ELUCIDATING THE RESPONSE OF PENNYCRESS AND CANOLA TO METAL LACTATES, AN EMERGING PLANT BIOSTIMULANT

EKOM-OBONG SOLOMON IDIO

AbstractThe goal of sustainable agriculture is to increase crop yields while maintaining soil health. Plant biostimulants have become more popular in recent years to improve plant growth, nutrient uptake, and stress tolerance without using copious amounts of inorganic fertilizers. Biostimulants that contain micronutrients conjugated to organic ligands, such as metal lactates, make micronutrients more available and change how plants respond physiologically, but little is known about how they affect plants at the molecular level. Metal lactates are organic salts formed by the reaction of metal ions with lactic acid. Chapter 1 provides a detailed literature review on micronutrients and their role in plant growth, while also discussing soil health and the emerging role of plant biostimulants. Chapter 2 discusses dose and transcriptomic responses to metal lactates in three Brassica species—pennycress (Thlaspi arvense L.), canola (Brassica napus L.) and Arabidopsis (Arabidopsis thaliana (L.) Heynh.). All three species were exposed to metal lactates at multiple concentrations using a plate-based assay to determine the optimal and toxic concentrations of five metal lactates and to these the hypothesis that pennycress would tolerate a higher concentration of these metal lactate compared to canola and Arabidopsis. The optimal and toxic concentrations of these metal lactates were determined using root growth length and root hair length as an indicator. Our results show that 1 µM metal lactate concentration is optimal as plants grew well in this concentration and 50 µM and 100 µM is considered toxic because canola and Arabidopsis root growth and root hair length were strongly inhibited. For the transcriptomic responses, which were carried out on Arabidopsis shoots and pennycress shoots and roots, plants were exposed to two treatment concentrations representing a low and high dose (1 µM and 50 µM, respectively), alongside untreated controls. Differential gene expression analyses were performed to evaluate transcriptional responses associated with each treatment. In Arabidopsis, our results showed that the low concentration treatment produced only a minimal transcriptional response, with eight differentially expressed genes detected relative to control. In contrast, the high concentration treatment triggered a large-scale transcriptional shift, with over five thousand differentially expressed genes. Gene Ontology (GO) enrichment analysis further revealed that the high concentration treatment was dominated by pathways associated with cell cycle progression, nuclear division, chromosome segregation, and DNA replication. These processes suggest disruption of normal cellular activities rather than a coordinated stress-response mechanism, indicating that high metal lactate exposure may interfere with normal growth and cell division processes in Arabidopsis. Similarly, in pennycress, the 1 µM concentration treatment showed little to no transcriptional response, whereas the 50 µM concentration resulted in a strong and distinct response in both shoot and root tissues. Gene Ontology (GO) enrichment analysis in pennycress shoots revealed pathways associated with defense metabolism and transport, including glucosinolate biosynthesis, glycosylation processes, and vascular transport. In the root, enriched pathways were mainly related to structural and metabolic processes such as cell wall biogenesis, cytoskeleton organization, and catabolic activities, indicating a tissue-specific response to metal lactates. As compared to Arabidopsis, pennycress showed a different suite of DEGs indicative of co-opting tolerance mechanisms. In Chapter 3, we describe a greenhouse study on a commercial metal lactate seed treatment (Commence) to evaluate its efficacy on cold tolerance and on shoot and root biomass on canola at 1oz and 2oz per acre treatment concentrations. Our results show that there were no significant differences between the treated group and control. This thesis provides insight into the physiological and molecular responses of plants to metal lactate-based biostimulants. Our findings highlight the importance of dosage in determining whether these compounds promote plant growth or induce stress responses and contribute to a better understanding of the role micronutrient-based biostimulants play in sustainable crop production.

ELUCIDATING THE RESPONSE OF PENNYCRESS AND CANOLA TO METAL LACTATES, AN EMERGING PLANT BIOSTIMULANT

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