A Novel MAPKK Involved in Cell Death and Defense Signaling

Transcriptome Analysis Reveals the Response Mechanism of Frl-Mediated Resistance to Fusarium oxysporum f. sp. radicis-lycopersici (FORL) Infection in Tomato

Tomato Fusarium crown and root rot (FCRR) is an extremely destructive soil-borne disease. To date, studies have shown that only plants with tomato mosaic virus (TMV) resistance exhibit similar resistance to tomato Fusarium oxysporum f. sp. radicis-lycopersici (FORL) and have identified a single relevant gene, Frl, in Peruvian tomato. Due to the relative lack of research on FCRR disease-resistance genes in China and elsewhere, transcriptome data for FORL-resistant (cv. ‘19912’) and FORL-susceptible (cv. ‘Moneymaker’) tomato cultivars were analysed for the first time in this study. The number of differentially expressed genes (DEGs) was higher in Moneymaker than in 19912, and 189 DEGs in the ‘plant–pathogen interaction’ pathway were subjected to GO and KEGG enrichment analyses. MAPK and WRKY genes were enriched in major metabolic pathways related to plant disease resistance; thus, we focused on these two gene families. In the early stage of tomato infection, the content of JA and SA increased, but the change in JA was more obvious. Fourteen genes were selected for confirmation of their differential expression levels by qRT-PCR. This study provides a series of novel disease resistance resources for tomato breeding and genetic resources for screening and cloning FORL resistance genes.

Involvement of potato (Solanum tuberosum L.) MKK6 in response to potato virus Y

Mitogen-activated protein kinase (MAPK) cascades have crucial roles in the regulation of plant development and in plant responses to stress. Plant recognition of pathogen-associated molecular patterns or pathogen-derived effector proteins has been shown to trigger activation of several MAPKs. This then controls defence responses, including synthesis and/or signalling of defence hormones and activation of defence related genes. The MAPK cascade genes are highly complex and interconnected, and thus the precise signalling mechanisms in specific plant–pathogen interactions are still not known. Here we investigated the MAPK signalling network involved in immune responses of potato (Solanum tuberosum L.) to Potato virus Y, an important potato pathogen worldwide. Sequence analysis was performed to identify the complete MAPK kinase (MKK) family in potato, and to identify those regulated in the hypersensitive resistance response to Potato virus Y infection. Arabidopsis has 10 MKK family members, of which we identified five in potato and tomato (Solanum lycopersicum L.), and eight in Nicotiana benthamiana. Among these, StMKK6 is the most strongly regulated gene in response to Potato virus Y. The salicylic acid treatment revealed that StMKK6 is regulated by the hormone that is in agreement with the salicylic acid-regulated domains found in the StMKK6 promoter. The involvement of StMKK6 in potato defence response was confirmed by localisation studies, where StMKK6 accumulated strongly only in Potato-virus-Y-infected plants, and predominantly in the cell nucleus. Using a yeast two-hybrid method, we identified three StMKK6 targets downstream in the MAPK cascade: StMAPK4_2, StMAPK6 and StMAPK13. These data together provide further insight into the StMKK6 signalling module and its involvement in plant defence.

The overexpression of a maize mitogen-activated protein kinase gene (ZmMPK5) confers salt stress tolerance and induces defence responses in tobacco

As sessile organisms, plants are exposed to potential dangers, including multiple biotic and abiotic stresses. The mitogen-activated protein kinase (MAPK) is a universal signalling pathways involved in these processes. A previous study showed that maize ZmMPK5 is induced by various stimuli at transcriptional and post-translational levels. In this study, ZmMPK5 was overexpressed in tobacco to further analyse its biological functions. Under salt and oxidative stresses, ZmMPK5-overexpressing lines displayed less severe damage and stronger growth phenotypes corresponding to a series of physiological changes. In addition, the transgenic lines accumulated less reactive oxygen species (ROS) and had higher levels of antioxidant enzyme activity and metabolites than wild-type (WT) plants following NaCl treatment. Quantitative RT-PCR revealed that the expression of ROS-related and stress-responsive genes was higher in transgenic plants than in WT plants. Furthermore, transgenic lines exhibited enhanced resistance to viral pathogens, and expressed constitutively higher transcript levels of pathogenesis-related genes, such as PR1a, PR4, PR5 and EREBP. Taken together, these results demonstrated that ZmMPK5 is involved in salt stress, oxidative stress and pathogen defence signalling pathways, and its function may be at least partly devoted to efficiently eliminating ROS accumulation under salt stress.