Relationship among symptom severity, virus accumulation and host gene regulation in Oryza glabberrima Steud. infected with Rice tungro spherical virus

In order to identify host genes whose expression patterns were altered and could be related to the differences in symptoms observed, two genotypes of O. sativa namely Taichung Native 1 (TN1) (RTSV susceptible) and TW16 (RTSV-resistant backcross line developed from Indonesian cultivar and TN1 as a recurrent parent), and four accessions of O. glaberrima (International Rice Germplasm Collection accession numbers 96790, 104545, 96793 and 102569 were used to evaluate their reaction with Rice tungro spherical virus (RTSV). TN1 and TW16 did not exhibit any recognizable symptoms while four accessions of O. glaberrima were significantly stunted. O. glaberrima 96790, 104545 and 96793 showed moderate stunting while accession number 102569 showed severe stunting when infected with RTSV. RTSV coat protein (CP) accumulation showed that among genotypes examined, accession number 96790 had the highest level of virus concentration. This observation did not correlate with the degree of symptom observed suggesting the symptom security in RTSV-infected plants is not simply related to the level of RTSV tiler per se and that other factors may be involved in symptom development processes. From six genotypes examined, three rice genotypes (TN1, Acc. numbers 96793 and 102569) with varying degree of response to RTSV infection were further examined for the gene expression analysis. Changes in gene expression revealed that expression of many genes involved in synthesis of cell wall as influenced by RTSV infection. Genes belonging to cellulose synthase (CESA5 and CESA6), cellulose synthase-like families (CSLA9 and CSLC7) and expansin (alpha-expansin 1 and 15 were remarkably suppressed in RTSV-infected plants showing stunted growth. The degree of changes in the expression intensities of genes examined is well correlated with symptom severity observed suggesting that efficient and persistent down regulation of cell wall-related genes involve in plant growth and development may be an important component of symptom development in certain host-plant virus interactions.