Cytokinins Act Synergistically with Salicylic Acid to Activate Defense Gene Expression in Rice

Hormone crosstalk is pivotal in plant–pathogen interactions. Here, we report on the accumulation of cytokinins (CK) in rice seedlings after infection of blast fungus Magnaporthe oryzae and its potential significance in rice–M. oryzae interaction. Blast infection to rice seedlings increased levels of N 6 -(Δ 2 -isopentenyl) adenine (iP), iP riboside (iPR), and iPR 5′-phosphates (iPRP) in leaf blades. Consistent with this, CK signaling was activated around the infection sites, as shown by histochemical staining for β-glucuronidase activity driven by a CK-responsive OsRR6 promoter. Diverse CK species were also detected in the hyphae (mycelium), conidia, and culture filtrates of blast fungus, indicating that M. oryzae is capable of production as well as hyphal secretion of CK. Co-treatment of leaf blades with CK and salicylic acid (SA), but not with either one alone, markedly induced pathogenesis-related genes OsPR1b and probenazole-induced protein 1 (PBZ1). These effects were diminished by RNAi-knockdown of OsNPR1 or WRKY45, the key regulators of the SA signaling pathway in rice, indicating that the effects of CK depend on these two regulators. Taken together, our data imply a coevolutionary rice–M. oryzae interaction, wherein M. oryzae probably elevates rice CK levels for its own benefits such as nutrient translocation. Rice plants, on the other hand, sense it as an infection signal and activate defense reactions through the synergistic action with SA.