We tested whether signaling pathways induced by systemin, oligosaccharide elicitors (OEs), and ultraviolet (UV)-B radiation share common components in Lycopersicon peruvianum suspension-cultured cells. These stress signals all induce mitogen-activated protein kinase (MAPK) activity. In desensitization assays, we found that pretreatment with systemin and OEs transiently reduced the MAPK response to a subsequent treatment with the same or a different elicitor. In contrast, MAPK activity in response to UV-B increased after pretreatment with systemin and OEs. These experiments demonstrate the presence of signaling components that are shared by systemin, OEs, and UV-B. Based on desensitization assays, it is not clear if the same or different MAPKs are activated by different stress signals. To identify specific stress-responsive MAPKs, we cloned three MAPKs from a tomato (Lycopersicon esculentum) leaf cDNA library, generated member-specific antibodies, and performed immunocomplex kinase assays with extracts from elicited L. peruvianum cells. Two highly homologous MAPKs, LeMPK1 and LeMPK2, were activated in response to systemin, four different OEs, and UV-B radiation. An additional MAPK, LeMPK3, was only activated by UV-B radiation. The common activation of LeMPK1 and LeMPK2 by many stress signals is consistent with the desensitization assays and may account for substantial overlaps among stress responses. On the other hand, MAPK activation kinetics in response to elicitors and UV-B differed substantially, and UV-B activated a different set of LeMPKs than the elicitors. These differences may account for UV-B-specific responses.

Systemin is a wound-signaling peptide that mediates defenses of tomato plants against herbivorous insects. Perception of systemin by the membrane-bound receptor SR160 results in activation of MAPKs, synthesis of jasmonic acid (JA), and expression of defense genes. To test the function of MAPKs in the response to systemin, we used virus-induced gene silencing (VIGS) in plants that overexpress the systemin precursor prosystemin (35S::prosys plants). These transgenic plants accumulate high levels of defense proteins and exhibit increased resistance to herbivorous insects. Cosilencing of the MAPKs MPK1 and MPK2 reduced MPK1/2 kinase activity, JA biosynthesis, and expression of JA-dependent defense genes. Application of methyl-JA restored the full defense response. These data show that MPK1 and MPK2 are essential components of the systemin signaling pathway and most likely function upstream of JA biosynthesis. MPK1 and MPK2 are 95% identical at the amino acid level. Specific VIGS of only MPK1 or MPK2 resulted in the same reduction of defense gene expression as cosilencing of MPK1 and MPK2, indicating that gene dosage effects may be important for MPK signaling. In addition, VIGS of the closely related MPK3 also reduced systemin-induced defense responses. The function of MPK1/2 and orthologs in pathogen-induced defenses is well established. Here we show that cosilencing of MPK1 and MPK2 compromised prosystemin-mediated resistance to Manduca sexta (Lepidoptera) herbivory, demonstrating that MPK1 and MPK2 are also required for successful defenses against herbivorous insects.

Mitogen-activated protein kinases (MAPKs), a family of Ser/Thr protein kinases, play an essential role in mediating biotic and abiotic stress responses in plants. In this study, we investigated 16 putative SlMAPK genes from tomato genome and compared them with those from Arabidopsis. The full cDNA sequences of 13 novel SlMAPKs in tomato were obtained through PCR ampilification. A comprehensive genome-wide analysis of SlMAPKs in tomato is presented, including their gene structure, phylogeny, genome localization, and expression profiles. Phylogenic analysis of the 16 SlMAPKs and 20 AtMAPKs from Arabidopsis indicated that the SlMAPK genes were clustered into four major groups, and genes within the same groups had similar exon-intron structures. All SlMAPK proteins in groups A, B and C had a Thr-Glu-Tyr (TEY) activation domain, whereas those in group D contained a Thr-Asp-Tyr (TDY) activation domain. The analysis of 5'-upstream region of SlMAPKs revealed a group of putative cis-acting elements related to stress responses. Expression analysis of SlMAPK genes using RT-PCR and real-time quantitative PCR demonstrated that all SlMAPK transcripts were able to be detected in at least one investigated tissue, and some of them exhibited tissue-specific expression patterns. The transcript abundance of nearly all SlMAPK genes was increased in response to heat stress treatment. Our data provided an insight into the evolution of the gene family and a useful reference for further functional analysis of MAPK family genes in tomato.