The Role of Polyphenols in Abiotic Stress Response: the Inﬂuence of Molecular Structure

plants Review The Role of Polyphenols in Abiotic Stress Response: The Influence of Molecular Structure Dunja Šamec 1,*, Erna Karalija 2 , Ivana Šola 3, Valerija VujˇcićBok 3 and Branka Salopek-Sondi 1 1 Ruder¯ BoškovićInstitute, Bijeniˇckacesta 54, 10000 Zagreb, Croatia; [email protected] 2 Faculty of Science, University of Sarajevo, Zmaja od Bosne 33–35, 71000 Sarajevo, Bosnia and Herzegovina; [email protected] 3 Department of Biology, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia; [email protected] (I.Š.); [email protected] (V.V.B.) * Correspondence: [email protected] Abstract: Abiotic stressors such as extreme temperatures, drought, flood, light, salt, and heavy metals alter biological diversity and crop production worldwide. Therefore, it is important to know the mechanisms by which plants cope with stress conditions. Polyphenols, which are the largest group of plant-specialized metabolites, are generally recognized as molecules involved in stress protection in plants. This diverse group of metabolites contains various structures, from simple forms consisting of one aromatic ring to more complex ones consisting of large number of polymerized molecules. Consequently, all these molecules, depending on their structure, may show different roles in plant growth, development, and stress protection. In the present review, we aimed to summarize data on how different polyphenol structures influence their biological activity and their roles in abiotic stress responses. We focused our review on phenolic acids, flavonoids, stilbenoids, and lignans. Keywords: abiotic stress; polyphenols; phenolic acids; flavonoids; stilbenoids; lignans Citation: Šamec, D.; Karalija, E.; Šola, I.; VujˇcićBok, V.; Salopek-Sondi, B. 1. Introduction The Role of Polyphenols in Abiotic In the last couple of decades, more scientific evidence has been found to support the Stress Response: The Influence of fact that climate and atmospheric changes can rapidly alter biological diversity [1] and crop Molecular Structure. Plants 2021, 10, production [2] around the world. Environmental stress can be defined as any change in 118. https://doi.org/10.3390/ growth condition(s) within the plant’s natural habitat that alters or disrupts its metabolic plants10010118 homeostasis. In general, we recognize two different types of stress: biotic (caused by insects, bacteria, or viruses) and abiotic. Abiotic stressors such as extreme temperatures, Received: 4 December 2020 drought, flood, light, salt, and heavy metals largely influence plant development and Accepted: 5 January 2021 crop productivity. Published: 8 January 2021 Plants, as sessile organisms, respond to stress conditions with changes in the gene Publisher’s Note: MDPI stays neu- expression pattern of proteins that control the biosynthesis of metabolites involved in tral with regard to jurisdictional clai- interactions between a plant and its environment. Polyphenols are an important class ms in published maps and institutio- of specialized metabolites that play crucial physiological roles throughout the plant life nal affiliations. cycle, including responses to stress. It is well known that the phenylpropanoid biosynthetic pathway is usually activated under harmful environmental conditions such as drought, extreme temperatures, salinity, heavy metal pollutions, and ultraviolet radiations, resulting in the accumulation of various phenolic compounds [3,4]. Copyright: © 2021 by the authors. Li- Polyphenols are the largest and the most studied group of plant-specialized metabo- censee MDPI, Basel, Switzerland. lites, which include more than 8000 molecules [5]. They are all biosynthesized through a This article is an open access article shikimate/phenylpropanoid pathway that produces a wide array of monomeric and poly- distributed under the terms and con- meric polyphenols [3]. The structure of phenolic compounds varies extensively, although ditions of the Creative Commons At- their common feature is the presence of one (simple phenolics) or more (polyphenols) hy- tribution (CC BY) license (https:// droxyl substituents, attached directly to one or more aromatic or benzene rings. According creativecommons.org/licenses/by/ to their structures, they may be grouped into phenolic acids, flavonoids, stilbenoids, and 4.0/). Plants 2021, 10, 118. https://doi.org/10.3390/plants10010118 https://www.mdpi.com/journal/plants Plants 2021, 10Plants, x FOR 2021 PEER, 10, xREVIEW FOR PEER REVIEW 2 of 25 2 of 25 Plants 2021, 10, 118 2 of 24 hough theirhough common their feature common is thefeature presence is the of presence one (simple of one phenolics) (simple phenolics) or more (polyp or morehe- (polyphenols) hydroxylnols) substituentshydroxyl substituents, attached ,directly attached to directly one or moreto one aromatic or more oraromatic benzene or rings. benzene rings. AccordingA toccording their structure to theirs, structure they mays, theybe grouped may be into grouped phenolic into acids, phenolic flavonoids, acids, flavonoids, stil- stil- benoids, andlignansbenoids lignans (Figure, and (Figure lignans1). In 1 general,) .