Oxford’s Department of Plant Sciences conducted a study on the ability of pea plants to allocate resources efficiently. The study found that pea plants are able to allocate sugar to symbiotic bacterial partners based on their effectiveness and conditional on the availability of better alternatives. The researchers hope that their insights might help reduce the need for artificial nitrogen-based fertilisers.

As global agriculture intensifies, the demand for nitrogen-based fertilisers is set to increase. They are produced by the Haber process, which uses natural gas to produce ammonia. The Haber process is highly energy-intensive, contributing to climate change as fossil fuels are burned to provide power. In addition, the use of these fertilisers has damaging effects on local ecosystems, creating ‘dead zones’ and harmful algal blooms.

Nitrogen is an essential nutrient for all plants, and is the limiting factor for much of global plant growth. However, most plants cannot absorb nitrogen from the air and are only able to absorb it from the soil via their roots. This is where legumes such as pea plants come in. 

Legumes are able to form mutually-beneficial (symbiotic) relationships with bacteria in the soil called rhizobia bacteria. These bacteria are able to convert nitrogen into ammonium, which plants are able to absorb and use – a process referred to as “fixing nitrogen”. In return, the legumes house the bacteria in specialised root nodules, and supply them with energy-rich sugars.

Most pea plants house multiple different rhizobia (soil bacteria) strains, which vary in their ability to fix nitrogen. Having only a limited sugar supply, pea plants need to ‘choose’ which strains to supply. 

Before the study, researchers already knew that pea plants cut off the sugar supply to non-fixing strains. However, there had been no investigation into the allocation between strains with different levels of effectiveness.

To research this allocation process, the researchers treated the plants with a genetically engineered strain with an intermediate ability to fix nitrogen. The pea plants responded to this strain differently based on the other available options: if only worse fixers were available, the plant supplied a lot of sugar and the bacteria-housing nodule grew large; if a better fixer was available, the sugar supply was cut off, and the nodule shrunk. 

However, sugar was not simply allocated in proportion to the nitrogen supplied. Instead, less efficient rhizobia strains were sanctioned early on when better strains became available, suggesting that pea plants have a sort of mechanism allowing them to compare the effectiveness of different bacteria.

Professor Phil Poole, a co-supervisor of the study said: “Understanding how plants manage their interactions with bacteria could help us select plants which are better at choosing effective bacterial partners. This could reduce the demand for nitrogen fertilisers”.

Dr Lindsay Turnbull, another co-supervisor, said: “This is a key development as previous research in this area used naturally-occurring bacteria which may have differed in many characteristics. In this study, the bacterial strains were genetically altered to provide different levels of nitrogen, so we can be sure that changes in the plant’s response are due to differences in their ability to supply nitrogen”.

Image credit Mikołaj Idziak on Unsplash

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