The post this week is part of a blog-swap with Sarah Shailes (@SarahShailes) of the Plant Scientist blog. You can read my post on plant defences against bacteria over at her blog.
Members of the legume family of plants (e.g. peas, soybean) can form symbioses with nitrogen-fixing bacteria known as rhizobia. In return for receiving nitrogen-containing compounds (e.g. ammonia) from the rhizobia, the plant supplies the rhizobia with sugars and a home in special organs in the plant root called nodules. To set up a symbiosis communication between the legume and rhizobia is required. The plant releases flavonoids and the rhizobia responds by producing Nod factors (NFs). The NFs activate signalling pathways in the plant root that lead to nodule formation and infection of the rhizobia. The whole process is known as nodulation.
Interestingly, the flavonoids released by the plant can also induce the activation of the type III secretion system in rhizobia (1). Type III secretion systems are commonly found in pathogenic bacteria where they act like a needle to inject “effector” molecules into the host to subdue defences during infection. So, what role is it playing in nodulation?
The evidence so far offers a mixed picture. With different bacteria-plant host pairings type III secretion systems seem to be able to help or hinder nodulation (2). Any hindrance of nodulation is probably caused by the plant host identifying effectors injected by the rhizobia as “dangerous” and the activation of plant defence. But how can effectors promote nodulation?
In a recent paper published in PNAS the authors studied the symbiosis between soybean and Bradyrhizobium elkani (2). B. elkani type III secretion (T3S) mutants formed fewer nodules on wildtype soybean than wildtype B. elkani. When NF signalling was abolished by using a plant NF receptor mutant (nfr1) and a B. elkani nodC mutant (cannot make NFs) there were even fewer nodules. Virtually no nodules formed at all in the soybean nfr1 mutant when a B. elkani nodC/T3S double mutant was used. The promotion of nodulation by type III secretion is therefore independent of NF recognition. Therefore, for soybean and B. elkani to form a symbiosis the NF signalling pathway or type III secretion system is required, but preferably both.
The group also carried out a microarray study of gene expression in soybean roots during nodulation. They found that two genes well-known for being up-regulated during NF signalling in legumes (NIN and Enod40) are also up-regulated by the rhizobial type III secretion system (2). So the effector(s) injected into soybean activate nodulation signalling downstream of NF recognition.
So the ability of a legume and a rhizobium species to form a symbiosis can be influenced positively or negatively depending on the effectors released by the type III secretion system. This is an exciting development and the research community are looking forward to finding out the identity of the effector(s) and their target(s) in legumes.
1. Wassem et al. (2008). TtsI regulates symbiotic genes in Rhizobium species NGR234 by binding to tts boxes. Molecular Micobiology.
2. Okazaki et al. (2013). Hijacking of leguminous nodulation signalling by the rhizobial type III secretion system. PNAS.
Don't forget to check out my post: Barring the gates: How plants defend against invading bacteria