Doctoral defence: Arun Kumar Devarajan "Microbes and climate change: insights from plant-microbe interactions in rice phyllosphere and soil microbiomes in subarctic grasslands“

On August 22, at 10:15 Arun Kumar Devarajan will defend his doctoral thesis "Microbes and climate change: insights from plant-microbe interactions in rice phyllosphere and soil microbiomes in subarctic grasslands“ for obtaining the degree of Doctor of Philosophy (in Microbial Biology).

Supervisors:
Associate Professor Marika Truu, University of Tartu
Professor Jaak Truu, University of Tartu
Professor Ivika Ostonen-Märtin, University of Tartu

Opponent:
Associate Professor Kim Yrjäla, University of Helsinki (Finland)

Summary:
Human activities, particularly the burning of fossil fuels and industrial processes, have led to an increase in the levels of greenhouse gases in the atmosphere. This has resulted in climate change, including global warming, droughts, and extreme weather events. Predictions indicate that by 2100, global temperatures could rise by 3-5°C, potentially causing irreversible damage to ecosystems. As a consequence of climate change, precipitation and air humidity are decreasing in many parts of the world, leading to reduced soil moisture and subsequently impacting agricultural crop yields, particularly cereals. 

Microorganisms in the soil play a crucial role in the nutrient cycle and help maintain soil stability. Similarly, various microbes inhabit different parts of plants. Some live on the leaves (phyllosphere), some live inside the plant (endophytes), and others live around the roots (rhizosphere). These plant inhabitants can be either beneficial, boosting plant health, or harmful, causing disease. This doctoral thesis explores the effect of climate change on microbial communities associated with plants and soil, drawing on findings from two detailed studies. The first study examined how bacteria from the leaf surfaces of drought-resistant rice could shield more vulnerable rice varieties from brief, moderate droughts. The study results showed that most stress-tolerant bacteria belong to Bacillus species. Notably, the Bacillus megaterium strain PB50 was especially effective, enhancing the genetic tolerance of susceptible rice to drought and also increasing its yield. A related study investigated how warming impacts microbial communities in the soils of subarctic grasslands, specifically in root-free soil and rhizosphere. 

The findings indicated that warming significantly changes the composition and function of these microbes. Notably, prolonged warming altered the balance of genes involved in denitrification, both in the rhizosphere and the root-free soil. This shift could lead to increased emissions of nitrous oxide, a greenhouse gas, from the soil. These studies contribute to a better understanding of the impact of climate change on plant-microbe-soil systems and allow for the prediction and reduction of the negative impacts of climate change on agriculture.