Date of Award

12-2025

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Plant and Environmental Science

Committee Chair/Advisor

Dr. Vidya Suseela

Committee Member

Dr. Nishanth Tharayil

Committee Member

Dr. William Bridges

Committee Member

Dr. Barbara Campbell

Abstract

Plants, as sessile organisms, are continuously exposed to biotic and abiotic stresses that negatively affect their growth and development, ultimately reducing the yield and quality of agricultural products. To mitigate these stresses, modern agriculture relies heavily on chemical inputs, such as fertilizers and pesticides. While these inputs provide a short-term solution, their prolonged use degrades soil health, disrupts ecological balance, and causes environmental pollution. Moreover, these agrochemicals are synthesized from finite, non-renewable sources that will eventually be depleted and are also economically unviable for farmers with small landholdings. To ensure global food security and promote the sustainability of agricultural systems, there is a dire need for an economically viable, environmentally friendly, and widely applicable alternative to these agrochemicals.

Arbuscular mycorrhizal fungi (AMF), which form symbiosis with up to 80% of terrestrial plants, could be a viable alternative to phosphorus (P) fertilizers. AMF are an obligate fungus that relies on plants for carbon to support its growth and reproduction, and in return, provides a range of nutrients, particularly P. It also offers many physiological and ecological benefits to the plants, as well as improving the soil structure. Plants release a diverse array of compounds in the rhizosphere, known as root exudates, to initiate symbiosis. Root exudate is a complex mixture of different compounds, which can be broadly classified into primary and secondary metabolites and inorganic ions. The composition of root exudates varies among plant species, within species, and across plant life stages, and is also influenced by external factors. Secondary metabolites, particularly strigolactones and flavonoids, act as chemoattractants and stimulate mitochondrial activity, hyphal growth, and branching of the AMF hyphae. Hyphal branching is a key morphological change, as it increases the probability of the hyphae locating the root and the number of connection points. It is known that the ability of individual or multiple strigolactone and flavonoid compounds to induce hyphal branching varies with their structure and concentration. However, the influence of the different compositions of secondary metabolites in root exudates on the establishment of plant-AMF symbiosis remains elusive.

In this study, we selected 100 genetically diverse sorghum accessions. We analyzed the chemical diversity of strigolactones and flavonoids in the root exudates of plants under nutrient-deficient conditions using liquid chromatography coupled with tandem mass spectrometry. The abundance of detected secondary metabolites, strigolactones (orobanchol, 5-deoxystrigol, and 19-hydroxy-carlactone), flavonoids (apigeninidin, 7-methoxy-apigeninidin, luteolinidin, and 5-methoxy-luteolinidin), Sorgoleone-360, and an unknown compound (m/z 287), varied significantly across accessions and regions of origin of these accessions. The abundance of orobanchol was substantially higher in the accessions from the West African region and the Guinea race. Interestingly, there were no overlapping accessions among the top ten producers of orobanchol and any of the flavonoids, and only one accession was common between the top ten producers of 5-deoxystrigol and certain flavonoids. Overall, the sorghum accessions exhibited distinct exudate profiles under nutrient-deficient conditions. We also analyzed secondary metabolites in root tissue samples and found that several were abundant in the sweet or cellulosic functional types. The mean abundance of metabolites in accessions from West Africa and Guinea was also markedly different from that of accessions from other regions. For the hyphal branching assay, we used root exudate samples from accessions with distinct exudate profiles and observed different orders of hyphal branching in the AMF species Gigaspora margarita. Overall, our results suggest that the chemical diversity of root exudates influences the hyphal branching in AMF. This research enhances our understanding of the pre-symbiotic stage of symbiosis and contributes to optimizing plant-AMF symbiosis for improved nutrient uptake, enhanced plant performance, and sustainable crop management.

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