Potyviral helper-component proteinase expressed in transgenic plants enhances titers of Potato leaf roll virus but does not alleviate its phloem limitation

Coinfection of Nicotiana benthamiana with Potato virus A (PVA, a potyvirus) and Potato leaf-roll virus (PLRV, a luteovirus) induces a synergistic inte raction manifested by enhanced titers of PLRV. The helper component protein ase (HC-Pro) of potyviruses is involved in viral vascular movement and supp ression of an antiviral defense mechanism in plants. Data of our study show ed that accumulation of PLRV in transgenic Ri. benthamiana expressing the W A HC-Pro was enhanced on average by 4.5-fold, as compared to a 6.0-fold enh ancement in wild-type N. benthamiana plants doubly infected with PVA and PL RV. Enhancement of PLRV accumulation was directly proportional to the conce ntration of the HC-Pro in leaves. In the HC-Pro-transgenic plants and wild- type plants, PLRV was almost exclusively confined to the phloem, but the HC -Pro-transgenic plants had a fourfold greater number of PLRV-infected cells within the phloem tissues, as revealed by immunohistochemical staining, in the leaves doubly infected with PLRV and PVA, PLRV was found to exit the p hloem in 25.0% of the veins, infecting all types of leaf cells, but, on ave rage, PLRV accumulation was not enhanced more than by sixfold at the whole- leaf level. Therefore, potLvirallluteoviral synergism seems to be based on two mechanisms. One of them is mediated by the HC-Pro and increases luteovi rus accumulation without allowing detectable egress from vascular tissue. T he other mechanism probably depends on additional potyviral proteins and al leviates the normal phloem limitation of PLRV, (C) 2001 Academic Press.