ABSTRACT

Nitrogen is essential for plant growth, yet most plants cannot use atmospheric nitrogen directly. Legumes such as clover, soybeans, and Medicago solve this problem through a remarkable symbiotic partnership with soil bacteria called rhizobia which forms an important cornerstone of agriculture. These bacteria live inside specialised root organs known as nodules and convert atmospheric nitrogen into forms that plants can use. In return, the plant supplies nutrients that support the bacteria.

This relationship is often described simply as an exchange of plant carbon for bacterial nitrogen. However, this view leaves important questions unanswered particularly during the early stages of nodule development, when nitrogen fixation has not yet begun, and later in the symbiotic process when bacteria are unable to utilise the nitrogen that they fix. 

In this director’s talk, I will reveal how a family of plant proteins known as UMAMIT transporters, which move amino acids between plant cells are critical to establish nitrogen fixation. Using the model legume Medicago truncatula, my group identified several UMAMIT genes that are specifically activated during the formation of nitrogen-fixing nodules. By creating plants lacking key members of this transporter family, we found that nodules formed but were much less effective at fixing nitrogen, and the plants showed symptoms of nitrogen starvation.

Further experiments revealed that one of these transporters localises to membranes surrounding the bacteria inside nodules, as well as other key membranes in the nodule and nodule vascular tissue. These results suggest it helps move amino acids across critical symbiotic interfaces to support the nitrogen fixing bacteria inside nodules as well as moving the fixed nitrogen from the nodule to the rest of the plant.

Our overall results indicate that UMAMIT transporters play an essential role in establishing an efficient nitrogen-fixing partnership. More broadly, the work highlights how legumes actively manage nutrient exchange with their microbial partners and reveals an evolutionary innovation that helps legumes support this vital agricultural and ecological process.

BIOGRAPHY

Michael obtained his BSc degree with first class Hons from the University of Queensland and his PhD in Molecular Biology from the ANU. Michael is an internationally recognised scientist whose career over the past 4 decades has combined fundamental discovery with practical innovation in agriculture and food security. His research has made major contributions to our understanding of the signalling pathways that regulate root development, nodulation, and nitrogen fixation.

During his career Professor Djordjevic has published more than 126 peer-reviewed papers, secured over $25 million in funding, collaborated widely with national and international partners, and co-invented patented technologies with agricultural potential. 

He is also highly regarded as a mentor and educator, with many of his former students going on to important roles in academia, industry, and government.