植物响应盐碱胁迫的生理分子和微生物学机制
doi: 10.11931/guihaia.gxzw202506024
王丽 1, 2 , 王剑峰 3 , 马兰民 2 , 李含笑 2 , 蒋中玉 2 , 梁佳 2 , 徐雯敏 2 , 马乐乐 2 , 李萍 2
1. 青海大学 畜牧兽医科学院,西宁 810016
2. 青海大学 生态环境工程学院,西宁 810016
3. 兰州大学 草种创新与草地农业生态系统全国重点实验室,兰州 730030
基金项目: 青海省“昆仑英才·高端创新创业人才”项目;国家自然科学基金面上项目(32371772);中央财政林草科技推广示范项目[青(2023)TG03];青海大学省部共建三江源生态与农牧业国家重点实验室开放基金项目(2024-KF-04);青海大学国家林业草原高寒草地适应性管理工程技术研究中心自主基金项目(2025GJLCGCZX002)。
Physiological, molecular, and microbiological mechanisms of plant response to saline-alkaline stress
WANG Li 1, 2 , WANG Jianfeng 3 , MA Lanmin 2 , LI Hanxiao 2 , JIANG Zhongyu 2 , LIANG Jia 2 , XU Wenmin 2 , MA Lele 2 , LI Ping 2
1. Academy of Animal Science and Veterinary, Qinghai University, Xining 810016, China
2. School of Ecological and Environmental Engineering, Qinghai University, Xining 810016, China
3. National Key Laboratory of Grassland Agro-ecosystem and Grass Variety Improvement, Lanzhou University, Lanzhou 730030, China
摘要
土壤盐碱化是影响土壤环境和农业生产的重要限制因素,会对植物的生长代谢活动产生不利影响。植物在长期进化中发展出复杂的机制以应对盐碱胁迫,如改变形态结构、激活抗氧化防御系统、调节渗透平衡、维持细胞离子稳态、调控植物激素水平、稳定细胞内pH值以确保正常生化反应。此外,植物还可通过调控基因和转录因子的表达,激活信号转导通路以响应盐碱胁迫。同时,植物根系分泌物作为化学信号,在根际环境中选择性招募特定有益微生物,重塑根际微生物群落的多样性、组成、结构及功能特性,构建出适应盐碱胁迫的根际环境。植物生长促进微生物(plant growth-promoting microorganisms,PGPM)因与植物的共生关系被认为是植物的第二基因组,可通过多种协同机制增强植物耐盐碱性。主要包括调节植物激素水平、控制渗透平衡、增强抗氧化防御系统、维持离子平衡、促进营养吸收、调节代谢途径,从而提高植物的耐盐碱性。PGPM还可以通过富集有益微生物以促进宿主植物生长,并增强其抗逆性。该文系统综述了植物应对盐碱胁迫的形态学、生理学、分子生物学及根际微生物学机制,重点阐明了根际微生物群在维持盐碱环境下植物生命活动中的关键作用,为盐碱地区可持续农业发展与生态修复提供了坚实的科学基础。
Abstract
Soil salinization is a significant limiting factor affecting soil environment and agricultural production, adversely impacting plant growth and metabolic activities. Through long-term evolutionary adaptation, plants have developed complex mechanisms to counteract saline-alkaline stress via intrinsic metabolic regulation. For instance, they alter the morphological structure of roots and leaves to increase plant water uptake and transpiration, activate antioxidant defense systems (enzymes and compounds) to scavenge stress-induced reactive oxygen species (ROS), accumulate various osmotic substances (proline, soluble sugar, soluble protein, and polyols) to regulate osmotic balance, maintain cellular ion homeostasis, modulate endogenous plant hormone levels (abscisic acid, gibberellic acid, auxin, and cytokinin) to coordinate growth and stress responses, and stabilize intracellular pH (HCO3-/CO32- stress) to ensure normal biochemical reactions. Moreover, plants respond to saline-alkaline stress by regulating the transcription of stress-responsive genes and transcription factors, coupled with triggering signal transduction pathways. Meanwhile, plant root exudates function as chemical signals to selectively recruit specific beneficial microorganisms in the rhizosphere. This recruitment process reshapes the diversity, composition, structure, and functional characteristics of the rhizosphere microbial community, constructing a rhizosphere environment adaptive to saline-alkaline stress. Plant growth-promoting microorganisms (PGPM), recognized as the “second genome” of plants owing to their close symbiotic relationship with plants, enhance plant saline-alkaline tolerance through various synergistic mechanisms. These primarily include regulating plant hormone levels, controlling osmotic balance, enhancing the antioxidant defense system, maintaining ion balance, promoting nutrient absorption, and regulating metabolic pathways, thereby improving the plant’s tolerance to salinity and alkalinity. In addition, PGPM can influence the assembly of the rhizosphere microbial community, promoting host plant growth and enhancing its stress tolerance by enriching beneficial microorganisms. This maintains their ability to promote plant growth and mitigate stress effects. This review systematically summarizes the morphological, physiological, molecular biology, and rhizosphere microbiological mechanisms by which plants respond to saline-alkaline stress. It highlights the pivotal role of PGPM in sustaining plant life under saline-alkaline conditions, provides a robust scientific foundation for sustainable agricultural development and ecological restoration in saline-alkaline regions.
