Chemical diversity on the leaf surface
The first encounter of foliar pathogens with their (potential) host plant occurs on the leaf surface. Here, specialized metabolites contribute to the decision whether the pathogen can enter the plant tissue or is warded off by the plant. Metabolites and enzymes from both plant and pathogen shape the chemical composition on the leaf surface and, thus, contribute to chemical communication between the partners.
Our work concentrates on two points:
- How are plant metabolites synthesized and exported to the leaf surface?
- How do metabolites contribute to defense?
We are using untargeted metabolite profiling by UPLC-ESI-QTOF-MS to identify compounds that accumulate on the leaf surface prior to and in response to inoculation by P. infestans. The structures of differentially occurring metabolites are elucidated by tandem MS and bioinformatic tools developed at the IPB by the group of Steffen Neumann.Functional characterization of selected metabolites includes growth and spore germination inhibition assays (Eschen-Lippold et al., 2009), using a GFP expressing P. infestans isolate (kindly provided by Felix Mauch, University of Fribourg, Switzerland).
So far, we have identified a MATE transporter from Arabidopsis, which is required for the export of specific hydroxycinnamic acid amides (HCAAs) that inhibit the germination of P. infestans zoospores (Fig. 1). The transfer of both the biosynthetic gene (ACT) and the transporter gene (DTX18) from the nonhost plant Arabidopsis into the host plant potato conferred the ability to accumulate high amounts of specific HCAAs on the leaf surface. Importantly, this correlated with an enhanced defense capacity against infection by P. infestans (Dobritzsch et al., 2016), showing the importance of antimicrobial surface metabolites for successful defense.
The ABC transporter PEN3, which is important for penetration resistance of Arabidopsis against P. infestans (Stein et al., 2006), was postulated to transport products of the PEN2-mediated conversion of 4-methoxyindole glucosinolates into the apoplast. We have identified a substrate of PEN3, whose synthesis is PEN2-dependent and which is transported in vitro in a PEN3-dependent manner (collaboration with Bernhard Westermann, IPB and Markus Geisler, University of Fribourg, Switzerland). Interestingly, this compound does not act as an antimicrobial substance, but enhances flg22-induced callose formation in Arabidopsis seedlings (Matern et al., 2019). Thus, defense-modulating compounds on the leaf surface contribute to the defense potential of plants.
In a DFG-funded project, we are presently analyzing the leaf surface metabolites from wild potato species that are resistant to P. infestans.
PAMP-triggered immunity in potato
Potato (Solanum tuberosum L.) is a host plant for P. infestans. Interestingly, the susceptible potato cultivar Désirée can be made more resistant by treatment with the oligopeptide Pep-13, a pathogen-associated molecular pattern (PAMP) from Phytophthora species (Brunner et al., 2002). Recognition of Pep-13 leads to the activation of strong local defense responses such as the accumulation of salicylic acid, jasmonic acid and hydrogen peroxide, as well as hypersensitive cell death (Halim et al., 2004; Halim et al., 2009; Fig. 2).If Pep-13-treated plants are subsequently infected by P. infestans, disease symptoms are not as severe as in control-treated plants and the pathogen biomass is significantly lower. To understand the mechanisms of this induced resistance, we identified genes that are activated in response to Pep-13 treatment by microarray analyses and RNA sequencing. Functional analyses of selected candidate genes are performed using transgenic plants with down-regulated expression of these genes.
Pep-13 is postulated to be a ligand for a yet unidentified plasma membrane receptor-like kinase (RLK). We have identified BAK1, a co-receptor for RLKs, to be important for early defense responses in potato. Interestingly, despite impaired formation of reactive oxygen species and reduced activation of MAP kinases, defense gene expression takes place in transgenic potato plants with reduced levels of BAK1 expression (Nietzschmann et al, submitted). Potato plants impaired in the expression of the syntaxin StSYR1 have a defect in the deposition of callose-containing papillae at the penetration sites, indicating the importance of vesicle fusion processes for cell wall fortification (Eschen-Lippold et al., 2012). In collaboration with Ingo Heilmann (MLU Halle), we are investigating the role of phosphatidylinositides for secretory processes during PAMP-triggered immunity.
