Bacterial and Fungal Biocontrol Agents for Plant Disease Protection: Journey from Lab to Field, Current Status, Challenges, and Global Perspectives

Plants are constantly exposed to various phytopathogens such as fungi, Oomycetes, nematodes, bacteria, and viruses. These pathogens can significantly reduce the productivity of important crops worldwide, with annual crop yield losses ranging from 20% to 40% caused by various pathogenic diseases. While the use of chemical pesticides has been effective at controlling multiple diseases in major crops, excessive use of synthetic chemicals has detrimental effects on the environment and human health, which discourages pesticide application in the agriculture sector. As a result, researchers worldwide have shifted their focus towards alternative eco-friendly strategies to prevent plant diseases. Biocontrol of phytopathogens is a less toxic and safer method that reduces the severity of various crop diseases. A variety of biological control agents (BCAs) are available for use, but further research is needed to identify potential microbes and their natural products with a broad-spectrum antagonistic activity to control crop diseases. This review aims to highlight the importance of biocontrol strategies for managing crop diseases. Furthermore, the role of beneficial microbes in controlling plant diseases and the current status of their biocontrol mechanisms will be summarized. The review will also cover the challenges and the need for the future development of biocontrol methods to ensure efficient crop disease management for sustainable agriculture.

Major Biological Control Strategies for Plant Pathogens

Food security has become a major concern worldwide in recent years due to ever increasing population. Providing food for the growing billions without disturbing environmental balance is incessantly required in the current scenario. In view of this, sustainable modes of agricultural practices offer better promise and hence are gaining prominence recently. Moreover, these methods have taken precedence currently over chemical-based methods of pest restriction and pathogen control. Adoption of Biological Control is one such crucial technique that is currently in the forefront. Over a period of time, various biocontrol strategies have been experimented with and some have exhibited great success and promise. This review highlights the different methods of plant-pathogen control, types of plant pathogens, their modus operandi and various biocontrol approaches employing a range of microorganisms and their byproducts. The study lays emphasis on the use of upcoming methodologies like microbiome management and engineering, phage cocktails, genetically modified biocontrol agents and microbial volatilome as available strategies to sustainable agricultural practices. More importantly, a critical analysis of the various methods enumerated in the paper indicates the need to amalgamate these techniques in order to improve the degree of biocontrol offered by them.

Genome analysis of plant growth-promoting rhizobacterium Pseudomonas chlororaphis subsp. aurantiaca JD37 and insights from comparasion of genomics with three Pseudomonas strains

Pseudomonas chlororaphis subsp. aurantiaca strain JD37 is a plant growth-promoting rhizobacterium (PGPR), which has important biotechnological features such as plant growth promotion, rhizosphere colonization and biocontrol activities. In present study, the genome sequence of JD37 was obtained and comparative genomic analysis were performed to explore unique features of the JD37 genome and its relationship with other Pseudomonas PGPR: P. chlororaphis PA23, P. protegens Pf-5 and P. aeruginosa M18. JD37 possessed a single circular chromosome of 6,702,062 bp in length with an average GC content of 62.75 %. No plasmid was detected in JD37. A total of 5003 functional proteins of JD37 were predicted according to the clusters of orthologous groups (COGs) database. The JD37 genome consisted of various genes involved in plant growth promotion, biocontrol activities and defense responses. Genes involved in the rhizosphere colonization and motility were also found in the genome of JD37, suggesting the common plant growth-promoting traits in PGPR. The identified resistance genes (e.g. those related to metal resistance, antibiotics, and osmotic and temperature-shock) and secondary metabolite biosynthesis revealed the pathways for metabolites it produced. Data presented in present study further provided valuable information on its molecular genetics and adaptive capacity in the rhizosphere niche.

Control of pyrimidine nucleotide formation in Pseudomonas aurantiaca

The control of pyrimidine nucleotide formation in the bacterium Pseudomonas aurantiaca ATCC 33663 by pyrimidines was studied. The activities of the pyrimidine biosynthetic pathway enzymes were investigated in P. aurantiaca ATCC 33663 cells and from cells of an auxotroph lacking orotate phosphoribosyltransferase activity under selected culture conditions. All activities of the pyrimidine biosynthetic pathway enzymes in ATCC 33663 cells were depressed by uracil addition to the minimal medium when succinate served as the carbon source. In contrast, all pyrimidine biosynthetic pathway enzyme activities in ATCC 33663 cells were depressed by orotic acid supplementation to the minimal medium when glucose served as the carbon source. The orotidine 5'-monophosphate decarboxylase activity in the phosphoribosyltransferase mutant strain increased by more than sixfold in succinate-grown cells and by more than 16-fold in glucose-grown cells after pyrimidine limitation showing possible repression of the decarboxylase by a pyrimidine-related compound. Inhibition by ATP, GTP, UTP and pyrophosphate of the in vitro activity of aspartate transcarbamoylase in ATCC 33663 was observed. The findings demonstrated control at the level of pyrimidine biosynthetic enzyme synthesis and activity for the P. aurantiaca transcarbamoylase. The control of pyrimidine synthesis in P. aurantiaca seemed to differ from what has been observed previously for the regulation of pyrimidine biosynthesis in related Pseudomonas species. This investigation could prove helpful to future work studying pseudomonad taxonomic analysis as well as to those exploring antifungal and antimicrobial agents produced by P. aurantiaca.

Isolation, Identification, and Decomposition of Antibacterial Dialkylresorcinols from a Chinese Pseudomonas aurantiaca Strain

A chemical investigation of a Chinese Pseudomonas aurantiaca strain has yielded a new benzoquinone (4) and furanone (5), in addition to the known dialkylresorcinols 1 and 2. Extensive decomposition studies on the major metabolite 1 produced an additional furanone derivative (6), a hydroxyquinone (7), and two unusual resorcinol and hydroxyquinone dimers (8 and 9). Structures were elucidated by nuclear magnetic resonance spectroscopy in combination with tandem mass spectrometry analysis. These studies illustrate the potential of artifacts as a source of additional chemical diversity. Compounds 1 and 2 showed moderate antibacterial activity against a panel of Gram-positive pathogens, while the antibacterial activities of the artifacts (4-9) were reduced.

Lahorenoic acids A-C, ortho-dialkyl-substituted aromatic acids from the biocontrol strain Pseudomonas aurantiaca PB-St2

Three new aromatic acids, named lahorenoic acids A (1), B (2), and C (3), have been isolated along with the known compounds phenazine-1-carboxylic acid (4), 2-hydroxyphenazine-1-carboxylic acid (5), 2-hydroxyphenazine (6), 2,8-dihydroxyphenazine (7), cyclo-Pro-Tyr (8), cyclo-Pro-Val (9), cyclo-Pro-Met (10), and WLIP (11) and characterized from the biocontrol strain Pseudomonas aurantiaca PB-St2. The structures of these compounds were deduced by 1D and 2D NMR spectroscopic and mass spectral data interpretation. Compounds 2, 4, and 7 showed moderate antibacterial activity against mycobacteria and other Gram-positive bacteria, while 4 was also found to exhibit cytotoxic and antifungal properties.