Abstract
The ability to solubilize insoluble inorganic pho- sphate compounds by Gluconacetobacter diazotrophicus was studied using different cul-ture approaches. Qualitative plate assays using tricalcium phosphate as the sole P-source showed that G. diazotrophicus produced solu-bilization only when aldoses were used as the C-source. Extracellular aldose oxidation via a pyrroloquinoline quinone-linked glucose dehy-drogenase (PQQ-GDH) is the main pathway for glucose metabolism in G. diazotrophicus. In batch cultures with 5 g l-1 of hydroxyapatite as the P-source and glucose as the C-source, more than 98% of insoluble P was solubilized. No solubilization was observed neither using glyc-erol nor culturing a PQQ-GDH mutant of G. di-azotrophicus. Solubilizaton was not affected by adding 100 mmol l-1 of MES buffer. Continuous cultures of G. diazotrophicus showed significant activities of PQQ-GDH either under C or P limi-tation. An intense acidification in the root envi-ronment of tomato and wheat seedlings inocu-lated with a G. diazotrophicus PAL5 was ob-served. Seedlings inoculated with a PQQ-GDH mutant strain of G. diazotrophicus showed no acidification. Our results suggest that G. di-azotrophicus is an excellent candidate to be used as biofertilizer because in addition to the already described plant growth-promoting abili-ties of this organism, it shows a significant mineral phosphate solubilization capacity.
Highlights
Phosphorus (P) is after Nitrogen, the most important nutrient limiting agricultural production
When plates were inoculated with strain MF105 (PQQ-GDH (-) mutant) neither the pyrroloquinoline quinone-linked glucose dehydrogenase (PQQ-GDH) substrates nor the other carbon sources showed a mineral phosphate solubilization (MPS) (+) phenotype
PQQ-GDH of G. diazotrophicus presented the common behaviour of a broad-substrate aldose dehydrogenase as reported for others PQQ-GDH [20]
Summary
Phosphorus (P) is after Nitrogen, the most important nutrient limiting agricultural production. Soils are often abundant in insoluble P, either in organic or inorganic forms, but deficient in soluble phosphates essential for growth of most plants and microorganisms. Converting soil insoluble phosphates (both organic and inorganic) to a form available for plants is a necessary goal to achieve sustainable agricultural production. A significant body of evidence has been developed to show that in gramnegative bacteria the expression of a direct extracellular oxidative pathway allows superior MPS capabilities [2]. Through this pathway ( called nonphosphorylating oxidation) glucose is oxidized to gluconic acid and 2-ketogluconic acid directly in the periplasmic space. On the other hand it has been reported that the buffering capacity of soils could limit P solubilization by microorganisms [4,5]
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