土壤盐碱化是影响土壤环境和农业生产的主要因素之一(Singh, 2022)。据估计,世界至少有1/3的耕地面积受到盐胁迫的影响(Shi & Gu, 2020)。此外,Wang等(2023)研究预测至2050年,全球范围内大约50%的农业土壤将因气候变化和不合理的土地利用方式而发生盐碱化。根据第三次全国土壤普查,我国盐碱化土地面积接近1亿hm2,占全球盐碱化土地面积的18%(凌磊,2021; Wang et al., 2024)。盐碱胁迫下,钠离子(Na+)和氯离子(Cl-)引起植物渗透胁迫,导致离子失衡,继而造成植物氧化损伤,营养失衡和器官衰老,甚至死亡(Yang & Guo, 2018)。因此,盐碱地修复对改善我国生态环境和提高粮食产量具有深远意义。目前,诸多研究已提出包括地下排水调控、化学改良剂施用和耐盐植物品种选育在内的一系列土壤盐碱化修复策略,其中植物生长促进微生物(plant growth-promoting microorganisms,PGPM)的应用是一项重要措施,该措施可减轻植物的盐毒害,并改善土壤健康(Ilangumaran & Smith, 2017; Sarkar et al., 2018)。已有研究表明,特定的微生物可通过介导离子稳态、调节植物激素水平、促进渗透物质积累、提高抗氧化活性和增强养分吸收能力等途径提高植物耐盐性(Li H et al., 2021)。例如,在盐碱胁迫下,植物体内乙烯的合成增加,过量的乙烯会抑制植物生长,而PGPM会产生1-氨基环丙烷-1-羧酸盐(ACC)脱氨酶,催化乙烯生物合成的前体物质(ACC)转化为氨和α-酮丁酸酯,从而降低乙烯含量(Riyazuddi et al., 2020)。此外,土壤中的PGPM群落多样性受植物根系分泌物的调控,植物在盐碱胁迫下可分泌特定的代谢物以招募根际促生微生物,从而改善土壤环境和植物代谢,减缓盐碱环境对植物的损伤(Sasse et al., 2018; Qu et al., 2024)。因此,通过耐盐植物和有益微生物相互作用改善盐碱土壤环境,对于治理修复盐碱地具有重要的科学意义和广阔的应用前景。目前,已有大量研究证明使用植物-微生物互作系统是改善盐碱地环境的重要方法,因此本文将综述植物响应盐碱胁迫的分子和微生物机制,以及PGPM促进植物抵抗盐碱胁迫的主要机制,以期为治理及修复盐碱化土壤提供科学依据。
1 植物响应盐碱胁迫的形态生理分子机制
盐碱土壤中的盐分主要由NaCl、Na2SO4、Na2CO3和NaHCO3构成,对植物的危害主要分为短期胁迫和长期胁迫两个阶段(鲍海娟等,2024)。第一阶段(短期胁迫):当植物初次处于高盐环境中时,根部吸收Na+会导致植物内部水势下降,生理干旱,进而导致生长速率下降,引发渗透胁迫(Munns & Tester, 2008)。第二阶段(长期胁迫):随着植物对Na+的持续吸收,Na+经木质部被转运到植物叶片和芽中,最终引发Na+在植物体内的过量积累。过量的Na+与K+竞争细胞代谢的结合位点,造成代谢紊乱,形成离子毒害(Liu et al., 2015Siddiqui et al., 2017)。此外,Na2CO3和NaHCO3通过渗透胁迫,在盐胁迫的基础上进一步提升环境pH值,破坏植物细胞内pH的稳定性,造成细胞膜破损,最终导致植物光合作用效率和根系活力下降(Kaiwen et al., 2020)。在长期的盐碱胁迫下,植物已进化出多种调控机制以增强其环境适应能力(Gong et al., 2016)(表1)。植物主要通过形态变化、代谢调节和基因调控等不同层面的反应以适应盐碱胁迫(图1)。
1.1 植物响应盐碱胁迫的形态学机制
