Evidence for a polyphosphatase-like enzyme catalyzing the hydrolysis of long-chain polyphosphates in the rhizosphere

Polyphosphates (poly-Ps), composed of two or more phosphate units, become plant-available only after hydrolysis to orthophosphate (ortho-P). While microbial polyphosphatase enzymes are well documented, no evidence exists for extracellular poly-P-hydrolyzing enzymes secreted by plants into the rhizosphere. This study aimed to evaluate plant capacity to hydrolyze long-chain and cyclic poly-P forms and to identify extracellular hydrolytic activity.

Six plant species were grown in sterile media supplemented with either cyclic poly-P or ortho-P to assess their capacity to hydrolyze and utilize different P sources. Species varied markedly in their ability to use poly-P. Lettuce displayed poor growth, while pepper achieved biomass levels comparable to ortho-P, providing direct evidence of rhizospheric hydrolytic activity. Hydrolysis assays using intact tissues confirmed significantly higher activity in pepper roots compared to lettuce, with leaves showing the lowest activity in both species.

Protein extracts from pepper roots were assayed for enzymatic activity. Heat treatment eliminated hydrolysis, confirming enzymatic mediation. Liquid chromatography enabled the isolation of a ∼20 kDa protein exhibiting high poly-P hydrolytic activity, exceeding that of known plant phosphatases. Mass spectrometry of the active fraction identified a Capsicum annuum protein (STH-21) with no close bacterial homologs, supporting its plant origin. The active fraction showed strong poly-P hydrolysis, with efficiency declining as chain length increased.

This study provides the first evidence of a polyphosphatase-like enzyme in vascular plants. The discovery of an extracellular, root-derived enzyme capable of long-chain poly-P hydrolysis challenges the prevailing view that plants depend solely on soil microorganisms for hydrolyzation of complex poly-Ps.