PhD Oral Exam - Subhikshaa Bagavathiappan, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Plant invasion has become a significant research topic in ecology and invasion biology since it threatens the integrity of natural ecosystems by displacing native plant groups and creating monocultures in newly invaded environments. Several theories have been put out to explain successful invasions, however, plant-microbe interaction is the least explored of all the potential hypotheses. Endophytic microbes play an important role in maintaining plant health, adaptability, and ecological interactions. In this study, I investigated the diversity, composition, and abiotic stress tolerance of bacterial and fungal endophytes associated with Lantana camara, an invasive plant species, and Lantana indica, its native congener, co-occurring in India. Using culture-independent amplicon sequencing (16S rRNA and ITS), culture-dependent isolation methods and abiotic stress tolerance assays, I examined the invasive and native plants to identify their distinct bacterial and fungal communities and the tolerance of fungi to abiotic stressors. Amplicon sequencing demonstrated Ascomycota to be the dominant phyla and Talaromyces as the dominant fungal genus in both L. camara and L.indica, while the bacterial communities were dominated by the family Clostridiaceae in L. camara and Pseudomonadaceae in L. indica. The native plants showed higher bacterial diversity and evenness compared to invasive plants suggesting a long-term adaptation to local biotic and abiotic conditions, whereas fungal communities showed no difference emphasizing that the endophytic fungi share similar community composition and functional adaptation. Culture dependent isolation of endophytic fungi revealed Ascomycota to be the dominant phyla. Both culture dependent and culture independent methods were compared to show how they can complement each other due to various bias adhered to these techniques. Abiotic stress tolerance assays on selected fungal isolates demonstrated species specific stress tolerance. Aspergillus spp. showing high salinity (2.5M NaCl) and drought (15% PEG), while Macrophomina phaseolina remained viable at 45 °C. These results suggest that endophytes can support host survival under abiotic stress, potentially facilitating plant growth. Overall, the thesis highlights that the endophytic communities differ between invasive and native Lantana species, and specific endophytes may contribute to stress resilience of the host. Understanding such microbial associations is essential in uncovering mechanisms underlying plant adaptation and invasion process.