Overexpression of a soybean nuclear localized type-III DnaJ domain-containing HSP40 reveals its roles in cell death and disease resistance.

Heat-shock proteins such as HSP70 and HSP90 are important molecular chaperones that play critical roles in biotic and abiotic stress responses; however, the involvement of their co-chaperones in stress biology remains largely uninvestigated. In a screen for candidate genes stimulating cell death in Glycine max (soybean), we transiently overexpressed full-length cDNAs of soybean genes that are highly induced during soybean rust infection in Nicotiana benthamiana leaves. Overexpression of a type-III DnaJ domain-containing HSP40 (GmHSP40.1), a co-chaperone of HSP70, caused hypersensitive response (HR)-like cell death. The HR-like cell death was dependent on MAPKKKα and WIPK, because silencing each of these genes suppressed the HR. Consistent with the presence of a nuclear localization signal (NLS) motif within the GmHSP40.1 coding sequence, GFP-GmHSP40.1 was exclusively present in nuclear bodies or speckles. Nuclear localization of GmHSP40.1 was necessary for its function, because deletion of the NLS or addition of a nuclear export signal abolished its HR-inducing ability. GmHSP40.1 co-localized with HcRed-SE, a protein involved in pri-miRNA processing, which has been shown to be co-localized with SR33-YFP, a protein involved in pre-mRNA splicing, suggesting a possible role for GmHSP40.1 in mRNA splicing or miRNA processing, and a link between these processes and cell death. Silencing GmHSP40.1 enhanced the susceptibility of soybean plants to Soybean mosaic virus, confirming its positive role in pathogen defense. Together, the results demonstrate a critical role of a nuclear-localized DnaJ domain-containing GmHSP40.1 in cell death and disease resistance in soybean.