosynthesis of peroxidases in an interplay or mediating role between them.