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Verticillium longisporum (Vl43) is a soilborne hemibiotrophic fungal pathogen causing stem striping on oilseed rape (OSR) and severe yield losses. Breeding for resistant varieties is the most promising approach to control this disease. Here, we report the identification of Hva22c as a novel susceptibility factor and its potential for improving OSR resistance. Hva22c is a member of the Hva22 gene family, originally described for barley (Hordeum vulgare). Several Hva22 members have been located at the endoplasmic reticulum. Hva22c is up-regulated in response to Vl43 in both Arabidopsis and OSR. We demonstrate that knock-out of Hva22c in OSR by CRISPR/Cas9 and its homolog in Arabidopsis by T-DNA insertion reduced plants' susceptibility to Vl43 infection and impaired the development of disease symptoms. To understand the underlying mechanism, we analysed transcriptomic data from infected and non-infected roots of hva22c knock-out and wild type plants. We identified a homozygous mutant with frame-shifts in all four BnHva22c loci displaying a vastly altered transcriptional landscape at 6 dpi. Significantly, a large set of genes was suppressed under mock conditions including genes related to the endomembrane systems. Among the up-regulated genes we found several defense-related and phytohormone-responsive genes when comparing mutant to the wild type. These results demonstrate that Hva22c is functionally required for a fully compatible plant-fungus interaction. Its loss of function reduces plant susceptibility, most likely due to endoplasmatic reticulum and Golgi dysfunction accompanied by additionally activated defense responses. These findings can help improve OSR resistance to V. longisporum infection. [ABSTRACT FROM AUTHOR]

Plants are confronted with various environmental stresses and develop sophisticated adaptive mechanisms. Our previous work demonstrated that the crosstalk of flg22 and ultraviolet (UV)‐B‐induced signalling cascades reprograms the expression of flavonol pathway genes (FPGs), benefiting plant defence responses. Although several transcription factors have been identified to be involved in this crosstalk, the underlying mechanism is largely unclear. Here, we analyzed microRNAs (miRNAs) and identified 126, 129 and 113 miRNAs with altered abundances compared to untreated control in flg22‐, UV‐B‐ and flg22/UV‐B‐treated seedlings, respectively. Two distinct modules were identified: The first consists of 10 miRNAs repressed by UV‐B but up‐regulated by flg22, and the second with five miRNAs repressed by flg22 but up‐regulated by UV‐B. In Arabidopsis, the knockdown of miR858a, a representative of module I, increased the abundance of CHS (a marker gene for FPGs), whereas its overexpression reduced CHS. Conversely, knockout of miR164b from module II decreased CHS and its overexpression increased CHS transcript levels. These data suggest a decisive role of miRNAs in the crosstalk. In the next, we described the interaction between miR858a and its target MYB111 (a positive regulator of FPGs) from module I in detail. We showed that MYB111 was profoundly post‐transcriptionally regulated by miR858a during the crosstalk, whose expression was specifically but antagonistically controlled by UVR8‐ and FLS2‐mediated signallings. Moreover, transcriptional monitoring using the GUS reporter gene demonstrates that miRNA‐mediated posttranscriptional regulation is the main driving force in reprogramming the expression of FPGs and regulates plant adaptation to multiple concurrent environmental stresses. Summary statement: MicroRNAs (miRNAs) act as 'master switches' in the crosstalk between flg22 and ultraviolet‐B‐induced signalling cascades. We show that miRNA‐mediated posttranscriptional regulation is the main driving force for reprogramming the expression of flavonol pathway genes to protect plants from multiple, concurrent environmental stresses. [ABSTRACT FROM AUTHOR]

SUMMARY: MicroRNAs negatively regulate gene expression by promoting target mRNA cleavage and/or impairing its translation, thereby playing a crucial role in plant development and environmental stress responses. In Arabidopsis, the MIR840 gene is located within the overlapping 3′UTR of the PPR and WHIRLY3 (WHY3) genes, both being predicted targets of miR840* and miR840, the short maturation products of MIR840. Gain‐ and loss‐of‐function of MIR840 in Arabidopsis resulted in opposite senescence phenotypes. The highest expression levels of the MIR840 precursor transcript pre‐miR840 were observed at senescence initiation, and pre‐miR840 expression is significantly correlated with a reduction in PPR, but not WHY3, transcript levels. Although a reduction of transcript level of PPR, but not WHY3 transcript levels were not significantly affected by MIR840 overexpression, its protein levels were strongly reduced. Mutating the cleavage sites or replacing the target sequences abolishes the miR840*/miR840‐mediated degradation of PPR transcripts and accumulation of WHY3 protein. In support for this, concurrent knockdown of both PPR and WHY3 in wild‐type plants resulted in a senescence phenotype resembling that of the MIR840‐overexpressing plant. This indicates that both PRR and WHY3 are targets in the MIR840‐mediated senescence pathway. Moreover, single knockout mutants of PPR and WHY3 show a convergent upregulated subset of senescence‐associated genes, which are also found among those induced by MIR840 overexpression. Our data provide evidence for a regulatory role of MIR840 in plant senescence. [ABSTRACT FROM AUTHOR]

In Arabidopsis cell culture, we have shown that flavonol metabolite accumulation depends on expression of the flavonol pathway genes (FPGs), which are up-regulated by UV-B irradiation but repressed during microbe-associated molecular pattern (MAMP) -triggered immunity (MTI) induced by the bacterial elicitor flg22. The suppression of flavonoid production during MTI is believed to allow the plant focusing its metabolism on the pathogen defense by directing phenylalanine resources from UV-B protective flavonol production towards production of phytoalexins and cell wall fortification by lignin incorporation. Here, we show that UV-B-induced FPGs are also suppressed by flg22 in Arabidopsis seedlings, demonstrating that this kind of 'crosstalk' is fully functional in planta. We believe that this system based on seedlings of the model plant Arabidopsis thaliana constitutes a valuable tool for further dissection of the underlying molecular mechanism, for example, by deploying gain/loss-of-function mutants. Furthermore, we observed some differences in the expression patterns of MYB transcription factors (TFs) as compared to data from the cell culture system. The new data suggest that in planta the TF MYB111 might play a more dominant role than the TF MYB12, which was strongly regulated in cell cultures. Thus, we can present an updated working model how this crosstalk might function. [ABSTRACT FROM AUTHOR]