Evidence for a Polyphosphatase-Like Enzyme Catalyzing the Hydrolysis of Long-Chain Polyphosphates in the Rhizosphere

Polyphosphates (poly-P), consisting of two or more phosphate residues, are not directly available to plants and must first be hydrolyzed to orthophosphate (ortho-P). Although microbial polyphosphatase activity is well established, there is currently no evidence for extracellular poly-P-hydrolyzing enzymes produced by plants in the rhizosphere. This study investigated the capacity of plants to hydrolyze and utilize long-chain and cyclic poly-P forms and sought to identify extracellular poly-P hydrolytic activity of plant origin.

Six plant species were cultivated under sterile conditions with either cyclic poly-P or ortho-P as the sole phosphorus source. Pronounced interspecific differences were observed in poly-P utilization. Lettuce exhibited limited growth on poly-P, whereas pepper achieved biomass levels comparable to those supplied with ortho-P, providing direct evidence of rhizospheric poly-P hydrolysis. Enzymatic assays using intact plant tissues revealed significantly higher hydrolytic activity in pepper roots than in lettuce, while leaves showed minimal activity in both species.

Protein extracts from pepper roots were further analyzed to characterize the enzymatic activity. Poly-P hydrolysis was abolished by heat treatment, confirming enzymatic involvement. Fractionation by fast protein liquid chromatography (FPLC) led to the isolation of an approximately 20 kDa protein displaying strong poly-P hydrolytic activity, exceeding that of known plant phosphatases. The enzyme preferentially hydrolyzed shorter poly-P chains, with activity declining as chain length increased.

These findings provide the first evidence for a polyphosphatase-like enzyme in vascular plants. The identification of an extracellular, root-derived enzyme capable of hydrolyzing long-chain poly-P challenges the prevailing paradigm that plants rely exclusively on soil microorganisms for the conversion of complex polyphosphates into bioavailable forms.