Plant hormones play a vital role in plant immune responses. However, in contrast to the relative wealth of information on hormone-mediated immunity in dicot plants, little information is available on monocot-virus defense systems. We used a high-throughput-sequencing approach to compare the global gene expression of Rice black-streaked dwarf virus (RBSDV)-infected rice plants with that of healthy plants. Exogenous hormone applications and transgenic rice were used to test RBSDV infectivity and pathogenicity. Our results revealed that the jasmonic acid (JA) pathway was induced while the brassinosteroid (BR) pathway was suppressed in infected plants. Foliar application of methyl jasmonate (MeJA) or brassinazole (BRZ) resulted in a significant reduction in RBSDV incidence, while epibrassinolide (BL) treatment increased RBSDV infection. Infection studies using coi1-13 and Go mutants demonstrated JA-mediated resistance and BR-mediated susceptibility to RBSDV infection. A mixture of MeJA and BL treatment resulted in a significant reduction in RBSDV infection compared with a single BL treatment. MeJA application efficiently suppressed the expression of BR pathway genes, and this inhibition depended on the JA coreceptor OsCOI1. Collectively, our results reveal that JA-mediated defense can suppress the BR-mediated susceptibility to RBSDV infection.

Net photosynthetic rate (PN), photorespiration (PR), chlorophyll (Chl) content, Chl fluorescence parameters, starch accumulation, and related key enzyme activities were determined during the grain-filling stage in two autotetraploid lines and corresponding diploid rice lines. The results showed that autotetraploid rice lines had a higher Chl content, PN, electron transport rate, maximum photochemical efficiency of PSII, actual photochemical efficiency of PSII, and lower PR in leaves than that in corresponding diploid rice lines during the grain-filling stage. It indicated that autotetraploid rice line had a high photosynthetic capacity and high light-utilization efficiency. The activities of ADP-glucose pyrophosphorylase, soluble starch synthase, and starch-branching enzyme in grains of autotetraploid rice lines were higher than those in grains of corresponding diploid rice lines during the grain-filling stage. Therefore, autotetraploid rice lines were more efficient than corresponding diploid rice lines in converting photosynthetic products into starch.

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