Phosphate solubilizing microorganisms vs. phosphate mobilizing microorganisms: What separates a phenotype from a trait?

Abstract

Soils are often high in insoluble mineral phosphates but deficient in the soluble orthophosphate (Pi) essential for the growth of most plants and microorganisms. In agricultural crop production, phos- phorous is second only to nitrogen in importance as a fertilizer amendment so that phosphorus fertilizers are the world’s second largest bulk agricultural chemical and, therefore, the second most widely applied chemical on Earth. There is a broad spectrum of mineral phosphate chemistries; but in arid to semiarid soils the predominant forms are the calcium phosphates. Calcium phosphates are soluble to varying degrees in the presence of the wide array of organic acids produced by microorganisms. Other biosol- ubilization mechanisms exist as well, so that conversion of mineral phosphates to Pi is generically attributed to microorganisms in most representations of global P cycling. With respect to plant growth, some workers have postulated that associations between plant roots and mineral phosphate solubilizing (MPS) microorganisms could play an important role in phosphorus nutrition in many natural and agroecosystems. As a result, an enormous amount of research has been conducted over the last 100 years involving isolation and characterization of MPS microorganisms from many soils with the goal of developing P biofertilizers that would accomplish much the same function as biological nitrogen fixation. To date, the results of these efforts have been problematic. In this review, we will attempt to identify the variables of state with respect to the MPS phenomenon in bacteria and briefly summarize the challenges that confront this field of research. Finally we will discuss our observation that, in Gram-negative rhizobacteria, extracellular oxidation of glucose to gluconic acid and 2)ketogluconic acid via the direct oxidation pathway provides the biochemical basis for highly efficacious calcium phosphate solubilization and may, in fact, be the basis for the evolution of mutualistic plant–bacteria relationships in some phosphate-limited soil ecosystems.