根部作为最先感应盐碱胁迫的器官,是植物发生逆境响应的初始部位,决定了盐碱胁迫对地上部分的危害程度(An et al., 2021)。盐碱胁迫对根系的伤害,主要表现在平均直径、总吸收面积和活力下降,甚至根部死亡(王学琴,2022)。Yi等(2007)研究发现,盐生植物梭梭(Haloxylon ammodendron)可通过增加轴根和主根的长度增加吸水面积,并通过提高根系活力和植物吸水量以适应盐碱胁迫。此外,植物根部在受到盐碱胁迫后,将信号迅速传递给地上部分,叶片随后关闭气孔以降低蒸腾作用,甚至减缓新叶发育以适应盐碱胁迫(Munns & Tester, 2008)。例如,Liu等(2022)发现水稻(Oryza sativa)在盐碱胁迫下,会降低叶片的蒸腾作用,减弱盐离子从根部向叶片转运,并减少植物内部水分散失,从而应对高盐胁迫;Waisel(1972)对盐生植物红树林(mangrove forest)的叶片形态和解剖结构分析时发现在盐碱胁迫下,盐生植物主要通过增加叶片肉质化程度,降低气孔密度与孔径、增厚表皮层和角质层,抵御盐胁迫引发的生理性干旱。综上所述,植物可通过改变地上部和地下部的结构和形态以适应盐碱胁迫。
1.2 植物响应盐碱胁迫的生理生化机制
1.2.1 激活抗氧化系统
NaCl可诱导植物ABI4基因的表达,从而增强RbohD的表达,并抑制VTC2的表达。活化的RbohD会促进活性氧(reactive oxygen species, ROS)的产生,而VTC2的抑制则会影响ROS的清除。因此,ABI4-RbohD/VTC2这一调控模块会积极促进植物中ROS的积累(Luo et al.,2021)。积累的ROS会导致DNA损伤、脂质过氧化、蛋白质氧化、酶失活和叶绿素降解,进而导致植物不能正常生长(Islam et al.,2016; Liu X L et al., 2019)。植物为抵御盐碱胁迫会启动抗氧化防御系统,主要包括酶类抗氧化系统与非酶类抗氧化系统两种。前者主要包括超氧化物歧化酶(superoxide dismutase, SOD)、过氧化物酶(peroxidase, POD)、过氧化氢酶(catalase, CAT)等(Madhu et al., 2022)。SOD是第一道防线,先清除超氧离子,将其转化成H2O2,之后POD和CAT负责将H2O2转化为无害的O2和H2O(De Andrade Santos et al., 2018; Lu et al., 2020)。Afzal等(2023)对不同耐盐性高粱品种进行盐胁迫实验发现,耐盐性品种叶片中SOD在高盐胁迫下增强了稳定性和活性,而CAT对盐胁迫浓度的敏感性更高且活性持续增加。盐生植物海马齿(Sesuvium portulacastrum)在盐碱胁迫下,可增强CAT、SOD、抗坏血酸过氧化物酶(ascorbate peroxidase, APX) 等抗氧化酶的活性,以清除叶片中过量的ROS,以在盐碱土壤修复中发挥重要作用(Muchate et al., 2016)。后者主要包括谷胱甘肽(glutathione, GSH)、抗坏血酸(ascorbic acid, ASA)、类胡萝卜素等抗氧化剂(Yin et al., 2019; Kamal et al., 2024)。这些化合物存在于植物的不同部位,与抗氧化酶共同作用,清除过量的ROS,以缓解盐碱胁迫引起的氧化损伤(许芳芳,2023)。Fu等(2017)对两个敏感性不同的玉米品种JY417(高耐盐性栽培品种)和XY335(盐敏感栽培品种)的研究表明,盐碱胁迫可增加ASA和GSH的含量,在维持蛋白质稳定性和生物膜系统结构完整性和防止膜脂过氧化方面发挥重要作用;并且,通过与SOD、APX、GPX和GR的合作,ASA和GSH构成一个循环系统,能有效清除自由基,维持细胞内活性氧代谢的平衡。Hamdani等(2017)发现,盐生植物滨藜(Atriplex halimus)的叶片同样可通过增加AsA含量和CAT活性清除ROS,从而达到有效缓解盐胁迫对其造成损伤的目的。
1 盐生植物种类及其耐盐机制
Table 1 Halophytes with their salt tolerance mechanisms
1 植物响应盐碱胁迫的生理生化机制
