The webinar, "Tiny but Mighty," hosted by the Horticultural Research Institute (HRI) and the Boxwood Blight and Insight Group (BBIG), focuses on the potential of the microbiome to enhance boxwood resilience to diseases and environmental stress. Dr. Xiaoping Li, a microbiome scientist at Virginia Tech, leads the discussion, explaining the importance of microorganisms in plant health.

Microorganisms are present everywhere, including in the above-ground parts of plants (the phyllosphere) and below-ground (roots, rhizosphere). These microbes, which include bacteria, fungi, archaea, protists, and algae, form a complex community called the microbiota. The concept of the microbiome includes the microbiota and their structural elements, such as viruses, and their activities. Plants and their associated microorganisms form an ecological unit called the holobiont.

The interaction between plants and their microbiome can be categorized into three types: pathogens, commensals, and beneficial. While some microbes are pathogens, a large proportion are either commensal or beneficial, aiding in plant growth and health. The microbiome can benefit plant growth in four main ways:

• Nutrient Acquisition: Microbes help plants access essential nutrients like nitrogen and phosphorus, which are often limited in soil. Some microbes fix atmospheric nitrogen into a usable form, while others break down organic matter to release nutrients.
• Modulating Seed Germination and Plant Growth: Microbes produce plant hormones that promote cell division and elongation, aiding in seed germination and plant growth. They can also attract pollinators by emitting or modifying visual, olfactory, and gustatory cues.
• Abiotic Stress Tolerance: Microbes help plants escape environmental stressors such as drought, heat, and salinity. They do this by producing biofilms, plant hormones, and osmo-protectants. Additionally, some microbes can reduce the concentration of ethylene, a stress hormone.
• Protection Against Pathogens and Pests: Microbes can produce antimicrobial compounds, compete with pathogens for resources, and activate the plant immune system.

Mycorrhizal fungi form a symbiotic relationship with plant roots, enhancing nutrient and water acquisition. They can also induce plant defense mechanisms. Many microorganisms are now being commercialized as biopesticides, biofertilizers, and biostimulants.

Microbial communities are affected by various biotic and abiotic factors, including plant species, compartment, age, agricultural activities, and climate conditions. High-throughput sequencing allows researchers to profile these microbial communities by examining microbial DNA.

Boxwood, once considered a no-maintenance plant, now faces many emergent diseases. Microbiome research may offer new strategies for understanding and protecting boxwood. The first step in this research is to capture the microbial communities associated with boxwood. Then, the functions of these microbiomes must be elucidated, leading to the development of microbiome-based applications.

Research has revealed a diverse microbial community in the boxwood phyllosphere, with proteobacteria and ascomycota being the dominant bacterial and fungal phyla, respectively. Core microbiomes have also been identified, which are consistently associated with boxwood and may have crucial functions. Some of the core microbes include Methylobacterium (bacteria) and Alternaria and Cladosporium (fungi), which may participate in plant nutrient acquisition and defense.

The study also explored how fungicides affect the boxwood microbiome. Repeated use of certain fungicides, like Daconil, can reduce fungal sensitivity to the chemicals. Fungicides can also alter the bacterial community structure. Similarly, antidesiccants, used to protect plants from water loss, can impact the microbiome, with fungal communities being more affected than bacteria.

The boxwood rhizosphere, the soil surrounding plant roots, contains diverse microbial communities, dominated by actinobacteria, proteobacteria, and firmicutes at the phylum level. High levels of Pseudonectria, a pathogen causing Volutella blight, have been found in the rhizosphere, suggesting soil could be a reservoir for pathogens. Core bacteria in the rhizosphere include Actinobacteria and Alpha proteobacteria, known for nitrogen fixation. Mycorrhizal fungi, which are essential to plant health, have also been identified, with Ceratobasidium, Hyalocifae, and Cystoderma species being the most abundant. Microbial networks constructed from blight-tolerant boxwood cultivars show more positive interactions among fungi and the presence of a hot taxon Amanita.

The webinar concluded with a discussion of the implications of this research, emphasizing that soil is a major reservoir of boxwood pathogens, that mycorrhizal fungal interactions may enhance blight tolerance, and that nursery practices influence the boxwood microbiome. Future research will focus on the functions of the microbiome at molecular levels and the interactions between boxwood and its microbiome.