Degradation of 2,4-D by plant growth-promoting Cupriavidus sp. DSPFs: role in mitigating herbicide toxicity in soil and enhancing crop production.

Repeated application of herbicides like 2,4-dichlorophenoxyacetic acid (2,4-D) in agriculture poses a significant environmental threat and health risk to non-target biota. Cupriavidus sp. strain DSPFs, isolated from Indian agricultural soil, utilizes 2,4-D and various other aromatics as the sole source of carbon and energy. It efficiently degrades high concentrations of 2,4-D (up to 0.3% wt/vol, i.e., 3,000 ppm) within 24 h with a maximum degradation rate of 105 mg L^-1 h^-1. Based on enzyme activity, whole-cell oxygen uptake, high-performance liquid chromatography (HPLC), and liquid chromatography-mass spectroscopy (LC-MS/MS) analyses, the strain was proposed to metabolize 2,4-D via 2,4-dichlorophenol (2,4-DCP) and 3,5-dichlorocatechol ortho ring-cleavage pathway. The key enzyme, 2,4-DCP monooxygenase (24DCPM), was purified to homogeneity and found to be homotetrameric with a native molecular mass of ~255 kDa with ~3.9 mol of FAD per mol of native protein. The enzyme exhibited high catalytic efficiency (K_cat/K_m) of 4.5 and 6.6 µM^-1 s^-1 with NADH and NADPH, respectively. Strain also displayed various plant growth-promoting traits like production of indoleacetic acid, ammonia, siderophores and mineral solubilization. Priming of Vigna radiata (mung bean) seeds with strain enhanced (30-47%) growth of seedlings in non-contaminated soil microcosms, indicating plant growth-promoting potential of the strain. Seed priming with strain significantly reduced 2,4-D toxicity to mung bean seedlings, suggesting the phytoprotective abilities of the strain. Bio-augmentation of 2,4-D-contaminated soil with strain DSPFs could remove >95% of 2,4-D (100 ppm) in 4 days with a degradation rate of 91 mg kg^-1 day^-1. Cupriavidus sp. DSPFs is a promising candidate for the eco-friendly clean-up of agricultural fields and enhancing crop productivity.

Importance: An agricultural soil isolate, Cupriavidus sp. strain DSPFs, is capable of degrading 2,4-dichlorophenoxyacetic acid (2,4-D), a widely used herbicide having deleterious effects on non-target crop plants and other biota. Strain DSPFs efficiently degrade relatively high concentrations of 2,4-D in minimal growth medium as well as in contaminated soil. This efficient degradation by strain can be attributed to the enhanced catalytic efficiency (low K_m and high V_max) of key enzyme 2,4-dichlorophenol-6-monooxygenase (2,4-DCPM). Strain DSPFs mitigate the toxicity caused by 2,4-D to crops and also promote plant growth. This tri-functional (bioremediator-phytoprotecting-plant growth promoting) bacterium has significant potential in the eco-friendly remediation of 2,4-D in agricultural fields to prevent groundwater contamination, reduce phytotoxicity of herbicides, and enhance crop productivity.