RNAs - a new frontier in crop protection.

Small RNA (sRNA)-mediated RNA interference (RNAi) is a regulatory mechanism conserved in almost all eukaryotes. sRNAs play a critical role in host pathogen interactions either endogenously or by traveling between the interacting organisms and inducing 'cross-Kingdom RNAi' in the counterparty. Cross-kingdom RNAi is the mechanistic basis of host-induced gene silencing (HIGS), which relies on genetically expressing pathogen-gene targeting RNAs in crops, and has been successfully utilized against both microbial pathogens and pests. HIGS is limited by the need to produce genetically engineered crops. Recent studies have demonstrated that double-stranded RNAs and sRNAs can be efficiently taken up by many fungal pathogens, and induce gene silencing in fungal cells. This mechanism, termed 'environmental RNAi', allows direct application of pathogen-gene targeting RNAs onto crops to silence fungal virulence-related genes for plant protection. In this review, we will focus on how we can leverage cross-kingdom RNAi and environmental RNAi for crop disease control.

A stable antimicrobial peptide with dual functions of treating and preventing citrus Huanglongbing.

Citrus Huanglongbing (HLB), caused by a vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas), is the most devastating citrus disease worldwide. Currently, there are no effective strategies to prevent infection or to cure HLB-positive trees. Here, using comparative analysis between HLB-sensitive citrus cultivars and HLB-tolerant citrus hybrids and relatives, we identified a novel class of stable antimicrobial peptides (SAMPs). The SAMP from Microcitrusaustraliasica can rapidly kill Liberibacter crescens (Lcr), a culturable Liberibacter strain, and inhibit infections of CLas and CL. solanacearum in plants. In controlled greenhouse trials, SAMP not only effectively reduced CLas titer and disease symptoms in HLB-positive trees but also induced innate immunity to prevent and inhibit infections. Importantly, unlike antibiotics, SAMP is heat stable, making it better suited for field applications. Spray-applied SAMP was taken up by citrus leaves, stayed stable inside the plants for at least a week, and moved systemically through the vascular system where CLas is located. We further demonstrate that SAMP is most effective on α-proteobacteria and causes rapid cytosol leakage and cell lysis. The α-helix-2 domain of SAMP is sufficient to kill Lcr Future field trials will help determine the efficacy of SAMP in controlling HLB and the ideal mode of application.