(Figure In general, phenolic 1). In phenolic general, compounds compoun phenolic mayds compoun bemay present beds presen may in plantst be in presenplants as free t in forms, plants as free formsbutas free, more but forms more often,, oftenbut they more, arethey foundoften are found, inthey conjugated arein conjugated found forms in conjugated form withs one with orform one mores or with sugarmore one residuessugar or more linked sugar β residues bylinkedresidues-glycosidic by linkedβ-glycosidic bonds by β to-glycosidic bonds a hydroxyl to abonds hydroxyl group to (O-glycosides) a grouphydroxyl (O -groupglycosides) or a carbon (O-glycosides) or atom a carbon of the or aromatica carbon atom of theringatom aromatic (C-glycosides). of the ring aromatic (C- Theglycosides). ring associated (C-glycosides). The sugars associatedcan The be associated sugars monosaccharides, can sugars be monosaccharides, can disaccharides, be monosaccharides, or even oligosaccharides [5]. disaccharidesdisaccharides, or even oligosaccharides, or even oligosaccharides [5]. [5]. Phenolic acidsPhenolic acids FlavonoidsFlavonoids StilbenoidsStilbenoids Polyphenols Polyphenols Lignans Lignans Figure 1. The basic division of the phenolic compounds. Figure 1. ThFiguree basic 1. divisionThe basic of division the phenolic of the compounds phenolic compounds.. PolyphenolicPolyphenolicPolyphenolic compounds compounds compounds are often are consideredare often often considered considered as a group as as of a a groupmolecules group of of molecules moleculeswith similar with with similar similar biologicalbiological biologicalactivity, and activity, activity, especially and and especially einspecially biological in in biological biologicalstudies, researchers studies, studies, researchers researchers measure total measure measure polyphe- total total polyphe- polyphe- nol contentnolnol as contentcontent a marker asas of a marker biological of of biologicala biologicalctivity. However a activity.ctivity., Howeverthe However, structure, the the ofstructure polyphenols structure of ofpolyphenols polyphenolssig- sig- nificantlysignificantly influencesnificantly influences their influences activity theirtheir and activity role activity in and biological and role role in biologicalprocesses in biological, processes and processes, consequently, and andconsequently, consequently,their , their involvementtheirinvolvement in involvement plant stress in plant inresponses. plant stress stress responses. Our responses. review Our aim Ourreviews to review provide aims aims to a compreheprovide to provide a nsivecomprehe a comprehensive sum-nsive sum- mary of thesummaryma currentlyry of the of theavaicurrently currentlylable literatureavai availablelable onliterature how literature different on how on howpolyphenolic different different polyphenolic structure polyphenolics structure(phe- structuress (phenolic acids,(phenolicnolic flavonoids, acids, acids, flavonoids, stilbenoids, flavonoids, stilbenoids, lignans) stilbenoids, influence lignans) lignans) plantinfluence influence stress plant response plant stress stresss .response responses.s. 2. Phenolic Acids 2. Phenolic2. AcidsPhenolic Acids A carboxyl group attached or linked to a benzene ring is a main feature of all phe- A carboxylA group carboxyl attached group or attached linked toor alinked benzene to aring benzene is a main ring featureis a main of featureall phe- of all phenolic acids. They are rarely found in free form, and are often linked by ether, acetal, or nolic acids.nolic They acids. are rarTheyely arefound rar elyin freefound form, in freeand form,are often and linkedare often by linkedether, acetalby ether,, or acetal, or ester bonds to structural components of a plant cell (cellulose, proteins, lignin), other ester bondsester to structuralbonds to structuralcomponents components of a plant of cell a plant(cellulose, cell (cellulose,proteins, lignin),proteins, other lignin), other smaller organic molecules (e.g., quinic, maleic, or tartaric acids, and glucose), other larger smaller organicsmaller molecules organic molecules(e.g., quinic, (e.g. maleic, quinic,, or maleictartaric, oracids, tartaric and acids,glucose), and other glucose), larger other larger polyphenolspolyphenols (flavonoids), (flavonoids), or or natural natural products products (e.g., (e.g., terpenes) terpenes) [ 6[6,7,7].]. Depending Depending on on their their polyphenolsstructure,

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