Akshit Puri

Paenibacillus polymyxa: A Prominent Biofertilizer and Biocontrol Agent for Sustainable Agriculture

Agricultural practice is currently moving from traditional chemical fertilizers and pesticides toward sustainable and environment-friendly biofertilizer and biocontrol agents. Paenibacillus polymyxa (previously Bacillus polymyxa) is an agriculturally important microbe widely studied for its plant growth-promoting abilities. P. polymyxa is an endospore-forming bacterium that could colonize a range of ecological niches. It is commonly found in the agricultural soils, especially in close association with plants, and has been isolated from diverse geographic locations. P. polymyxa is renowned for its ability to act as a biocontrol agent against a wide array of plant pathogens. It can produce antibiotic compounds like polymyxin and antifungal compounds like fusaricidin that can suppress the growth of pathogens in both lab and field conditions. Apart from being a potent biocontrol agent, P. polymyxa strains are also known widely for their ability to fix atmospheric nitrogen, solubilize phosphate, and produce phytohormones; thus they could be used as effectual biofertilizers in commercial agriculture. The aim of this chapter is to provide an overview about both direct and indirect plant growth promotion accomplished by P. polymyxa in a wide variety of agricultural crops, through extensive reviewing of old and recent studies.

Plant Growth Promotion by Endophytic Bacteria in Nonnative Crop Hosts

Studies highlighting the colonization and plant growth-promoting ability of endophytic bacteria inoculated into nonnative plant hosts reviewed and presented in this chapter. Endophytic bacteria, especially those related to the genus Bacillus, Burkholderia, Enterobacter, Gluconacetobacter, Herbaspirillum, Paenibacillus, Pseudomonas have been reported to form endophytic colonies in roots and shoot of nonnative plant hosts. Marker genes like green fluorescent protein have also been used widely to view the sites of colonization in real time. Apart from colonizing a nonnative plant host, these endophytic bacteria are also involved in promoting host plant growth and acting as a biocontrol agent against pathogenic fungi. Such endophytes have a great potential in future for sustainable agriculture since they could be used in a range of environmental and biological conditions.

Beneficial Effects of Bacterial Endophytes on Forest Tree Species

Since their discovery, beneficial bacteria living inside the plant tissues (known as bacterial endophytes) have been studied widely in agricultural crop species. But their ecology and effects on tree species in a forest ecosystem could be very different yet intriguing. In this chapter, studies highlighting the isolation of bacterial endophytes, re-inoculation and detection of the endophytic population in the host tree, and benefits provided to the host tree through direct and indirect mechanisms have been reviewed. Important tree species including those belonging to the genus Pinus, Populus, and Picea have been reported widely to harbor bacterial endophytes belonging to the genus Bacillus, Paenibacillus, and Pseudomonas and possibly obtain benefits like nitrogen fixation and increased biomass production from them. Nitrogen-fixing bacterial endophytes are the most commonly studied beneficial microbes of forest tree species, and thus have been reviewed in detail in this chapter.