Poly-β-hydroxybutyrate (PHB) biosynthesis, tricarboxylic acid activity and PHB content in chickpea (Cicer arietinum L.) root nodule

An experiment was conducted to assess the relationship between poly-β-hydroxybutyrate (PHB) biosynthesis and tricarboxylic acid (TCA) activity in desi and kabuli chickpea (Cicer arietinum L.) genotypes. The specific activities of enzymes of PHB metabolism viz., β-ketothiolase (PHB-A), acetoacetyl coenzyme A reductase (PHB-B) and PHB synthase (PHB-C), and those of tricarboxylic acid cycle (citrate synthase (CS) and malate dehydrogenase (MDH) under symbiosis were measured in bacteroids and compared with the PHB accumulation in the nodule and the root. The significant positive correlation was observed between shoot and nodule mass and PHB-A, PHB-B, and PHB-C activities. However, nodule and shoot weights were not significantly correlated with PHB content either in the roots or nodules. The same was true for PHB levels and citrate synthase activity. MDH activity showed a significant negative correlation with nodule PHB. A marked variation and an age dependant increase in malate dehydrogenase activity were measured. A higher capacity for malate oxidation by an increased MDH is likely alter the balance between malate decarboxylation and oxidation, resulting in a higher steady-state concentration of oxaloacetate and that may favor the utilization of acetyl-CoA in the TCA cycle rather than for the synthesis of PHB.