Microorganisms in Plant Growth and Pathogens

Microorganisms and Plant Growth

• Streptomyces species: Filamentous microbes that colonize soil, roots, and aerial plant parts. Produce antibiotics, volatile organic compounds (giving soil its smell), and lytic enzymes. Act as biocontrol agents and biofertilizers. Their spore formation and survival in adverse conditions make them competitive.

• Plant growth-promoting bacteria: Beneficial bacteria that enhance plant growth and resistance to abiotic stresses (drought, salt). Can also be used in phytoremediation (remediating metal-contaminated soil). Examples include Bacillus amyloliquefaciens GB03 and bacteria isolated from other tolerant plants.

• Arbuscular mycorrhizal fungi (AMF): Specifically, Rhizophagus irregularis CD1 enhances plant growth, especially cotton, and resistance to Verticillium dahliae wilt. This fungus may inhibit V. dahliae growth via unknown volatiles.

Plant Pathogens and Host Specificity

• Fungal Plant Pathogens: A significant threat to plant health. Many are highly host-specific. Elucidating the host specificity factors for pathogen proliferation and disease formation is a crucial research topic.

• Biotrophic Plant Pathogenic Fungi: Live in close contact with plant tissue, feeding on living plant parts and subvert plant defenses. One strategy is secreting effector proteins that interact with plant proteins to the pathogen's advantage. Examples include Microbotryum lychnidis-dioicae (anther smut) and Fusarium proliferatum (tomato disease).

• Host specificity factors: Identification often involves comparing molecular characteristics of different strains or species. This usually results in a list of target genes potentially involved in host-specific virulence, requiring further verification and validation. Combining classical genetics, genomics, and transcriptomics (of both pathogen and host) is crucial.

• Bacterial Plant Pathogens: Pseudomonas syringae pv. actinidiae (Psa) causes bacterial canker. Study of kiwifruit infected with Psa involved transcriptome analysis revealing upregulated genes involved in defense compounds, plant immunity, and metabolic processes.

• Fungal Pathogens and Rhodopsins: Some phytopathogenic and phyto-associated ascomycetes contain rhodopsin-encoding genes. Fusarium fujikuroi, a rice plant pathogen, has two rhodopsins (CarO and OpsA). CarO, a light-driven proton pump, is shown to be positively regulated by indole-3-acetic acid and sodium acetate. Deletion of the CarO-encoding gene leads to hypervirulence.