EFFECT OF CULTIVATION TEMPERATURE ON THE SPONTANEOUS DEVELOPMENT OF DELETIONS IN SOILBORNE WHEAT MOSAIC FUROVIRUS RNA-2

Intraplant movement of soilborne wheat mosaic virus (SBWMV) and the spontaneous development of sequence deletions in SBWMV RNA 2 were monitored in individual plants of Triticum aestivum cultivars Galahad and Vona inoculated mechanically or with viruliferous zoospores of Polymyxa graminis and grown at different temperatures. Movement of SBWMV RNAs 1 and 2 (either full-length or deleted forms) from foliage to roots was detected 2 days after mechanical inoculation. Plants infected by P. graminis and maintained at high temperatures (25-30 C) showed extensive deletions in the readthrough (RT) domain of the coat protein-RT gene within 4-12 wk. In contrast, plants kept at 17 C over the same period contained only full-length RNA 2 molecules (3,593 nucleotides). Thus, in addition to prolonged cultivation at 15-17 C and/or serial mechanical transfers to healthy plants, higher temperatures resulted in truncated forms of SBWMV RNA 2. Our observations confirm that only full-length SBWMV RNA 2 is transmitted to wheat roots by viruliferous P. graminis from field soil and that there is no intraplant barrier to the movement of deleted forms of RNA 2 between roots and leaves. Deleted forms of SBWMV RNA 2 appear to cause more severe symptoms only after mechanical inoculation to young, healthy plants. The significance of these observations for events in fields of winter wheat or barley, especially those expressing a temperature-sensitive host resistance gene against SBWMV, are discussed. These studies may also help elucidate the mechanism of spontaneous RNA deletion.