Identification of a damage-associated molecular pattern (DAMP) receptor and its cognate peptide ligand in sweet potato (Ipomoea batatas)

Sweet potato (Ipomoea batatas) is an important tuber crop, but also target of numerous insect pests. Intriguingly, the abundant storage protein in tubers, sporamin, has intrinsic trypsin protease inhibitory activity. In leaves, sporamin is induced by wounding or a volatile homoterpene and enhances insect resistance. While the signalling pathway leading to sporamin synthesis is partially established, the initial event, perception of a stress-related signal is still unknown. Here, we identified an IbLRR-RK1 that is induced upon wounding and herbivory, and related to peptide-elicitor receptors (PEPRs) from tomato and Arabidopsis. We also identified a gene encoding a precursor protein comprising a peptide ligand (IbPep1) for IbLRR-RK1. IbPep1 represents a distinct signal in sweet potato, which might work in a complementary and/or parallel pathway to the previously described hydroxyproline-rich systemin (HypSys) peptides to strengthen insect resistance. Notably, an interfamily compatibility in the Pep/PEPR system from Convolvulaceae and Solanaceae was identified.

ANNEXIN1 mediates calcium-dependent systemic defense in Arabidopsis plants upon herbivory and wounding

Cellular calcium (Ca) transients are endogenous signals involved in local and systemic signaling and defense activation upon environmental stress, including wounding and herbivory. Still, not all Ca²⁺ channels contributing to the signaling have been identified, nor are their modes of action fully known. Plant annexins are proteins capable of binding to anionic phospholipids and can exhibit Ca channel-like activity. Arabidopsis ANNEXIN1 (ANN1) is suggested to contribute to Ca transport. Here, we report that wounding and simulated-herbivory-induced cytosolic free Ca elevation was impaired in systemic leaves in ann1 loss-of-function plants. We provide evidence for a role of ANN1 in local and systemic defense of plants attacked by herbivorous Spodoptera littoralis larvae. Bioassays identified ANN1 as a positive defense regulator. Spodoptera littoralis feeding on ann1 gained significantly more weight than larvae feeding on wild-type, whereas those feeding on ANN1-overexpressing lines gained less weight. Herbivory and wounding both induced defense-related responses on treated leaves, such as jasmonate accumulation and defense gene expression. These responses remained local and were strongly reduced in systemic leaves in ann1 plants. Our results indicate that ANN1 plays an important role in activation of systemic rather than local defense in plants attacked by herbivorous insects.

Pyrrolic and Dipyrrolic Chlorophyll Degradation Products in Plants and Herbivores

The degradation of chlorophyll, the omnipresent green pigment, has been investigated intensively over the last 30 years resulting in many elucidated tetrapyrrolic degradation products. With a comparison to the degradation of the structurally similar heme, we hereby propose a novel additional chlorophyll degradation mechanism to mono- and dipyrrolic products. This is the first proof of the occurrence of a family of mono- and dipyrrols in leaves that are previously only known as heme degradation products. This product family is also found in spit and feces of herbivores with specific metabolomic patterns reflecting the origin of the samples. Based on chromatographic and mass spectrometric evidence as well as on mechanistic considerations we also suggest several tentative new degradation products. One of them, dihydro BOX A, was fully confirmed as a novel natural product by synthesis and comparison of its spectroscopic data.

Plant-Plant Communication: Is There a Role for Volatile Damage-Associated Molecular Patterns?

Damage-associated molecular patterns (DAMPs) are an ancient form of tissue-derived danger or alarm signals that initiate cellular signaling cascades, which often initiate defined defense responses. A DAMP can be any molecule that is usually not exposed to cells such as cell wall components, peptides, nucleic acid fragments, eATP and other compounds. DAMPs might be revealed upon tissue damage or during attack. Typically, DAMPs are derived from the injured organism. Almost all eukaryotes can generate and respond to DAMPs, including plants. Besides the molecules mentioned, certain volatile organic compounds (VOCs) can be considered as DAMPs. Due to their chemical nature, VOCs are supposed to act not only locally and systemically in the same plant but also between plants. Here, we focus on damage-induced volatiles (DIVs) that might be regarded as DAMPs; we will review their origin, chemical nature, physiochemical properties, biological relevance and putative function in plant-plant communications. Moreover, we discuss the possibility to use such airborne DAMPs as eco-friendly compounds to stimulate natural defenses in agriculture in order to avoid pesticides.

Beneficial and Pathogenic Arabidopsis Root-Interacting Fungi Differently Affect Auxin Levels and Responsive Genes During Early Infection

Auxin (indole-3-acetic acid, IAA) is an important phytohormone involved in root growth and development. Root-interacting beneficial and pathogenic fungi utilize auxin and its target genes to manipulate the performance of their hosts for their own needs. In order to follow and visualize auxin effects in fungi-colonized Arabidopsis roots, we used the dual auxin reporter construct DR5::EGFP-DR5v2::tdTomato and fluorescence microscopy as well as LC-MS-based phytohormone analyses. We demonstrate that the beneficial endophytic fungi Piriformospora indica and Mortierella hyalina produce and accumulate IAA in their mycelia, in contrast to the phytopathogenic biotrophic fungus Verticillium dahliae and the necrotrophic fungus Alternaria brassicicola. Within 3 h after exposure of Arabidopsis roots to the pathogens, the signals of the auxin-responsive reporter genes disappeared. When exposed to P. indica, significantly higher auxin levels and stimulated expression of auxin-responsive reporter genes were detected both in lateral root primordia and the root elongation zone within 1 day. Elevated auxin levels were also present in the M. hyalina/Arabidopsis root interaction, but no downstream effects on auxin-responsive reporter genes were observed. However, the jasmonate level was strongly increased in the colonized roots. We propose that the lack of stimulated root growth upon infection with M. hyalina is not caused by the absence of auxin, but an inhibitory effect mediated by high jasmonate content.