Identification and characterisation of new plant peptide hormones that control legume nodulation and nitrogen fixation

A project undertaken at the Centre for Integrative Legume Research, School of Agriculture and Food Sciences, The University of Queensland, and supervised by Dr Brett Ferguson and Prof. Peter Gresshoff

Legume crops include a wide range of economically and environmentally important food, feed and biofuel species.  Almost all have the ability to enter into a highly-specialized symbiotic relationship with nitrogen-fixing rhizobia bacteria.  The relationship results in the formation of new organs that form on the host roots, called nodules (Figure 1), which house the rhizobia and provide the ideal environment for biological nitrogen fixation.  This gives legume plants a tremendous growing advantage, and improves soil conditions, whilst also helping to reduce nitrogen fertiliser requirements in agriculture.  Identifying and understanding the molecular factors that drive nodulation and nitrogen-fixation is of great importance and could help to alleviate the current, excessive use of nitrogen-based fertilisers in agriculture.  This could be achieved by optimising the nodulation process and by establishing critical targets for breeding and engineering programs that aid in generating superior crop varieties. 

Exciting new developments have recently revealed a number of novel peptide hormones that act to control nodule development in soybean, the world’s most economically significant legume species.  Our group discovered three genes encoding soybean peptides that negatively regulate nodule development, including two Rhizobia Induced CLE (RIC1 and RIC2) peptides and a Nitrate Induced CLE (NIC1) peptide.  This project will aid in further characterising these critical regulatory peptides and will provide the foundation for isolating and detecting the peptides.  This will be achieved by engineering transgenic roots in isolation that produce the peptides in large quantities, growing these roots indefinitely in tissue culture (Figure 2), and subsequently using them to extract, purify and detect the peptides.  Outcomes will enhance the current understanding of nodulation control mechanisms and could aid in future studies focused on optimising legume nodulation and nitrogen fixation in agriculture.    

An overview of these critical nodulation-suppressing CLE peptides was recently published, with an acknowledgement to the Hermon Slade Foundation for financial support:

Hastwell AH, Gresshoff PM, Ferguson BJ (2015) The structure and activity of nodulation-suppressing CLE peptide hormones of legumes. Functional Plant Biology, doi:10.1071/FP14222.
(Invited ASPS Goldacre Paper)
(Awarded the March 2015 issue cover image).

 
Figure 1. Mature nodules of soybean roots formed in symbiosis with nitrogen-fixing Bradyrhizobium japonicum.

Figure 2. Transgenic roots of soybean plants engineered in isolation to over-express nodulation-suppressing peptide hormones and grown indefinitely in tissue culture.