Microbial Colonization of the Host Plant: Cellular and Molecular Mechanisms of Symbiosis

Abstract

Nitrogen is an essential element for all plants, animals, and microorganisms in the Earth’s biosphere. For millions of years, nitrogen was available through soil minerals; dead organisms; and biological nitrogen fixation by microorganisms, with symbiotic nitrogen fixation in particular being the most efficient source. Humans have used legumes and symbiotic nitrogen fixation in all continents for thousands of years without knowing anything about symbiosis. Back in Roman times, in the literature on soil improvement, the positive impact on yields and soil quality after planting legumes was already noted. The 20th century industrial technology of chemical synthesis increased the availability of nitrogenous fertilizers. Thanks to this, crop yields improved significantly, and the human mortality rate from starvation reduced. Nitrogen fertilizers are expensive and not quite ecologically friendly, but, considering the size and growth of the human population, they are strictly necessary. A trend in technology of recent times has been the search for environmentally friendly methods to replace the “dirty” technologies used since the beginning of the technological era. This has led to the development of research in biological nitrogen fixation [1]. Over the past 30–40 years, studies on legume–rhizobium symbiosis have obtained a vast amount of information regarding the structure of root nodules and the dynamics of symbiosis formation. The scientific community working in this area of research was fascinated by the logic of the successive steps in the establishment of symbiosis and by the cytological beauty of root nodules, as it was almost like Newton’s celestial mechanics. The advent of new methods of genetics and molecular biology revealed complexity in seemingly simple processes such as inoculation, plant selection of microsymbionts, and nodule formation. In return, researchers gained the ability to manipulate both plant and bacterial genes instead of being passive spectators [1,2]. Major efforts have been made to decipher Nod factor (NF) signaling pathways and the mechanisms of the perception of NF in susceptible regions of young roots, as well as the regulation of nodule formation [2,3,4].