Characterisation of the effects of a major QTL of the partial resistance to Rice yellow mottle virus using a near-isogenic-line approach

Two quantitative trait loci (QTL12 and QTL7) acting in epistatis and associated with Rice yellow mottle virus (RYMV) infection were introgressed from the traditional Oryza sativa japonica cultivar Azucena into the improved indica cultivar IR64. The response to RYMV infection was followed over time in this nearly isogenic line (NIL). In leaves, the virus coat protein content was detected by ELISA and RNA by RT-PCR. In tissues, the coat protein was localised by immuno-fluorescence labelling and RNA by in situ hybridisation. We found (i) that Azucena combined a tolerance-characterised by mild symptoms despite generalised distribution and large accumulation of the virus-and a partial resistance, (ii) that partial resistance under the control of QTL12 and QTL7, dissociated in the NIL from tolerance and any other morphological or physiological Azucena traits, was expressed similarly in a japonica and in an indica genetic background, (iii) that partial resistance was transient and consisted in a 1-week delay in virus accumulation and symptom expression; passed this delay, partial resistance brokedown, and (iv) a similar delay in virus detection was observed at tissue levels, in the vascular bundle-sheath layers. Impaired virus movement caused by the absence or mutation of a QTL12-encoded plant factor necessary for virus transport through vascular bundle sheaths is proposed to explain this partial resistance. (Résumé d'auteur