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 interaction manifested by enhanced titers of PLRV. The helper component proteinase (HC-Pro) of potyviruses is involved in viral vascular movement and suppression of an antiviral defense mechanism in plants. Data of our study showed that accumulation of PLRV in transgenic N. benthamiana expressing the PVA HC-Pro was enhanced on average by 4.5-fold, as compared to a 6.0-fold enhancement in wild-type N. benthamiana plants doubly infected with PVA and PLRV. Enhancement of PLRV accumulation was directly proportional to the concentration 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 phloem in 25.0% of the veins, infecting all types of leaf cells, but, on average, PLRV accumulation was not enhanced more than by sixfold at the whole-leaf level. Therefore, potyviral/luteoviral synergism seems to be based on two mechanisms. One of them is mediated by the HC-Pro and increases luteovirus accumulation without allowing detectable egress from vascular tissue. The other mechanism probably depends on additional potyviral proteins and alleviates the normal phloem limitation of PLRV.