Background: Mitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants. In Arabidopsis, perception of microbe-associated molecular patterns (MAMPs) activates the MAPKs MPK3, MPK4 and MPK6. Increasing information depicts the molecular events activated by MAMPs in plants, but the specific and cooperative contributions of the MAPKs in these signalling events are largely unclear. Results: In this work, we analyse the behaviour of MPK3, MPK4 and MPK6 mutants in early and late immune responses triggered by the MAMP flg22 from bacterial flagellin. A genome-wide transcriptome analysis reveals that 36% of the flg22-upregulated genes and 68% of the flg22-downregulated genes are affected in at least one MAPK mutant. So far MPK4 was considered as a negative regulator of immunity, whereas MPK3 and MPK6 were believed to play partially redundant positive functions in defence. Our work reveals that MPK4 is required for the regulation of approximately 50% of flg22-induced genes and we identify a negative role for MPK3 in regulating defence gene expression, flg22-induced salicylic acid accumulation and disease resistance to Pseudomonas syringae. Among the MAPK-dependent genes, 27% of flg22-upregulated genes and 76% of flg22-downregulated genes require two or three MAPKs for their regulation. The flg22-induced MAPK activities are differentially regulated in MPK3 and MPK6 mutants, both in amplitude and duration, revealing a highly interdependent network. Conclusions: These data reveal a new set of distinct functions for MPK3, MPK4 and MPK6 and indicate that the plant immune signalling network is choreographed through the interplay of these three interwoven MAPK pathways.

Background: Pepper is one of the most cultivated crops worldwide, but is sensitive to salinity. This sensitivity is dependent on varieties and our knowledge about how they can face such stress is limited, mainly according to a molecular point of view. This is the main reason why we decided to develop this transcriptomic analysis. Tolerant and sensitive accessions, respectively called A25 and A6, were grown for 14 days under control conditions and irrigated with 70 mM of NaCl. Biomass, different physiological parameters and differentially expressed genes were analysed to give response to differential salinity mechanisms between both accessions. Results: The genetic changes found between the accessions under both control and stress conditions could explain the physiological behaviour in A25 by the decrease of osmotic potential that could be due mainly to an increase in potassium and proline accumulation, improved growth (e.g. expansins), more efficient starch accumulation (e.g. BAM1), ion homeostasis (e.g. CBL9, HAI3, BASS1), photosynthetic protection (e.g. FIB1A, TIL, JAR1) and antioxidant activity (e.g. PSDS3, SnRK2.10). In addition, misregulation of ABA signalling (e.g. HAB1, ERD4, HAI3) and other stress signalling genes (e.g. JAR1) would appear crucial to explain the different sensitivity to NaCl in both accessions. Conclusions: After analysing the physiological behaviour and transcriptomic results, we have concluded that A25 accession utilizes different strategies to cope better salt stress, being ABA-signalling a pivotal point of regulation. However, other strategies, such as the decrease in osmotic potential to preserve water status in leaves seem to be important to explain the defence response to salinity in pepper A25 plants.