Microbial survival strategies in desiccated roots of Myrothamnus flabellifolia

Shandry Mmasetshaba Tebele ^1* Rose Marks ² Jill Margaret Farrant ^1*

• ¹ University of Cape Town, Cape Town, South Africa
• ² University of Illinois at Urbana-Champaign, Champaign, Illinois, United States

Root-associated microbiomes are critical to plant vigor, particularly under drought stress. The spatial dynamics of microbial community diversity and composition are strongly influenced by plant root and environmental factors. While the desiccation tolerance of the resurrection plant Myrothamnus flabellifolia using leaf tissue has been previously investigated, the transcriptional responses of its rootassociated microbiomes under desiccation remain completely unexplored. Here, we conducted metatranscriptome sequencing on root samples of M. flabellifolia collected in the field across four states: dry, desiccated, partially hydrated, and fully hydrated. Bacterial transcripts dominated the root metatranscriptome across all conditions. Desiccated roots exhibited a significant increase in transcripts from Actinomycetota, whereas fully hydrated roots showed an enrichment of Pseudomonadota. Under desiccation, root-associated bacteria upregulated genes involved in antioxidant systems, trehalose biosynthesis, and hormonal regulation, highlighting their adaptive mechanisms to withstand extreme water loss. In contrast, the bacterial transcriptional response in hydrated roots was characterized by genes linked to peptidoglycan biosynthesis, sugar transporters, and chemotaxis. Taken together, our findings indicate that root-associated bacteria deploy defense mechanisms analogous to those of their host plant to adapt to extreme drought stress, highlighting their crucial role in plant resilience.Plants and microbiota have evolved adaptive mechanisms to combat the negative impacts of drought.Recent studies highlight the critical interplay of plant microbe interactions in improving performance under drought conditions (Gholizadeh et al., 2024;Shaffique et al., 2022). For instance, microbes associated with roots of plants from semi-arid environments prone to extreme drought, function synergistically with plants to enhance drought tolerance (Du et al., 2024). A deeper understanding of the complex interactions between plant roots, microorganisms, and the adaptive mechanisms they employ is crucial for understanding survival in the face of drought.Under drought stress, specific microbes dominate the soil microbiome. Specifically, monoderm bacteria, which are primarily gram-positive, increase under drought, possibly due to their thick peptidoglycan cell walls, while diderm lineages (gram-negative) decrease during drought (Xu et al., 2018). Changes in the community composition also impact the function of the soil microbiome under drought. A recent metatranscriptome study demonstrated that moderate drought activated oxidative, osmotic and heat stress-related genes in rhizosphere bacteria such as Burkholderiales and