Epiphytic Microbes of Roots: Diversity and Significance

Microbes residing on the surface of roots are termed as root epiphytic microbes. They contribute majorly in biogeochemical mechanisms of soil. There are various approaches for deciphering root epiphytes which are either culture dependent or culture independent methods. Plants play a decisive role in selecting root epiphytic community which is driven by factors such as nature of soil, host plant, and root exudates. Root surface harbors both prokaryotic and eukaryotic microbes. Proteobacteria and Actinobacteria are major phyla present as root epiphytes. Alphaproteobacteria and Gammaproteobacteria predominate among bacteria. Pseudomonas, Burkholderia, Erwinia, Acinetobacter, and Sphingomonas are the major genera isolated from different plant root surfaces. Few other genera such as Bacillus, Agrobacterium, Streptomyces, Klebsiella, Nocardia, etc. are also found on some of the root surfaces. Fungi of class Ascomycota represents eukaryotic root epiphytic community. Epiphytes play a significant role in plant metabolism. Many epiphytic microbes produce phytohormones such as auxin, gibberellins as well as other growth promoting substances that contribute in plant growth. Root epiphytes involve in root defense via mechanisms such as biofilm formation, releasing antimicrobial substances, or competing for space and nutrients with pathogens. Epiphytic microbes play a role in framing root morphology and root structure thereby molding shape of roots.