Q.80 At root nodules, which of the following C4 organic acid(s) is/are transported across
the symbiosome membrane and into bacteroids?
(A) Succinate
(B) Pyruvate
(C) Malate
(D) Fumarate

Succinate, malate, and fumarate are the primary C4 organic acids transported across the symbiosome membrane into bacteroids in root nodules, while pyruvate is not. These acids serve as key carbon sources for bacteroid respiration during symbiotic nitrogen fixation in legumes.​

Option Analysis

(A) Succinate: Correct. Succinate, a C4 dicarboxylic acid, crosses the symbiosome membrane via dicarboxylate transporters (DCT/OCT) and supports bacteroid energy needs for nitrogen fixation. Mutants defective in succinate transport form ineffective nodules.​

(B) Pyruvate: Incorrect. Pyruvate (C3) is not a C4 organic acid and lacks evidence for symbiosome membrane transport into bacteroids; it may form internally but is not plant-supplied this way.​

(C) Malate: Correct. Malate, the main C4 dicarboxylate, is synthesized via PEPC-mediated CO2 fixation, transported across the symbiosome, and metabolized by bacteroids, often as the primary substrate.​

(D) Fumarate: Correct. Fumarate transports similarly to succinate and malate via DCT systems, providing carbon to bacteroids, though less dominant than malate in some symbioses.​

Answer: A, C, D​

In root nodules of legumes, C4 organic acids transported across the symbiosome membrane into bacteroids play a crucial role in symbiotic nitrogen fixation. These acids, produced by plant phosphoenolpyruvate carboxylase (PEPC), fuel bacteroid respiration as rhizobia convert atmospheric N2 into ammonia.​

Key C4 Acids in Symbiosis

• Malate: Primary substrate; moves from infected cells via symbiosome dicarboxylate transporters (DCT), supporting highest N2 fixation rates.​

• Succinate: Essential C4 dicarboxylic acid; transport mutants yield white, ineffective nodules, confirming its role.​

• Fumarate: Transported alongside others via OCT/DCT proteins; aids bacteroid metabolism.​

Pyruvate (C3) is excluded, as transport focuses on C4 dicarboxylates.​

Transport Mechanism

The symbiosome membrane, derived from plant plasma, hosts DCT systems driving C4 acid uptake into bacteroids using pH/electrical gradients. This exchange sustains energy for nitrogenase.​

Relevance for CSIR NET

Understanding C4 organic acids in root nodules is vital for exams; mutants prove their necessity—malate/succinate/fumarate enable fixation, pyruvate does not.​