Response Mechanism of Plants to Drought Stress
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horticulturae Review Response Mechanism of Plants to Drought Stress Xinyi Yang, Meiqi Lu, Yufei Wang, Yiran Wang, Zhijie Liu and Su Chen * State Key Laboratory of Tree Genetics and Breeding, Northeast Forestry University, Harbin 150040, China; [email protected] (X.Y.); [email protected] (M.L.); [email protected] (Y.W.); [email protected] (Y.W.); [email protected] (Z.L.) * Correspondence: [email protected] Abstract: With the global climate anomalies and the destruction of ecological balance, the water shortage has become a serious ecological problem facing all mankind, and drought has become a key factor restricting the development of agricultural production. Therefore, it is essential to study the drought tolerance of crops. Based on previous studies, we reviewed the effects of drought stress on plant morphology and physiology, including the changes of external morphology and internal structure of root, stem, and leaf, the effects of drought stress on osmotic regulation substances, drought-induced proteins, and active oxygen metabolism of plants. In this paper, the main drought stress signals and signal transduction pathways in plants are described, and the functional genes and regulatory genes related to drought stress are listed, respectively. We summarize the above aspects to provide valuable background knowledge and theoretical basis for future agriculture, forestry breeding, and cultivation. Keywords: drought stress; osmotic regulation; LEA protein; ROS; signaling; drought-responsive gene Citation: Yang, X.; Lu, M.; Wang, Y.; Wang, Y.; Liu, Z.; Chen, S. Response 1. Introduction Mechanism of Plants to Drought Stress. Horticulturae 2021, 7, 50. Drought is one of the most important factors restricting agricultural production, https://doi.org/10.3390/ which seriously affects crop yield [1,2]. Moreover, as one of the main restraining factors in horticulturae7030050 the process of plant growth, drought can hinder plant respiration, photosynthesis, and stom- atal movement; thus, affecting plant growth and physiological metabolism. In response to Academic Editors: Stefania Toscano, drought stress, plants activate their drought response mechanisms, such as morphological Giulia Franzoni and Sara Álvarez and structural changes, expression of drought-resistant genes, synthesis of hormones, and osmotic regulatory substances to alleviate drought stress. To better reveal the mech- Received: 20 February 2021 anism of drought resistance of plants, based on a lot of previous work, we summarized Accepted: 11 March 2021 the status quo and progress of studies on the morphological structure, physiological and Published: 13 March 2021 biochemical mechanism changes, internal signal transduction system, and molecular regu- lation mechanism of plants under drought stress in recent years. Under drought conditions, Publisher’s Note: MDPI stays neutral plants sense water stress signals and produce signal molecules, such as abscisic acid (ABA), with regard to jurisdictional claims in Ca2+, inositol-1, 4, 5-triphosphate (IP3), cyclic adenosine 50-diphosphate ribose (cADPR), published maps and institutional affil- NO, etc., and directly or indirectly lead to the morphological and physiological changes of iations. plants through signal transduction. Indirectly, drought stress signals induce the expression of downstream genes. Functional gene products, such as proline (pro), glycine betaine (GB), soluble sugar (SS), late embryogenesis abundant (LEA) proteins, and aquaporin (AQP) can be involved in plant metabolism and, thus, affect plant state. Regulatory gene Copyright: © 2021 by the authors. products, such as calcium-dependent protein kinases (CDPKs), mitogen-activated protein Licensee MDPI, Basel, Switzerland. kinases (MAPKs), HD-zip/bZIP, AP2/ERF, NAC, MYB, and WRKY can cause changes in This article is an open access article plant morphology or physiology by regulating signal transduction pathways or acting as distributed under the terms and transcription factors to regulate the expression of downstream genes, and further enable conditions of the Creative Commons plants to successfully survive in the arid environment (Figure1). Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Horticulturae 2021, 7, 50. https://doi.org/10.3390/horticulturae7030050 https://www.mdpi.com/journal/horticulturae Horticulturae 2021, 7, x FOR PEER REVIEW 2 of 38 duction pathways or acting as transcription factors to regulate the expression of down- Horticulturae 2021, 7stream, 50 genes, and further enable plants to successfully survive in the arid environment 2 of 36 (Figure 1). Figure 1. The processFigure of 1.plantThe drought-tolerance process of plant drought-tolerance development. development. We will elaborateWe willfrom elaborate the following from four the following parts. The four first parts. is the Theeffect first of drought is the effect stress of drought stress on the externalon morphology the external and morphology internal stru andcture internal of plants. structure The ofsecond plants. part The elaborates second part elaborates the physiologicalthe and physiological biochemical and responses biochemical from responsesthe perspectives from theof osmotic perspectives regulation of osmotic regula- metabolism, drought-inducedtion metabolism, protein drought-induced metabolism, protein and reactive metabolism, oxygen and metabolism. reactive oxygen Here metabolism. we summarizeHere some we important summarize drought-regulating some important drought-regulatingsubstances and we also substances briefly andsum- we also briefly marize the generationsummarize and thescavenging generation process and of scavenging reactive oxygen process species of reactive (ROS). The oxygen third species (ROS). part is the signalThe transduction third part is pathway the signal in transductionplants. We describe pathway common in plants. signals We describein detail common sig- and elucidationnals of intracellular in detail and signal elucidation transduc oftion intracellular pathways. signal The fourth transduction part is about pathways. the The fourth molecular regulationpart is aboutmechanism the molecular of plants. regulationFrom the perspective mechanism of of genes, plants. the Fromanabolism the perspective of and regulationgenes, mechanisms the anabolism of osmotic and regulation regulation-related mechanisms substances, of osmotic drought-induced regulation-related substances, proteins, signalingdrought-induced path-related proteins,substances, signaling and transcription path-related factors substances, are summarized and transcription re- factors are spectively. Allsummarized of the advances respectively. indicate Allthat of it the is of advances great significance indicate that to it study is of great the effects significance to study of drought stressthe on effects plants of droughtand explore stress the on mechanism plants and of explore drought the tolerance. mechanism of drought tolerance. 2. Effects of Drought2. Effects Stress of Drought on Plant Stress Morphological on Plant Morphological Characteristics Characteristics When plants areWhen subjected plants areto drought subjected stre toss, drought they will stress, first theyrespond will to first changes respond in to changes in external form externaland internal form structure. and internal The structure.most significant The most effect significant of water effect loss is of that water the loss is that the plant grows slowlyplant and grows even slowly dies. Studies and even have dies. shown Studies that have plants shown under that abiotic plants stress under can abiotic stress adapt to changingcan adaptenvironmental to changing factors environmental through phenotypic factors through plasticity. phenotypic Therefore, plasticity. under Therefore, the influence ofunder the environment, the influence xerophytes of the environment, have formed xerophytes certain morphological have formed certain charac- morphological characteristics in the process of evolution, and adapted themselves to drought in their teristics in the process of evolution, and adapted themselves to drought in their ontoge- ontogenetic development under these characteristics. The drought-resistant plants have netic development under these characteristics. The drought-resistant plants have morpho- morphological and structural characteristics that were adapted to the arid environment in logical and structural characteristics that were adapted to the arid environment in terms terms of leaves, stems, roots, and so on. of leaves, stems, roots, and so on. 2.1. Drought Stress and the External Form of Plants The obvious symptoms of water deficit during the vegetative period are plant height decreased, leaf wilting, number and area of leaves changed. Plant height, severely affected by drought, is closely related to cell enlargement and leaf senescence. The decrease in plant Horticulturae 2021, 7, 50 3 of 36 height is mainly due to decreased cell expansion, increased leaf shedding, and impaired mitosis under drought conditions. Some studies have reported that plant height of lily [3], maize [4], cane [5], and rice [6] decreased significantly under drought stress. In addition to the changes in plant height, different organs of plants also differ significantly in morphology. As an indicator of the degree of water shortage in direct response, leaves are the main organs for plant assimilation and transpiration. Plant leaves generally adopt smaller leaf areas, larger leaf thickness, and higher leaf tissue density to adapt