Immune signaling in arabidopsis thaliana upon perception of bacterial and viral molecular patterns with a special emphasis on roots

In order to recognize a vast variety of attackers, plants possess a plethora of sophisticated detection systems. Perception of microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) by the plant pattern recognition receptors (PRRs) leads to subsequent initiation of defense responses, a process collectively referred to as pattern-triggered immunity (PTI). PTI has been extensively studied in plant leaves, especially of the model organism Arabidopsis thaliana, whereas the mechanisms underlying PTI in roots so far attracted less attention. However, since a vast number of plant pests are soil-borne and attack roots in order to propagate and colonize whole plants, understanding the mechanisms underlying basic defense at the root level is of high interest for the development of new tools to combat root pathogens of crop plants.
It has been demonstrated that recognition of flg22, the conserved epitope of the bacterial flagellin protein, leads to tissue-specific defense responses in roots. In order to investigate the cause for this tissue-specific induction of downstream responses, several approaches were employed during the course of this work. By studying the cellular localization of the PRR recognizing flg22, FLAGELLIN-SENSING 2 (FLS2), we were able to depict an expression map of FLS2 in wild-type Arabidopsis plants. Our study revealed that FLS2 was expressed in a highly tissue-specific manner in roots and shoots and that the FLS2 promoter activity was inducible upon environmental stimuli as well as during developmental processes, changing not only in intensity in expressing tissues but also in tissue-specificity. These results indicate an important role of the tissue-specific PRR localization in immunity mechanisms.
In a parallel study, we expressed FLS2 under the control of several root tissue-specific promoters, which allowed us to analyze the competence of these tissues to detect flg22. Unexpectedly, all investigated root tissues were able to perceive externally applied flg22. In fact, PTI responses could be activated in intact roots as well as in dissected roots, suggesting that the peptide is able to penetrate through the different tissue layers. Remarkably, the expression level of the receptor was not the major parameter determining the magnitude of the immune response output. Thus, we postulated that perception of flg22 by certain tissues leads to stronger PTI responses potentially indicating why plants restrict immune receptor accumulation to tissue-specific locations possibly in order to balance the outcome of the defense activation.
Due to the fact that many developmental or immunity processes in plants depend on systemic communication between different plant organs and that beneficial root microbes are known to prime and enhance resistance in aerial plant tissues, we hypothesized that MAMP perception by roots might induce a signaling event from roots to shoots. In order to address the potential existence of such systemic alarm signals, various methods were implemented. However, we encountered several technical limitations mainly concerning elicitor diffusion. Therefore, we focused on the development of an improved application method for studying systemic root-to-shoot signaling in Arabidopsis plants. Our system proved suitable to perform systemic signaling analysis and revealed that at the transcriptional level no systemically activated defense gene modifications were detectable in distal shoots of root-treated plants in our conditions.
Like root pathogens, also viruses constitute a major threat in agro-economy and are responsible for immense crop losses. The basal defense response against viruses is thought to be mediated by RNA silencing, a process by which viral replication intermediates are cleaved and degraded by the plant silencing machinery through the recognition of virus-derived small RNAs. Intriguingly, a recent study conducted in our lab demonstrated a role of BRASSINOSTEROID INSENSITIVE1 (BRI1)-ASSOCIATED RECEPTOR KINASE 1 (BAK1), a coreceptor of several PRRs involved in immunity and development, in antiviral defense. These results indicated that PTI may also contribute to antiviral resistance but the exact recognition process remained elusive. Because dsRNA produced during viral replication has been shown to act as a PAMP in animals, we decided to test whether dsRNA is perceived as a viral PAMP in planta as well. We found that natural as well as synthetic dsRNA is indeed perceived as a PAMP by Arabidopsis, leading to the activation of typical PTI responses. Remarkably, dsRNA application also promoted protection of Arabidopsis plants against viral infection.
Taken together, this study provides new insights into the recognition mechanisms of bacteria- and virus-associated molecular patterns by different plant organs and contributes to elucidate the molecular defense strategy of plants against agriculturally important diseases.