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SUMMARYThe two closely related Arabidopsis transcription factors, WRKY18 and WRKY40, play a major and partly redundant role in PAMP-triggered basal defense. We monitored the transcriptional reprogramming induced by the powdery mildew fungus, Golovinomyces orontii, during early stages of infection with respect to the role of WRKY18/40. Expression of >1300 Arabidopsis genes was differentially altered already 8 hours post infection (hpi), indicating rapid pre-penetration signaling between the pathogen and the host. We found that WRKY18/ 40 negatively affects pre-invasion host defenses and deduced a subset of genes that appear to be under WRKY18/40 control. A mutant lacking the WRKY18/40 repressors executes pathogen-dependent but exaggerated expression of some defense genes leading, for example, to strongly elevated levels of camalexin. This implies that WRKY18/40 act in a feedback repression system controlling basal defense. Moreover, using chromatin immunoprecipitation (ChIP), direct in vivo interactions of WRKY40 to promoter regions containing W box elements of the regulatory gene EDS1, the AP2-type transcription factor gene RRTF1 and to JAZ8, a member of the JA-signaling repressor gene family were demonstrated. Our data support a model in which WRKY18/40 negatively modulate the expression of positive regulators of defense such as CYP71A13, EDS1 and PAD4, but positively modulate the expression of some key JA-signaling genes by partly suppressing the expression of JAZ repressors.

The spot blotch disease of wheat is caused by Bipolaris sorokiniana, which is an anamorph (teleomorph Cochliobolus sativus). The disease mainly occurs in warm, humid wheat-growing regions, and the Eastern Gangetic Plains (EGP) of South Asia is a hotspot. Significant progress has been made in recent years in characterizing the host-pathogen interaction. The study of the pathogen's life cycle and diversity have been an active area of research. A number of resistance sources have also been identified, characterized and used for breeding. Although immunity has not been observed in any genotype, cultivars displaying a relatively high level of resistance have been developed and made available to farmers. Further progress will require regular use of marker-assisted breeding, genomic selection, gene editing and transgenic interventions. This review summarizes the current state of knowledge about genetic and breeding efforts on the wheat-B. sorokiniana pathosystem and discusses ways in which emerging tools can be used for future research to understand the mechanism involved in infection and for developing cultivars exhibiting a high level of resistance.

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SummarySmall RNAs are important regulators of plant development and resistance to viruses. To determine whether small RNAs mediate defense responses to herbivore attack, we silenced the expression of three RNA-directed RNA polymerases (RdRs) in the native tobacco Nicotiana attenuata by virus-induced gene silencing. Larvae of the leaf-chewing solanaceous specialist Manduca sexta grew faster on the RdR1-silenced plants than on empty vector (EV) controls; silencing RdR3 and 2 had little to no effect on larval performance. NaRdR1 transcripts were strongly elicited when puncture wounds were treated with M. sexta oral secretions (OS) to simulate herbivore attack, and with SA and JA, phytohormones that are elicited by herbivore attack. Stably silencing RdR1 by transforming N. attenuata with an inverted-repeat RdR1 construct produced plants (irRdR1) that grew normally but were highly susceptible to both M. sexta larvae and the cell-content-feeder Tupiocoris notatus. When irRdR1 lines were planted into N. attenuata's native habitat in the Great Basin Desert (Utah, USA), they were highly susceptible to herbivore attack, due to deficiencies in direct rather than indirect defenses. Microarray analysis revealed the downregulation of ADC and ODC genes, which supply substrates for synthesizing the chemical defense compound nicotine; irRdR1 lines failed to accumulate nicotine after attack. We conclude that RdR1 mediates herbivore resistance, and infer that the small RNAs produced by RdR1 are probably involved in orchestrating some of the rapid metabolic adjustments required for plants to survive herbivore attack in their natural habitats. The experiment highlights the value of carrying out 'real-world' tests of gene function early in the discovery process.