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The Bundle Sheath-Phloem Interface of Cucumis sativus Is a Boundary to Systemic Infection by Tomato Aspermy Virus

The progress of infection of two cucumoviruses in cucumber plants was analyzed immunohistochemically. Strain Fny of cucumber mosaic virus (CMV, FFF) was found to infect cucumber tissues systemically by 6 days postinoculation (dpi), while a reassortant virus with RNAs 1+2 of Fny-CMV plus RNA3 of strain 1 of tomato aspermy virus (FFT) was unable to move long distance and infect cucumber plants systemically. FFF infection of the vasculature was detected 6 dpi in the phloem of a low percentage of both minor (order VII--VI) and major (order V--IV) veins. At 9 dpi, infection was detected in phloem cells of about 50% of both minor and major veins. FFT colonization of inoculated cotyledons followed a pathway similar to that of FFF, but virus accumulation was never detected in vascular tissues. In minor or major veins, FFT infection was arrested at the bundle sheath (BS), and at 9 dpi was not detected in intermediary or other phloem cells. Thus, our data indicate that the BS-phloem interface is a boundary for the systemic movement of these viruses in cucumber, and provide evidence of a functional difference between the plasmodesmata connecting mesophyll cells, as well as mesophyll and BS cells, which allow the movement of both FFF and FFT, from the plasmodesmata connecting BS and phloem, which allow movement of FFF but not of FFT.