Advances in salt tolerance molecular mechanism in tobacco plants

Sun et al. Hereditas (2020) 157:5 https://doi.org/10.1186/s41065-020-00118-0 REVIEW Open Access Advances in salt tolerance molecular mechanism in tobacco plants Haiji Sun1, Xiaowen Sun1, Hui Wang1 and Xiaoli Ma2* Abstract Tobacco, an economic crop and important model plant, has received more progress in salt tolerance with the aid of transgenic technique. Salt stress has become a key research field in abiotic stress. The study of tobacco promotes the understanding about the important adjustment for survival in high salinity environments, including cellular ion transport, osmotic regulation, antioxidation, signal transduction and expression regulation, and protection of cells from stress damage. Genes, which response to salt, have been studied using targeted transgenic technologies in tobacco plants to investigate the molecular mechanisms. The transgenic tobacco plants exhibited higher seed germination and survival rates, better root and shoot growth under salt stress treatments. Transgenic approach could be the promising option for enhancing tobacco production under saline condition. This review highlighted the salt tolerance molecular mechanisms of tobacco. Keywords: Salt tolerance, Transgenic technology, Gene, Tobacco Background Abiotic stress is the most harmful factor concerning the growth and productivity of crops worldwide, leading to enhanced accumulation of osmolytes, reduced photosynthesis, closure of stomata, and induction of stressresponsive genes [1–5]. Salt stress is one of the major abiotic stresses that have been related to the significant economic impact caused by the loss of arable land and the decline of agricultural productivity [6–8]. Salt stress caused the crop damages via ion balance, osmotic regulation and removal of reactive oxygen species [9–12]. Inducing these pathways through short-term exposure to low-salt stress, a process known as salt adaptation, can improve plant resistance to salt [13–15]. However, tolerance to soil salinity levels varies between plant species. Tobacco (Nicotiana tabacum L.) is one of the main industrial crops and is widely grown in many countries. Tobacco is forming complex defenses to resist salt stress that rely on a variety of mechanisms [16–19]. Generally, salt stress in tobacco can be divided into ion toxicity, such as destroying plasma membrane structure, hindering the absorption of mineral elements, etc. and the * Correspondence: 2 Central laboratory, Jinan Central Hospital Affiliated to Shandong University, Jinan 250013, China Full list of author information is available at the end of the article secondary stress effect, such as oxidative stress, drought stress, etc. [20, 21]. In this review, the recent advances on the mechanism of salt tolerance in tobacco were summarized in order to provide data for the study of salt tolerance and the adjustment of planting layout in tobacco. Ion transport genes related to tobacco salt tolerance The activities of ion transporters or antiporters localized in the plasma membrane and vacuolar membrane are essential for tobacco growth and development [22–24]. Intracellular regionalization of toxic ions using specific transporter proteins is a key pattern used by tobacco to maintain a moderate cytosolic K+/Na+ ratio in the cytosol. The high-affinity potassium ion transporter protein selectively absorbs K+ from the environment to balance the ratio of Na+/K+ in cells and prevent the toxicity of excessive Na+ content to cells [25–28]. Constitutive expression of potassium transporter OsHAK5 in culturedtobacco BY2 (Nicotiana tabacum cv. Bright Yellow 2) cells enhanced the accumulation of K+ but not Na+ in the cells during salt stress and conferred increased salt tolerance to the cells, suggesting that the plasmamembrane localized Na+ insensitive K+ transporters could be used as a tool to enhance salt tolerance in tobacco [29]. Na+ transporter protein (SKC) can transport © The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Sun et al. Hereditas (2020) 157:5 Na+ exclusively, but does not participate in the transport of other cations such as K+, and plays an important role in resisting abiotic stress [30–32]. The survival rate and root length of SbSKC1 transgenic tobacco plants under NaCl stress were significantly higher than those of the control [33]. The activities of superoxide dismutases (SOD), catalase (CAT), and pero-xidase (POD) enzymes were increased, and the salt tolerance of transgenic tobacco plants was strengthened [34]. Na+/H+ reverse proteins are mainly located in the vacuole membrane and cytoplasmic membrane, which are called vacuolar Na+/H+ reverse transporter (V-type and P-type) [35]. Na+/H+ antiporters (NHXs) are integral membrane transporters that catalyze the electroneutral exchange of K+/Na+ for H+ and are implicated in cell expansion, development, pH/ion homeostasis and salt tolerance [36, 37]. Different NHX isoforms have been overexpressed in variety of plant species showed substantial salt tolerance. NHX1 had functions in regulating the pH in the vacuole and cellular ROS level, which could prime the antioxidative system [38, 39]. Arabidopsis AtNHX1, the first tonoplast Na+/H+ exchanger identified in plants, mediates Na+/H+ exchange activity in plant vacuoles [40]. Overexpression of AtNHX confers salt tolerance in Arabidopsis plants and salt tolerance correlates with increased vacuolar Na+/H+ exchange activity and vacuolar sodium accumulation. LfNHX1 protein sequence showed high similarity with NHX1 homologs reported from other halophyte plants. The overexpression of LfNHX1 gene under CaMV35S promoter conferred salt and drought tolerance in tobacco plants [41, 42]. NbNHX1 silencing led to a lower pH in the vacuole and a lower cellular ROS level in N. benthamiana, which was coupled with a decreased NAD(P) (H) pool and decreased expression of ROSresponsive genes [43]. Overexpression of SeNHX1 intensified the compartmentation of Na + into vacuole under salt stress and improved the ability of eliminating ROS after pathogen attack, which then enhanced salt tolerance and disease resistance simultaneously in tobacco [44]. SeNHX1, AtNHX1, sbNHX1 and NbNHX1 transgenic tobaccos exhibited more biomass, longer root length, and higher Na+/H+ ratio under NaCl treatment, indicating enhanced salt tolerance [45]. Osmotic regulation genes related to tobacco salt tolerance Betaine is a water-soluble alkaloid in plants and has (...truncated)