Fig. 1 Physiological and biochemical mechanisms of plant response to saline-alkaline stress
1.2.2 调节渗透系统
盐碱胁迫下,植物细胞外大量离子积累导致渗透势升高,细胞吸水困难,代谢失衡,最终造成植物生理性干旱,生长速率降低。植物通过合成积累各种渗透物质(小分子有机化合物,包括脯氨酸、可溶性糖、可溶性蛋白质和多元醇),提高细胞吸水或保水能力,从而维持细胞渗透压及正常生理代谢活动(魏天娇,2021)。Muchate等(2016)研究发现,盐生植物海马齿可通过增加叶片中脯氨酸、甘氨酸甜菜碱和总可溶性糖浓度维持叶片渗透平衡以响应盐碱胁迫。其中,脯氨酸是植物中重要的渗透剂,其含量升高已被用作评判植物响应盐碱胁迫的生理标志。Ahanger等(2019)研究发现,施用外源物质水杨酸和一氧化氮可增加黄花木槿(Vigna angularis)的脯氨酸含量,并增强糖的合成途径,以缓解盐碱胁迫带来的不利影响。此外,Khedr等(2003)对盐生植物海水仙(Pancratium maritimum)的耐盐碱机制进行研究发现,脯氨酸除具有渗透保护作用以外,还可上调应激保护蛋白的合成,从而提高植物的耐盐碱性。可溶性糖同样是调节植物渗透压的重要物质,如盐生植物梭梭根部中葡萄糖、蔗糖和甘油等糖和糖醇水平的升高有利于渗透调节(Panda et al., 2021)。脯氨酸和可溶性糖类物质通常共同作用,以调节植物的渗透平衡。例如,耐盐碱高粱(Sorghum bicolor)和小麦(Triticum aestivum)可通过增加脯氨酸、可溶性糖和山梨醇含量改善植物在盐碱胁迫下的生长(Guo et al., 2015;Sun et al., 2019)。
1.2.3 调节离子平衡
有研究发现,随着土壤盐碱程度的增加,大量Na+流入细胞质会导致膜电位降至静息电位以下,进而激活K+流出通道,并破坏K+/Na+的稳态平衡(Li et al., 2010Falhof et al., 2016)。K+与卡尔文循环、糖酵解、细胞初级代谢等多条途径相关的酶活性密切相关,在盐碱胁迫下,Na+在细胞内大量积累,与K+离子竞争酶活性位点,严重影响了细胞的正常代谢活动(Zhao et al., 2020)。植物主要通过将Na+排出胞外和固存在液泡中以适应盐碱胁迫(魏天娇,2021)。植物细胞中,Na+的转运主要由H+-ATP酶和H+-VPPase产生的质子驱动。在盐生植物中,盐碱胁迫可通过显著提高H+-ATP酶和H+-VPP酶的活性增强Na+从细胞质流入胞外质体和液泡的能力(Ye et al., 2019)。此外,盐碱胁迫使Ca2+含量显著增加,进而触发SOS信号通路,实现对Na+的排除,减少Na+在细胞质中的积累(Yang et al., 2019)。钙依赖性SOS2/SOS3激酶复合物可调控液泡膜上的NHX1离子反转运体,将Na+隔离在液泡内(Bahmani et al., 2015)。
1.2.4 调节激素水平
植物激素在抵御盐碱胁迫过程中发挥着重要作用,其中脱落酸(abscisic acid, ABA)是调节植物抗氧化防御系统最重要的激素之一(Sahay et al., 2019)。Chen等(2022)研究发现,ABA可显著降低盐胁迫下水稻细胞中的Na+含量,从而提高水稻抗氧化酶活性,以维持内源性激素平衡,缓解逆境对植物的损伤。赤霉素(gibberellic acid, GA)也是重要的植物抗逆激素。Li等(2019)研究发现,耐盐碱水稻品种的内源性GA含量受盐碱胁迫的抑制作用显著低于敏感性品种,说明植物对盐碱胁迫的耐受机制与GA含量密切相关。此外,生长素(auxin, IAA)和细胞分裂素(cytokinin, CK)在根尖大量积累以应对高pH环境(Xu et al., 2012)。Liu X等(2019)发现,在碱性胁迫下,苹果(Malus domestica)砧木根系中IAA相关基因ARF5、GH3.6、SAUR36和SAUR32以及与CK相关基因IPT5的表达显著上调,可提高IAA和CK含量,从而增强苹果砧木的耐碱性。乙烯(ethylene, ET)作为IAA生物合成相关基因的重要调节因子,增加ET含量可促进IAA在根部的大量积累,进而缓解盐碱胁迫对根部伸长的抑制作用(Li et al., 2015)。通常,各种植物激素共同作用以抵御盐碱对植物的胁迫。Guo等(2020)研究发现,盐地碱蓬(Suaeda salsa)在盐碱胁迫下,与ABA、GA、IAA生物合成和植物激素信号转导相关基因的显著上调,有助于植物生长。
