Biochemical Defenses of Plants

Dear Colleagues,

Plants are often referred to as ‘nature’s chemists’ since they collectively biosynthesize hundreds of thousands of specialized metabolites. Plant-specialized metabolites have direct economic value as pharmaceuticals, food additives, and other industrial uses, yet they have a prominent role in defending plants, including major crop species, against important agricultural pests and pathogens. These include pathogenic bacteria, fungi, oomycetes, phytoplasma, viruses, viroids, other plant species, and macro- and microscopic animals such as insects and nematodes. Thus, understanding the biochemical defences of plants, including their biochemical diversity, biosynthesis, degradation, complex signalling, and regulatory networks, and their roles in mediating interactions with other organisms, will inform genetic engineering or breeding strategies to enhance their production to support sustainable agriculture and the supply of plant-derived small-molecule-type pharmaceuticals.

This Special Issue of Plants will highlight all novel aspects of the biochemical defences of plants at the genetic, molecular, biochemical, cellular, organismal, and ecological levels. Scientists from all over the world are invited to submit original research and review articles on topics related to the biochemical defences of plants.

Dr. Nik Kovinich
Prof. Dr. Philippe Jeandet
Dr. Clive Lo
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Plants is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

• defence mechanisms
• biochemical defence
• disease resistance
• antimicrobial activity
• biological activity
• defence metabolites
• specialized metabolites
• secondary metabolites
• phytoalexins
• phytoanticipins
• elicitors
• pathogens
• microorganisms
• insects
• nematodes

Title: Timing is everything: Metabolic partitioning of suberin-destined carbon
Authors: JESSICA L. SINKA and MARK A. BERNARDS
Affiliation: Department of Biology, Western University, London, ON, N6A 5B7, Canada
Abstract: Suberin is a cell wall-associated biopolymer that possesses both poly(phenolic) and poly(aliphatic) elements assembled into chemically and spatially distinct domains. Domain-specific monomers are formed via a branched pathway between phenolic and aliphatic metabolisms. Transcript accumulation data (RNAseq), from the early stages of wound-induced suberization, revealed highly coordinated, temporal changes in regulation of the ‘branches’. Notably, phenolic-associated transcripts accumulated first which could indicate a preference toward phenolic production early on post-wounding. To better understand the dynamics of suberin monomer biosynthesis and assembly, we quantified the allocation of carbon between phenolic and aliphatic metabolisms in a wound-induced suberin model. To do so, [13C]-glucose was administered to wound-healing potato tuber discs at different times post-wounding and patterns of heavy carbon incorporation into primary metabolites assessed. During the early stages of wound healing, carbon from [13C]-glucose was rapidly incorporated into phenolic-destined metabolites and sucrose, while at later stages it was shared between phenolic- and aliphatic-destined metabolites. Labelled carbon was ultimately found in both the poly(phenolic) and poly(aliphatic) domains, regardless of when the [13C]-glucose was applied. Because suberin is an innate physical barrier that confers resistance to drought, pathogens, and desiccation during crop storage, understanding it’s temporal regulation can help inform strategies for crop enhancement through genetic engineering and/or marker-assisted breeding.

Title: Altered metabolism in knockdown lines of two HXXXD/BAHD acyltransferases in wound-healing potatoes
Authors: SAU KEI (LORENA) YEUNG1, INDIRA QUERALTA CASTILLO1, JESSICA L. SINKA1, EMILY DRENNAN1, GEORGE DEL GROSSO1, ISABEL MOLINA2, SANGEETA DHAUBHADEL1,3, MARK A. BERNARDS1
Affiliation: 1Department of Biology, Western University, London, ON, N6A 5B7, Canada, 2Department of Biology, Algoma University, Sault St. Marie, ON, P6A 2G4, Canada, 3London Research and Development Centre, Agriculture and Agri-Food Canada, 1391 Sandford St, London, Ontario, N5V 4T3, Canada
Abstract: Suberin is a phenolic-lipophilic biopolymer that functions as a physical barrier to protect plants from environmental stressors such as desiccation and pathogen infection. It is composed of two spatially distinct domains, the poly(phenolic) domain and the poly(aliphatic) domain. Using RNAseq data, we identified StFHT (Fatty ω-hydroxyacid/fatty alcohol Hydroxycinnamoyl Transferase) and two uncharacterized potato (Solanum tuberosum L.) HXXXD/BAHD acyltransferases, designated StHCT (HydroxyCinnamoyl Transferase) and StACT (ACyl Transferase), as highly expressed upon wounding. While StFHT has been previously characterized, StHCT and StACT have not. Despite being more closely related to hydroxycinnamoyl-quinate hydroxycinnamoyl transferases than StFHT, analysis from independently generated RNAi-silenced StFHT lines and StHCT lines revealed the same suberin phenotype of decreased alkyl ferulate ester production and reduced esterified ferulic acid in wound-induced potato tubers of both genotypes. StACT-RNAi knockdown lines, however, were indistinguishable from wild-type (WT). To assess the upstream effects of impaired ferulate ester production, metabolites were extracted from suberizing empty vector (WT control), StFHT-RNAi, and StHCT-RNAi microtubers over 8 days post wounding. Overall, StFHT knockdown lines accumulated alternative ferulate conjugates (esp. feruloyltyramine), while StHCT knockdown lines accumulated p-coumaroyl derivatives. These data support the function of STHCT in upstream hydroxycinnamic acid biosynthesis and the critical role of phenolic metabolism in suberin formation.

