Formaldehyde assimilation through coordination of the glyoxylate pathway and the tricarboxylic acid cycle in broad bean roots.

Formaldehyde (HCHO) assimilation in broad bean (Vicia faba L. cv. YD) roots was investigated using C-labeled HCHO followed by C-NMR analysis. Results revealed that HCHO was first oxidized to HCOOH in the roots treated with 2 mM HCHO in a time-dependent manner. Subsequently, a massive signal peak of [2, 4-C]citrate (Cit) and a signal peak of [2, 3-C]succinate (Su) were observed in accompany with an enhancement in the signal intensity of [3-C]Cit. The data suggested that the glyoxylate pathway and the tricarboxylic acid (TCA) cycle functioned simultaneously in the subsequent assimilation of HCOOH. The yield of [2, 4-C]Cit accounted for more than 80% of the total metabolites. The activity of isocitrate lyase (ICL), a key enzyme in the glyoxylate pathway, was stimulated by HCHO in a dosage-dependent manner. As a result, [2, 4-C]Cit production was increased significantly in YD roots treated with high concentrations (4 and 6 mM) of HCHO. Moreover, induction of the ICL activity by methanol resulted in a simultaneous elevation in the production of [2, 4-C]Cit and [3-C]Cit in methanol-pretreated roots under 2 mM HCHO stress. Pretreatment of roots with cyclosporin A, which hinders the transport of C-enriched compounds into mitochondria, caused a notable decline in the signal peak and yield of [2, 4-C]Cit and consequently induced a notable accumulation of [2, 3-C]Su and an increase in the HCO production (generated from HCOOH oxidation) in HCHO-treated roots. These results suggested that the glyoxylate pathway and the TCA cycle function coordinately in HCHO assimilation in broad bean roots.