Chapter 34 : Plant Growth-Promoting Agents

Abstract:

This chapter focuses on the use of the relatively small groups of plant growth-promoting agents that have been known for a long time. Among the organisms that have received most attention in the field are the bacteria Pseudomonas spp., Azospirillum spp., and Enterobacter spp., together with the fungus Trichoderma spp.; the specific illustrations of their applications are therefore considered in the chapter. The main groups of plant growth regulators are outlined. The chapter focuses on the use of the relatively small groups of plant growth-promoting agents that have been known for a long time. The synthesis of ethylene can be inhibited by the enzyme 1-aminocyclopropane-1-car boxylate (ACC) deaminase. Many of the stimulatory activities of microorganisms depend on tests that have not been characterized in chemical terms; for example, the Trichoderma growth stimulation falls into this category. This is sometimes because there has been no attempt to characterize, but it might also be that the mode of action involves nonestablished growth regulators. The chapter shows that the antibiotic-producing bacterium enhanced nodulation of pea roots by Rhizobium leguminosarum in comparison with a non-2,4-diacetylphloroglucinol (DAPG)-producing deletion mutant. It is suggested that DAPG can act as a plant hormone-like substance. It is likely that in the pool of soil microbial biodiversity there will be other microbial secondary metabolites capable of such activity which might be harvested to optimize crop productivity or microbially mediated phytoremediation processes.