Abstract
Excessive use of fungicides in agriculture may result in substantial accumulation of active residues in soil, which affect crop health and yield. We investigated the response of Raphanus sativus (white radish) to fungicides in soil and potential beneficial interactions of radish plants with fungicide-tolerant plant growth-promoting rhizobacteria (PGPR). The PGPR were isolated from cabbage and mustard rhizospheres. Morphological and biochemical characteristics measured using standard methods, together with analysis of partial 16S rRNA gene sequences, revealed that fungicide-tolerant PGPR, isolates PS3 and AZ2, were closely related to Pseudomonas spp. These PGPR survived in the presence of high fungicide concentrations i.e., up to 2400 μg mL−1 carbendazim (CBZM) and 3200 μg mL−1 hexaconazole (HEXA). Bacterial isolates produced plant growth stimulants even under fungicide stress, though fungicides induced surface morphological distortion and alteration in membrane permeability of these bacteria, which was proved by a set of microscopic observations. Fungicides considerably affected the germination efficiency, growth, and physiological development of R. sativus, but these effects were relieved when inoculated with PGPR isolates. For instance, CBZM at 1500 mg kg−1 decreased whole dry biomass by 71%, whole plant length by 54%, total chlorophyll by 50%, protein content by 61%, and carotenoid production by 29%. After applying isolate AZ2 for white radish grown in CBZM (10 mg kg−1)-amended soil, it could improve plant growth and development with increased whole plant dry weight (10%), entire plant length (13%) and total chlorophyll content (18%). Similarly, isolate PS3 enhanced plant survival by relieving plant stress with declined biomarkers, i.e., proline (12%), malondialdehyde (3%), ascorbate peroxidase (6.5%), catalase (18%), and glutathione reductase (4%). Application of isolates AZ2 and PS3 could be effective for remediation of fungicide-contaminated soil and for improving the cultivation of radish plants while minimizing inputs of fungicides.
Highlights
Raphanus sativus, a member of the Brassicaceae family, is an important vegetable crop in India and Southeast Asia; both leaves and tubers of the radish plant are consumed
Some Gram-negative bacteria have been reported to withstand high levels of fungicides; for example, Pseudomonas putida PS 9 isolated from the mustard rhizosphere exhibited variable tolerance to four fungicides at concentrations ranging from 1400–3200 μg mL−1 [52]
To overcome fungicidal stress and to safeguard the host plant, three mechanisms may be employed by plant growth-promoting rhizobacteria (PGPR), i.e., degradation of fungicides by enzymes, accumulation within cells followed by complexation with cellular constituents, and genetic mutation [90]
Summary
Raphanus sativus (white radish), a member of the Brassicaceae family, is an important vegetable crop in India and Southeast Asia; both leaves and tubers of the radish plant are consumed. Considering the importance of radish in the Indian diet, the nutrient storehouse of soils; the deleterious impacts of fungicides on the functional composition of soil microbiota, soil fertility and crop productivity; and the lack of adequate information on fungicidal response to radish, this study was undertaken with the following objectives i.e., to: (i) isolate and identify PGPR isolates from different rhizosphere soils; (ii) evaluate fungicidal tolerance by selected PGPR isolates; (iii) determine the production of growth-regulating bio-stimulants under fungicide pressure; (iv) assess the fungicidal toxicity to R. sativus under both in vitro bioassays and greenhouse conditions; (v) evaluate the remediation potential of identified PGPR on biochemical attributes of R. sativus; and (vi) determine the production of stressor molecules (proline and malondialdehyde) in fungicide-treated and PGPR-inoculated plants
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