Rice is an economically important crop because it is the staple food for more than half of the world's population [1]. Future population growth will necessitate increased rice production, for which cultivation techniques must be adapted to biotic and abiotic stresses [2, 3]. Previous studies have estimated that up to 40% of rice production is lost annually owing to diseases [4]. Rice blast disease, caused by Pyricularia oryzae (synonym of Magnaporthe oryzae), is a critical disease in rice cultivation that must be controlled. Numerous fungicides are used in agricultural production to control diseases. However, the emergence of fungicide-resistant strains has become a serious problem, necessitating the development and application of technologies that increase disease resistance in plants. To protect themselves from pathogenic attacks, plants have evolved unique defense mechanisms that are governed by plant hormones, such as salicylic acid (SA) and jasmonic acid (JA) [5, 6]. SA is synthesized at the site of pathogen infection and functions as a signal for inducing systemic acquired resistance (SAR) throughout the plant body [7–10]. SAR is effective against various types of pathogens and protects plants from further attacks. Chemicals that activate SAR have been developed and are widely used to control rice blast disease in rice paddy fields. Probenazole and its active metabolite, 1, 2-benzisothiazol-3(2H)-one-1,1-dioxide (BIT), are SAR-inducing agents that activate SA biosynthesis in plants [11–13]. One of the key genes for rice SAR, OsWRKY45, has been identified to act downstream of SA and is used as an SAR marker gene [14, 15]. Overexpression of the transcription factor OsWRKY45 confers strong disease resistance for rice [16].
Strigolactones (SLs) and karrikin are butanolide-type compounds (Fig. 1) with diverse effects on plant growth and development [17, 18]. SLs regulate shoot branching and root morphology in plants [19–21]. SLs are involved in biotic interactions, promoting symbiosis with mycorrhizal fungi and the settlement of parasitic weeds [22, 23]. Karrikin, a compound found in smoke emitting from burning wood, affects seed germination rates, seedling photomorphogenesis, and stress tolerance [24].
Various SL analogs have been characterized to elucidate SL functions and develop strategies for controlling parasitic weeds [25, 26]. The canonical, natural SL structure includes a tricyclic lactone (ABC ring) and a methylfuranone ring (D ring) connected by an enol ether. The representative synthetic SL analog, rac-GR24, has a four-ring structure similar to canonical SLs (Fig. 1). The smaller compound, rac-4-bromodebranone (rac-4BD), which is an ether linkage of a substituted phenol and D ring, is easier to synthesize than rac-GR24 [27]. Furthermore, rac-4BD was found to have stronger activity for shoot branching inhibition in rice than rac-GR24 [27].
The mechanism of SL signaling and biosynthesis in rice has been studied through the analysis of dwarf (d) and high-tillering dwarf mutants [28, 29]. The causative gene for the SL-insensitive mutant d14 encodes the SL receptor, an α/β-fold hydrolase family protein. The D14 protein accepting SL interacts with the F-box protein D3 [30, 31] to form the Skp1-Cullin-F-box (SCF) E3 ubiquitin ligase complex with the SKP1 protein OSK1 [32, 33]. The D14-SCFD3 complex transmits the SL signal by inducing the ubiquitination and degradation of the D53 protein by the proteasome system. Additionally, the D3 protein interacts with the karrikin receptor D14L in a karrikin-dependent manner, whereas the internal ligands for D14L have not been identified [34]. Thus, D3 is a common pivotal factor in the SL- and karrikin-mediated signaling pathways.
The effects of SL on plant-pathogen interactions have been reported in recent years [35, 36]. In rice, SL biosynthesis mutant d17 and SL receptor mutant d14 showed higher susceptibility to P. oryzae than the wild type [35]. We previously demonstrated that SL signaling enhanced the expression of SAR marker genes in response to bacterial infection and enhanced disease resistance in Arabidopsis using rac-GR24 and an SL-biosynthesis inhibitor [36]. Therefore, SL signaling has positive effect on disease resistance and treatment with an SL analog is a promising tool for plant disease control. However, further research is necessary to determine whether activation of the SL-mediated signal influences rice immune systems. On the other hand, karrikin has been found to induce disease resistance in Arabidopsis [37]. Therefore, F-box protein MAX2, Arabidopsis D3 ortholog, is crucial for SL- and karrikin-mediated immune systems in Arabidopsis, which is also presumed to function in rice. Although D3- and MAX2-related signalings by SLs and Karrikins in growth and development have been extensively studied, their function and underlying mechanisms in the immune system are poorly understood. In this study, we analyzed the effects of the SL analog rac-4BD on defense gene expression and disease resistance in rice, with the aim of demonstrating the importance of the D3- and MAX2-related immune systems and providing a chemical tool to elucidate their mechanisms.