First report of cDNA clone-launched infection of maize plants with the polerovirus maize yellow mosaic virus (MaYMV).

An East African isolate of the maize-associated polerovirus, maize yellow mosaic virus (MaYMV) was previously shown to cause leaf reddening on singly infected maize plants (Zea mays). Here we describe the construction of a full-length infectious clone of an East African isolate and, for the first time, show infectivity of clone-derived transcripts in the primary host, maize, through vascular puncture inoculation (VPI), as well as in the dicotyledonous research model plant species, Nicotiana benthamiana, through agrobacterium inoculation. Characteristic leaf reddening symptoms were observed in a subset of maize plants inoculated with clone-derived transcripts, and infection was confirmed by RT-PCR and Northern blot analyses. In N. benthamiana plants, infections were entirely asymptomatic even at high virus titers, as was also reported for the cloned Chinese isolate. In this study, however, we demonstrated that N. benthamiana can serve as a clone launching platform for maize infection, as VPI of sap of infected N. benthamiana plants into maize kernels resulted in infection and the typical red leaf symptoms. We further demonstrated that the cloned East African isolate virus was aphid transmissible to maize, with experimental transmission rates up to 97 %, comparable to that shown previously for the native virus. Interestingly, our data additionally showed a definitive correlation of leaf reddening symptoms with increased expression of chalcone synthase, thus suggesting upregulation of the flavonoid biosynthesis pathway as the molecular basis for symptom induction in maize. As the first report of experimental infection of maize with transcripts from a cloned polerovirus, this work constitutes a breakthrough for studies on molecular maize-polerovirus-aphid interactions.