Smart Plants!

The trend these days is to have smart houses and smart phones and other “smart” things. But many forget that there are some really smart plants too… namely legumes!

They are smart because they can utilize nitrogen from the atmosphere, where it is found at concentrations as high as 80%, to cover their nutritional needs. This unique ability of legumes among cultivated plants arises from their capability to form symbiotic relationships with some bacteria species, commonly called rhizobia. The rhizobia possess an enzyme called nitrogenase which catalyzes the conversion of aerial nitrogen into plant-available N forms. The plants can utilize most of this nitrogen and in return they provide energy through photosynthesis to the bacteria. Thus, legumes do not need nitrogen fertilization as soon as they form symbiotic relationships with rhizobia. In contrast, in non-legume crops, the farmers add synthetic fertilizers to replenish the nitrogen removed from the soil through the harvested products. Unfortunately, the production of synthetic nitrogen fertilizers entails high energy consumption and extensive greenhouse gas (GHG) emissions, while their use results in excessive runoff of nitrates into water reservoirs.

TRUE Case Study 21 at the Agricultural University of Athens uses beans and grafting technique to obtain efficient rootstock/scion genotypes in terms of symbiotic nitrogen fixation and GHG emissions. The most important plant part in legume–rhizobia symbiosis is the shoot, which provides photosynthates to support the nodules, and produces signal molecules sent to the root. Grafting produces plants with different genotypes in root and shoot, while it is an excellent tool to investigate shoot signal essential for controlling root nodule formation and development. The process of nodulation is costly to the plant in terms of energy and resources required to form and maintain the nodules. To balance these costs with the benefit of acquiring fixed nitrogen, the plants limit the number of nodules via a systemic feedback inhibition signaling network called the autoregulation of nodulation.

Symbiotic N-fixation is dependent on the rootstock and scion genotype, the Rhizobium strain, the availability of N in the root zone, and several environmental conditions. Having these in mind, the first experiment conducted at the greenhouse of the Agricultural University of Athens was aimed to find the best nutrient solution (concerning nitrogen concentration) for successful bacteria inoculation and possible bacteria combinations, while the goal of the current experiments is to identify compatible rootstock/scion combinations.