Bloody plants! How soybean uses haemoglobins to obtain nitrogen

What do you think of when you hear the word haemoglobin? If you’re anything like me then blood, oxygen, iron and medical tests come to mind. However, there is much more to the story of haemoglobins than vertebrate circulatory systems. Haemoglobins and similar molecules to them are found across a variety of other organisms, including in plants. A subset of the haemoglobins found in plants, known as leghaemoglobins, are known to be essential for the commercially and ecologically-important symbioses formed by legumes with nitrogen-fixing bacteria. In their recent paper in Annals of Botany, Du, Gao and colleagues from Fujian Agriculture and Forestry University in China investigate the presence and expression of haemoglobin genes in soybean. The data they present highlight one of the ways soybean likely maximises the efficiency of the nitrogen-fixing symbioses it forms, and how this interaction can be helped to function best in the field.

Leghaemoglobins are produced in the nitrogen-fixing nodules of leguminous plants. They function to deliver low amounts of oxygen to the nitrogen-fixing bacteria in these nodules. This process is tightly controlled as too much oxygen inhibits proper nitrogen fixation. These leghaemoglobins give nitrogen-fixing nodules of many legume species a slight red-pink colour. However, in old nodules, green leghaemoglobins are also produced and are associated with nodule senescence (death). These green leghaemoglobins may be a breakdown product of the red leghaemoglobins.

In order to understand the lifecycle of soybean nodules, Du, Gao and colleagues monitored various nodule properties as they developed. Concurrent with other results, they find that red pigmentation in nodules peaked alongside high nodule growth. Nitrogen fixing activity also peaked alongside red pigmentation and nodule growth at around 30 days after inoculation. Nodule nitrogen fixing activity subsequently decreased, and was accompanied by green leghaemoglobin production. Nodule senescence is associated both with nodule ageing and with high availability of external nitrogen sources. The authors confirm that this for soybean nodules and find that in high nitrogen availability nodule produce high amounts of green leghaemoglobin and have low nitrogen fixation levels.

To understand more about how soybean may fine tune nitrogen acquisition in varying external nitrogen availabilities, Du, Gao and colleagues identify seven haemoglobins encoded in the soybean genome. Five of these have the characteristics of leghaemoglobins. The authors find four of these genes to be highly expressed in nodules, consistent with roles in supporting proper nitrogen fixation. The expression of these four genes peaked at, or a few days before, the point of maximum nitrogen fixation in conditions of low external nitrogen availability. In high external nitrogen availability conditions, by contrast, expression of all four genes was much reduced.

As the most commonly cultivated legume, soybean is of high importance when considering how we might optimise acquisition of nitrogen in the field by leguminous crops. When external nitrogen availability is low, soybean and other legumes use symbiotic interactions with nitrogen-fixing bacteria to obtain usable nitrogen. By contrast, when external nitrogen availability is high initiation of new nodules is reduced, expression of leghaemoglobins is reduced and existing nodules enter senescence. In other words, legumes don’t need to bother entering or maintaining symbioses to obtain usable nitrogen when it is otherwise in high availability. As Du, Gao and colleagues show, soybean is capable of dynamically responding to this upon sudden transition to high nitrogen availability, with its root nodules entering senescence. Du, Gao and colleagues identify leghaemoglobin genes in soybean with expression patterns and responses consistent with roles in supporting nitrogen fixation during low external nitrogen availability, and that these genes are down-regulated during high external nitrogen availability.

This study provides a foundation for furthering our molecular understanding of how nitrogen fixation is optimised and dynamically maintained in the arguably most important leguminous crop. It also highlights the interesting outstanding question of what other functions plant haemoglobins have aside from supporting nitrogen-fixing symbioses. Du, Gao and colleagues putatively identify two of these in soybean and other work has linked such proteins with plant responses to stress. There’s much more to haemoglobins than just blood it seems.