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 sp ontaneous development of sequence deletions in SBWMV RNA 2 were monito red in individual plants of Triticum aestivum cultivars Galahad and Vo na 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. g raminis 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 mechanic al transfers to healthy plants, higher temperatures resulted in trunca ted forms of SBWMV RNA 2. Our observations confirm that only full-leng th SBWMV RNA 2 is transmitted to wheat roots by viruliferous P. gramin is from field soil and that there is no intraplant barrier to the move ment of deleted forms of RNA 2 between roots and leaves. Deleted forms of SBWMV RNA 2 appear to cause more severe symptoms only after mechan ical inoculation to young, healthy plants. The significance of these o bservations 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 mecha nism of spontaneous RNA deletion.