Silica deposition by a strongly cationic proline-rich protein from systemically resistant cucumber plants

Infection of one leaf of cucumber (Cucumis sativa) plants can render other leaves resistant to various pathogens. This so-called systemic acquired resistance (SAR) can be functionally mimicked by certain chemicals. All these treatments enhanced expression of a gene encoding a novel proline-rich protein (PRP1) which has C-terminal repetitive sequences containing an unusually high amount of lysine and arginine residues. Antibodies raised against a synthetic peptide derived from four of the repetitive sequences cross-reacted mainly with a cell wall polypeptide of an apparent MW of 8 kDa. The protein accumulated upon SAR induction, though it does not appear to take part in oxidative protein cross-linking, at least in the hypocotyl tissue. The synthetic peptide derived from the repetitive sequences was able to polymerize orthosilicic acid to insoluble silica, a property not resulting directly from the primary protein sequence, but rather from the high positive charge density. Our results suggest that during induction of SAR, the synthesis of a strongly cationic PRP prepares the cell walls for enhanced silica deposition which is known to participate in cell wall reinforcement, localized at the site of attempted penetration of fungi into epidermal cells. Constitutive accumulation of related PRPs may function in silica deposition during certain developmental stages, a process important for various physiological functions of green plants.