Functions of the lethal leaf-spot 1 gene in wheat cell death and disease tolerance to Puccinia striiformis

Pheophorbide a oxygenase (PaO) is a key enzyme in chlorophyll catabolism that is known to suppress cell death in maize and Arabidopsis. The catalytic activity of PaO in chlorophyll degradation has been clearly demonstrated, but the function of PaO in the regulation of cell death and plant-microbe interactions is largely unknown. In this study, we characterized a PaO homologue in wheat of the lethal leaf-spot 1 gene, TaLls1, that was induced in leaves infected by Puccinia striiformis f.sp. tritici (Pst) and wounding treatment. The TaLls1 protein contains a conserved Rieske [2Fe-2S] motif and a mononuclear iron-binding site typical of PaOs. Silencing of TaLls1 by virus-induced gene silencing in wheat led to leaf cell death without pathogen attacks, possibly due to the accumulation of pheophorbide a (upstream substrate of PaO), indicating a suppressor role of TaLls1, while overexpression of TaLls1 also triggered cell death in both tobacco and wheat leaves, probably owing to the accumulation of the red chlorophyll catabolite (downstream product of PaO). Further deletion mutant analysis showed that the conserved Rieske domain, but not the iron-binding site, was essential for cell death induction. These results thus suggest a threshold for TaLls1 in maintaining cell homeostasis to adapt in various stresses, and shed new light on the role of TaLls1 in cell death regulation. Furthermore, silencing of TaLls1 in wheat did not change the disease symptoms but enhanced tolerance to Pst via an significant increase in H2O2 generation, elevated cell death occurrence, and upregulation of pathogenesis-related genes.