Fighting Plant Diseases Through the Application of Bacillus and Pseudomonas Strains

Abstract

Plant growth-promoting bacteria (PGPBs) are capable of colonizing plants and influencing their growth by direct or indirect mechanisms. The direct mode of action occurs when metabolites or compounds synthesized by microorganisms are provided to the plant- for example phytohormones- or when the bacteria facilitate the plant?s uptake of certain nutrients from the environment. In the indirect form of promotion, bacteria protect the plant against phytopathogenic organisms through the induction of systemic resistance and/or by the synthesis of antimicrobial compounds. The use of beneficial microorganisms as biopesticides offers a promising alternative to the use of chemical pesticides and an environmentally friendly strategy for agriculture. The PGPBs most studied and exploited as biocontrol agents are the species of Bacillus and fluorescent Pseudomonas. These strains produce a wide variety of metabolites involved in the biologic control of phytopathogenic fungi -for example, extracellular enzymes, siderophores, antibiotics, hydrogen cyanide and volatile organic compounds, among others. Antibiosis is one mechanism of biologic control that is well-characterized in Bacillus and Pseudomonas strains both genetically and biochemically. Among antibiotics identified in these two genera are include the cyclic lipopeptides such surfactin, iturin and fengycin in the bacilli and phenazines, 2,4-diacetyl phloroglucinol, pyoluteorin and pyrrolnitrin in the pseudomonads. This chapter focusses on the ability of these PGPR to improve plant health in addition to the mechanisms involved in their biocontrol capabilities.