Interaction of endophytic microbes with legumes

Abstract

Large numbers of bacterial and fungal endophytes have been reported from different plant tissues: roots, nodules, leaves, flowers and sprouts of legumes, with numbers ranging from few to more than 150. Endophytes can accelerate seedling emergence, promote plant establishment under adverse conditions and enhance plant growth. Endophytic microbes promote plant growth by helping plants in acquiring nutrients, e.g. via nitrogen fixation, phosphate solubilization or iron chelation, by preventing pathogen infections via antifungal or antibacterial agents, by outcompeting pathogens for nutrients by siderophore production, or by establishing the plant's systemic resistance. Further growth promotion is affected by producing phytohormones such as auxin or cytokinin, or by producing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which lowers plant ethylene levels. For establishment of endophytes in different tissues, endophytic microbes must be compatible with the host plants and able to colonize the tissues of the host plants without being recognized as pathogens. A particular bacterium or fungus may affect plant growth and development using one or more of these mechanisms, and they may use different mechanisms at various times. The population density of endophytes is highly variable, depending mainly on the microbial species and host genotypes, developmental stage and environmental conditions. Genotypic and cultivar specific endophytes have also been reported. The quantum benefit derived by plants from an endophyte and vice versa is still not clear. It seems that the endophytic genus or species best adapted for living inside a plant is naturally selected. Here, we concentrate on soil or rhizosphere-derived endophytes recruited out of a large pool of soil or rhizospheric microbes. Some endophytes are more aggressive colonizers and displace others, but seeming lack of strict specificity has been observed. However, the processes of host-microbe signaling and colonization and the mechanisms leading to mutual benefits are less-well characterized. It is still not clear which population of microorganisms (endophytes or rhizospheric) promotes plant growth and the way the interactions among endophytes influence plant productivity. Though attempts to know the molecular ecology and interactions are underway, a high amount of progress is required to fully understand the mechanism of establishment, the way interactions take place in planta, between different microbes and plants and exlusive benefits by endophytes and plants.

A large diversity of non-rhizobial endophytes found in legume root nodules in Flanders (Belgium)

“…Previous studies indicate that endophytes can have three main functions inside root nodules, firstly as true rhizobial endosymbionts capable of inducing nodulation and performing nitrogen fixation (Valverde et al, 2005;Ardley et al, 2012). Secondly, as helper bacteria increasing plant health and/or yield when co-inoculated with rhizobia (Annapurna et al, 2013;Tariq et al, 2014), and finally, as opportunistic endophytes thriving in this nitrogen riche nodule environment (Dudeja et al, 2012). Therefore, nodules seem to be an excellent environment for a wide variety of bacteria.…”

“…S1). As Dudeja et al (2012) mentioned, most endophytes originate from the rhizosphere or phyllosphere and only a small number may be transmitted via seed. We postulate that the host plant may select the appropriate NRE from the environment and hence characteristic NRE for certain legume species were found.…”

“…However, our study reveals an even more complex interaction between traditional rhizobia and NRE bacteria for legumes growing in the temperate climate in Western Europe. Most of the plants harboured Bacillus and Pseudomonas species in their root nodules together with traditional rhizobia, which is becoming a common observation (Dudeja et al, 2012). The function of these NRE as helpers still needs to be confirmed but the fact that these specific co-occurrence patterns exist, might suggest that not only the plant selects certain NRE but also the rhizobia inside the root nodules prefer certain NRE.…”

Beneficial Bacillus Strains Improve Plant Resistance to Phytopathogens: A Review

“…Some endophytes showed specificity for a genotype or even cultivar [10]. In many cases, the biocontrol bacteria found in association with cultivated plants have an increased antagonistic potential against those phytopathogens specific for the same host.…”

“…However, due to the large mixture of colonizers and complex mechanisms of plant-microbe interactions it is not conclusively clear which bacteria is more involved in the plant benefit [10]. Since all PGPB species that colonize the host contribute to the wellness of the plant, are involved in a complex interrelation, it is quite difficult to differentiate each beneficial rate.…”