Exploration of arbuscular mycorrhiza to improve rice growth and production with a focus on zinc transport and translocation routes

Soil degradation, driven by overexploitation, excessive use of fertilizers, and pollution, has become a critical global challenge. This degradation disrupts soil structure, reduces organic matter, and alters nutrient cycles, threatening sustainable food production systems. Among the various threats posed by soil degradation, trace metal pollution is particularly concerning due to its impacts on crop yield and food safety. Trace metals, such as zinc (Zn), present a dual function due to their critical yet potentially harmful roles. As a vital micronutrient for both plants and humans, Zn deficiency hampers plant growth and reduces nutritional quality, whereas excessive Zn levels cause toxicity and environmental damage. Balancing Zn availability in soils is essential, particularly for rice, a staple crop, feeding over half the world's population. Many rice-growing regions face Zn-deficient or Zn-polluted soils, necessitating sustainable strategies to maintain rice productivity and ensure global food security.

Among such strategies, the role of soil microbes, especially arbuscular mycorrhizal fungi (AMF), has gained attention. AMF form symbiotic relationships with plant roots, effectively expanding the root system and enhancing nutrient uptake. This study aimed to investigate how AMF influences rice growth, nutrient dynamics, and stress responses under varying Zn conditions. Specifically, the research explored the interactions between rice genotypes and AMF species to uncover differential impacts on Zn uptake, translocation, and overall nutrient homeostasis under Zn-deficient and Zn-excess conditions. By examining these interactions, this study contributes valuable insights into the role of AMF in enhancing Zn management and supporting sustainable rice cultivation.

Chapter 2 presents a pot experiment evaluating the potential of commercial AMF inoculum to support rice growth and production under three soil conditions. Measurements included rice biomass, grain yield, tissue Zn content, AMF colonization rates, and the expression of Zn transporter genes. The findings demonstrate that AMF inoculation restored rice growth and grain production in Zn-amended soils, with Zn supplementation promoting symbiosis development. Shoot ionomes exhibited high sensitivity to Zn supply levels, with AMF and mock-inoculated plants responding differently. AMF inoculation also interacted with Zn responsive expression of OsHMA transporters, which mediate root-to-shoot Zn translocation. Collectively, these results suggest that AMF colonization impacts Zn transport routes, enhancing rice adaptability to variable Zn conditions.

Chapter 3 investigated the symbiotic compatibility of four rice genotypes — Delmatti, Dular, GLPA, and Nipponbare — with the AMF species Rhizophagus irregularis (DAOM197198) through a pot experiment. This study focused on mycorrhizal colonization rates and the expression of key zinc transporter genes from the ZIP and HMA families without any nutritional constraints. The results demonstrate that R. irregularis inoculation significantly enhances arbuscular mycorrhiza (AM) colonization and balances Zn supply to their host plant. In particular, drought tolerant aus-variety, Dular showed lower colonization potential in absence of any nutritional constraints and the low phytic acid variety, GLPA showed a positive response in grain production. Specifically, Zrt/Irt-like proteins (ZIPs), heavy metal ATPases (HMAs), transporters associated with Zn detoxification and root-to-shoot translocation exhibited variety specific expression patterns which were absent in AMF inoculated plants.

Chapter 4 focused on the responses of wild-type rice (Nipponbare) and a mutant (∆CYCLOPS3) to three AMF species — Rhizophagus irregularis (MUCL41833), Rhizophagus clarus (MUCL46238), and Rhizophagus intraradices (MUCL49410) — under two Zn concentrations (20 and 120 mg/kgsoil). The result showed that all three AMF species effectively colonized the roots of WT rice. However, there was no significant relation between AMF colonization and beneficial growth effect. The antioxidative activity showed that AMF can modify ROS homeostasis, even in mutant rice lines with impaired colonization. Additionally, these findings suggest a complex interplay between AMF symbiosis and Zn homeostasis signaling pathways, where AMF contribute to Zn tolerance by modifying reactive oxygen species (ROS) metabolism, and Zn translocation. This study reinforces that AMF and Zn stress responses are interconnected at different levels and provides a basis for future research aiming at unwiring their signaling networks.

Overall, this thesis demonstrates that AMF symbiosis contributes to rice adaptation under Zn stress by modulating Zn uptake and transport pathways. The outcomes offer insights for sustainable rice breeding and soil management strategies.