Title: Bioactive immunoadjuvant saponins from /Quillaja lancifolia/ act as antifungal and herbivore deterrents
Authors: Yendo, A.C.A.; Matsuura, H.N.; Colling, L.C.; De Costa, F.; Vargas, L.R.B.; Martinelli, J.A.; Graichen, F.; Vainstein, M.H.; Landell, M.F.; Fett-Neto, A.G.
Affiliation: Federal University of Rio Grande do Sul, Brazil
Abstract: Saponins from leaves of /Quillaja lancifolia/, a native species from southern Brazil, show immunoadjuvant activity in several experimental vaccine formulations. The accumulation of the immunoadjuvant saponin fraction QB-90 is induced by several stresses and stress signaling molecules in cultured leaf disks and seedlings, suggesting that these terpenes may be players in defense responses. An investigation of the potential inhibitory role of /Q. lancifolia /saponins on plant pathogenic fungi and two herbivore models was carried out. The results revealed that /Q. lancifolia /saponins had antifungal activity against several phytopathogenic fungi, including /Bipolaris micropus, Curvularia inaequalis /and /Fusarium incarnatum/. In addition, the same saponins acted as deterrents against the generalist insect and mollusk herbivores /Spodoptera frugiperda/ and /Helix aspersa/, respectively. Significant reductions in consumption of leaf area and weight (larvae) were recorded. Data support a role for /Q. lancifolia/ saponins in plant defense against fungi and herbivores, thereby having potential as natural control agent against plant pests or as molecular platforms for new molecule development.

Title: Phosphite containing nickel and potassium potentiates soybean defense against infection by Phakopsora pachyrhizi
Authors: Fabricio Rodrigues
Affiliation: Universidade Federal de Viçosa
Abstract: Soybean is considered one of the most profitable crops among the legumes grown worldwide. The occurrence of rust epidemics, caused by the fungus Phakopsora pachyrhizi, has contributed significantly to great yield losses and an abusive spray of fungicides. Within this context, this study investigated the potential of using a phosphite of nickel (Ni) and potassium (K) (referred to as induced resistance [IR] stimulus) to induce soybean resistance against infection by P. pachyrhizi. Plants were sprayed with water (control) or with the IR stimulus and non-inoculated or inoculated with P. pachyrhizi. Urediniospores germination was significantly reduced by 99% by the IR stimulus as its rates ranged from 2 to 15 mL/L in vitro. Rust severity was significantly reduced by 68-78% from 7 to 15 days after inoculation (dai). The area under the disease progress curve significantly decreased by 74% for IR stimulus-sprayed plants compared to water-sprayed plants. For inoculated plants, K and Ni foliar concentrations were significantly higher for IR stimulus treatment than for the control treatment. Infected plants sprayed with IR stimulus had their photosynthetic apparatus (great pool of photosynthetic pigments and lower values for some chlorophyll a fluorescence parameters) preserved, associated with less cellular damage (lower concentrations of malondialdehyde, hydrogen peroxide, and anion superoxide) and more production of phenolics and lignin than plants from the control treatment. In response to infection by P. pachyrhizi, the defense-related genes (PAL2.1, PAL3.1, CHIB1, LOX7, PR-1A, PR10, ICS1, ICS2, JAR, ETR1, ACS, ACO, and OPR3) were up-regulated from 7 to 15 dai for IR stimulus-sprayed plants in contrast to plants from the control treatment. Collectively, these findings provide a global picture of the enhanced capacity of IR stimulus-sprayed plants to efficiently cope with fungal infection at both biochemical and physiological levels without discarding the direct effect of this IR stimulus on urediniospores germination.