Responses of Oryza sativa japonica sub-species to infection with Rice yellow mottle virus

The distribution of Rice yellow, mottle virus (RYMV) was compared in the pa rtially resistant upland rice Oryza sativa japonica cv. Azucena and in the susceptible O. sativa indica cv. IR64, which are the two parents of a doubl ed haploid population studied for several traits, including genetic determi nants of resistance to RYMV infection. Symptom expression, distribution and accumulation of viral coat protein and nucleic acid were studied in inocul ated leaves, systemically infected leaves and spices, and in leaf host tiss ues. Initially, the resistance was apparent as delayed virus detection and multiplication which led to a lower virus content in inoculated leaves and in systemically infected organs. Later, the resistance was less apparent as virus titers became progressively similar in the two cultivars. Then, tole rance was observed, as symptom expression was less pronounced in cv. Azucen a than in IR64, despite similar virus contents. In host tissues, partial re sistance was associated with the delayed detection of virus in the bundle s heaths (mestomes) of Azucena, and, later, the mestome invasion paralleled t he decline in resistance. The mature leaves of both cultivars always escape d infection. Partial virus resistance resulted from the expression of quant itative trait locus on chromosome 12, whereas tolerance is a characteristic of O. japonica, including the expression of QTL(1). An impaired cell to ce ll movement through the mestome, possibly reinforced by slow vascular movem ent, are proposed to explain the partial resistance. (C) 2000 Academic Pres s.