Brassinosteroids Antagonize Gibberellin- and Salicylate-Mediated Root Immunity in Rice      

Abstract

Brassinosteroids (BRs) are a unique class of plant steroid hormones that orchestrate myriad growth and developmental processes. Although BRs have long been known to protect plants from a suite of biotic and abiotic stresses, our understanding of the underlying molecular mechanisms is still rudimentary. Aiming to further decipher the molecular logic of BR-modulated immunity, we have examined the dynamics and impact of BRs during infection of rice (Oryza sativa) with the root oomycete Pythium graminicola. Challenging the prevailing view that BRs positively regulate plant innate immunity, we show that P. graminicola exploits BRs as virulence factors and hijacks the rice BR machinery to inflict disease. Moreover, we demonstrate that this immunesuppressive effect of BRs is due, at least in part, to negative cross talk with salicylic acid (SA) and gibberellic acid (GA) pathways. BR-mediated suppression of SA defenses occurred downstream of SA biosynthesis, but upstream of the master defense regulators NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 and OsWRKY45. In contrast, BR alleviated GA-directed immune responses by interfering at multiple levels with GA metabolism, resulting in indirect stabilization of the DELLA protein and central GA repressor SLENDER RICE1 (SLR1). Collectively, these data favor a model whereby P. graminicola coopts the plant BR pathway as a decoy to antagonize effectual SA-and GA-mediated defenses. Our results highlight the importance of BRs in modulating plant immunity and uncover pathogen-mediated manipulation of plant steroid homeostasis as a core virulence strategy.

Brassinosteroid Regulates Cell Elongation by Modulating Gibberellin Metabolism in Rice

“…Thus, BR might also promote GA biosynthesis to promote cell elongation in Arabidopsis. Moreover, in rice root, exogenous GA was found to repress BR biosynthesis, whereas BR was found to promote the expression of GA2ox-3, the GA inactivation gene (De Vleesschauwer et al, 2012). The authors speculated that BR regulates GA metabolism to inhibit root growth, which is consistent with our finding.…”

“…While BR has been widely used as a plant growth modulator in the field for over 30 years (Khripach et al, 2000), its effects on plants are perplexing, and conflicting results have been reported regarding its roles in several processes (Hu et al, 2000;Sasse, 2003;De Vleesschauwer et al, 2012;Wang, 2012;Serna, 2013). In contrast with its growth-promoting effect, exogenous BR application frequently leads to retarded growth; however, the detailed mechanism is unclear.…”

“…Several rice genes have been shown to participate in both BR and GA responses, including SPINDLY, GSR1 (for GA-stimulated transcript in rice), and DWARF1 (D1) (Ueguchi- Tanaka et al, 2000;Shimada et al, 2006;Wang et al, , 2009Oki et al, 2009a). In rice root, BR application appears to repress the levels of active GA by inhibiting the expression of GA20ox-3, a GA biosynthetic gene, and by simultaneously promoting the expression of GA2ox-3, a GA inactivation gene (De Vleesschauwer et al, 2012). These results suggest that BR and GA have a complex crosstalk depending on species or tissue.…”

Intervention of Phytohormone Pathways by Pathogen Effectors

“…As shown in Figure 2, extensive crosstalk is known to occur between phytohormone signaling pathways Manners, 2008, 2012;Robert-Seilaniantz et al, 2011;De Vleesschauwer et al, 2012;Naseem and Dandekar, 2012;B. Kim et al, 2013;Nahar et al, 2013).…”

Pattern of CsICE1 expression under cold or drought treatment and functional verification through analysis of transgenic Arabidopsis

“…Cross-talk occurs between different hormone pathways. OsGSR1 is induced by exogenous GA, while exogenous BR can repress the induction effect of GA on OsGSR1 (De Vleesschauwer et al, 2012). Normally, ABA induces the expression of the proline synthesis gene P5CS1, while BR represses the effect of ABA on P5CS1 (Abrahám et al, 2003).…”