Periodic Reporting for period 1 - GAiNS (Gibberellic acid signaling and dynamics during arbuscular mycorrhizal symbiosis and rhizobial-legume symbiosis)
Reporting period: 2020-09-01 to 2022-08-31
The use of nitrogen-based fertilizers is harmful for the environment, expensive for small-share farm holders in many parts of the world, but also important for producing high-yielding crops. The legume family of plants (soy, peas, peanuts) can acquire their own nitrogen from associations with bacteria in the soil. This plant-bacteria relationship is interesting from a biological perspective but also has potential to be applied to other species, like rice and wheat, to reduce to yield-reliability on nitrogen-based fertilizer. Legumes interact with nitrogen-acquiring bacteria by housing them within cells of a specialized organ termed a nodule. The objective of this project is to understand how legumes use a plant growth hormone called Gibberellin to regulate nodule development. I am doing this by using a cutting-edge technology to measure how much Gibberellin is in each cell under the microscope during nodule development. I developed this technology in the model legume called Medicago truncatula. I’ve spatially mapped Gibberellin during nodule development and found surprising but clear patterns of Gibberellin gradients in nodules. I’ve also developed Gibberellin visualization tools in barley, an important crop to many European farmers. Most plants, including barley, can associate with soil fungi within their roots to acquire nutrients, like Phosphorus, from the soil. Despite the differences in microorganisms, these two symbioses are quite similar and share common host genes that enable interaction. I have thus spatially-mapped Gibberellin patterns during fungi interactions in barley roots, finding surprising and clear accumulation patterns. These studies provide an interesting frame work to compare and contrast how Gibberellin is functioning in symbiosis and will help inform Gibberellin-based breeding targets for producing more sustainable crops.
With funding from the REA, I developed a transgenic M. truncatula line that contains a genetically-encoded Gibberellin biosensor. Because the sensor was developed in a different species, the functioning of the sensor needed to be verified. A suite of tests, including inoculating the sensor with biosynthetic inhibitors or bioactive Gibberellin have confirmed it is functioning as a Gibberellin biosensor (Figure 1). Using these lines, I have assessed the pattern of Gibberellin accumulation during symbiosis. I defined clear patterns of Gibberellin at all stages, from initiating nodules to mature nodules. The results (which are unpublished) are unexpected and raise many interesting questions. Following up on this pattern, I have introduced the biosensor into mutants that are deficient in creating Gibberellin or have defects in nodule organogenesis. Beneficial fungal-interactions have also been studied using the Gibberellin-encoded biosensor. A monocot-optimized Gibberellin sensor has been established in Hordeum vulgare (barley). Experiments testing functionality of the sensor demonstrate that it functions as expected. Initial studies inoculating barley with fungi show a surprising pattern (unpublished), which will be followed up on in future studies.The work is currently still in progress but is expected to be a part of a manuscript submitted for publication within the next 6-9 months. While the pandemic has hampered opportunities for presentations in live conferences, I have presented the functioning and patterning of the GA sensor in medicago and barley at four conferences, two virtual (~45 participants each), and two in person (~50 participants each).
Another plant hormone called cytokinin is also important in nodule formation. I have developed a stable genetically-encoded cytokinin signaling sensor and am currently generating the tools to address the relationship between Gibberellin and Cytokinin in nodule development. Most of the cereal crops we eat are deficient in Gibberellin and have been selected for this – they are smaller, taking fewer resources in the field and also produce more seed. In this selective breeding of the Green Revolution, nitrogen-based fertilizer was used and we must consider if we have accidentally selected away from some benefits of Gibberellin adequacies for plants.