We further identified the ABC transporter ABCG1 to be required for suberin formation in potato. Suberin is a lipophilic biopolymer that acts as a transpiration barrier. Transgenic plants with decreased ABCG expression cannot form suberin in tuber skin, leading to highly disorganized periderm and enhanced water loss. Metabolite analyses revealed major alterations in the composition of suberin and the accumulation of potential suberin precursors in methanolic extracts of tuber skin, suggesting that the ABC transporter is required for the export of suberin precursors (Landgraf et al., 2014). Within the graduate program Agripoly (MLU Halle, Anhalt University of Applied Sciences), we are presently analysing the role of suberin for pathogen defense using CRISPR-Cas9-edited potato plants.
References:
- Bednarek P, Pislewska-Bednarek M, Svatos A, Schneider B, Doubsky J, Mansurova M, Humphry M, Consonni C, Panstruga R, Sanchez-Vallet A, Molina A, and Schulze-Lefert P. (2009). A glucosinolate metabolism pathway in living plant cells mediates broad-spectrum antifungal defense. Science 323, 101-106.
- Brunner F, Rosahl S, Lee J, Rudd JJ, Geiler C, Kauppinen S, Rasmussen G, Scheel D, and Nürnberger T. (2002). Pep-13, a plant defense-inducing pathogen-associated pattern from Phytophthora transglutaminases. EMBO J 21, 6681-6688.
- Dobritzsch M, Lübken T, Eschen-Lippold L, Gorzolka K, Blum E, Matern A, Marillonnet S, Böttcher C, Dräger B, and Rosahl S. (2016). MATE transporter-dependent export of hydroxycinnamic acid amides. Plant Cell 28, 583-596.
- Eschen-Lippold L, Draeger T, Teichert A, Wessjohann L, Westermann B, Rosahl S, and Arnold N. (2009). Anti-oomycete activity of γ-oxocrotonate fatty acids against P. infestans. J Agric Food Chem 57, 9607-9612.
- Eschen-Lippold L, Landgraf R, Smolka U, Schulze S, Heilmann M, Heilmann I, Hause G, and Rosahl S. (2012). Activation of defense against Phytophthora infestans in potato by down-regulation of syntaxin gene expression. New Phytol 193, 985-996.
- Halim VA, Hunger A, Macioszek V, Landgraf P, Nürnberger T, Scheel D, and Rosahl S. (2004). The oligopeptide elicitor Pep-13 induces salicylic acid-dependent and-independent defense reactions in potato. Physiol Mol Plant Pathol 64, 311-318.
- Halim VA, Altmann S, Ellinger D, Eschen-Lippold L, Miersch O, Scheel D, and Rosahl S. (2009). PAMP-induced defense responses in potato require both salicylic acid and jasmonic acid. Plant J 57, 230-242.
- Landgraf R, Smolka U, Altmann S, Eschen-Lippold L, Senning M, Sonnewald S, Weigel B, Frolova N, Strehmel N, Hause G, Scheel D, Böttcher C, and Rosahl S. (2014). The ABC transporter ABCG1 is required for suberin formation in potato tuber periderm. Plant Cell 26, 3403-3415.
- Lipka V, Dittgen J, Bednarek P, Bhat R, Wiermer M, Stein M, Landtag J, Brandt W, Rosahl S, Scheel D, Llorente F, Molina A, Parker J, Somerville S, and Schulze-Lefert P. (2005). Pre- and postinvasion defenses both contribute to nonhost resistance in Arabidopsis. Science 310, 1180-1183.
- Matern A, Böttcher C, Eschen-Lippold L, Westermann B, Smolka U, Döll S, Trempel F, Aryal B, Scheel D, Geisler M, and Rosahl S. (2019). A substrate of the ABC transporter PEN3 stimulates bacterial flagellin (flg22)-induced callose deposition in Arabidopsis thaliana. J Biol Chem 294, 6857-6870.
- Stein M, Dittgen J, Sanchez-Rodriguez C, Hou BH, Molina A, Schulze-Lefert P, Lipka V, and Somerville S. (2006). Arabidopsis PEN3/PDR8, an ATP binding cassette transporter, contributes to nonhost resistance to inappropriate pathogens that enter by direct penetration. Plant Cell 18, 731-746.