1.2.5 HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}应激
盐碱土壤中的NaHCO3和Na2CO3是提高土壤pH值的主要成分。通过分泌有机酸(抗坏血酸、柠檬酸、乙酸和苹果酸等)来维持细胞的pH和离子平衡是植物响应盐碱胁迫的主要策略之一(Fang et al., 2021)。然而,大量证据表明,植物对HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}胁迫的反应与对高pH胁迫并不完全相同。截至目前,已经发现一些转运体参与了HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}应激。例如,在拟南芥(Arabidopsis thaliana)中,过表达GsBOR2和GsSLAH3可增强其对NaHCO3和KHCO3的抗性,而过表达GsBOR2或GsSLAH3基因的转基因拟南芥对高pH胁迫没有抗性。此外,还发现一些转录因子(如GsERF6、GsERF71、GsbZIP67和HD-Zip Gshdz4)可提高植物在盐碱胁迫下对HCO\begin{equation}_{3}^{-}\end{equation}的耐受性(Cao et al., 2022)。例如,在紫花苜蓿中,过表达GsbZIP67通过提高植物对HCO\begin{equation}_{3}^{-}\end{equation}的耐受性,正向调节植物对盐碱的耐受性(Wu et al., 2018)。Ca2+信号通路也参与了植物对HCO\begin{equation}_{3}^{-}\end{equation}胁迫的响应,但与盐胁迫和渗透胁迫无关。例如,在大豆中,GsCML27编码一种钙调蛋白样家族蛋白,其表达可被HCO\begin{equation}_{3}^{-}\end{equation}、盐胁迫和渗透胁迫诱导,而在拟南芥中异位表达GsCML27可提高植物对HCO\begin{equation}_{3}^{-}\end{equation}的耐受性,但却降低了对盐和渗透胁迫的耐受性(Chen et al., 2015)。总之,植物对HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}胁迫的适应与高pH胁迫并不完全相同。与高pH值相比,植物可能采用了更复杂的机制来应对HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}胁迫。然而目前,对植物应对盐胁迫或高pH胁迫的研究较为深入,而对植物如何应对HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}胁迫的研究较少。因此,利用耐pH或耐盐或敏感的植物探索不同植物对盐碱联合胁迫的响应机制,可为植物在盐碱土壤中的生长发育提供新的科学依据。
1.3 植物响应盐碱胁迫的分子机制
1.3.1 调控抗盐碱基因的表达
植物响应盐碱胁迫需要多基因协同表达,这些基因通过参与渗透调节、离子稳态、抗氧化系统等多种途径调控植物生理活动以抵御盐碱胁迫。盐碱胁迫诱导上调与植物抗盐碱相关基因的表达。例如,Du等(2019)在烟草(Nicotiana tabacum)中过表达MsGSTU8、NtP5CsNtLEA5和NtLEA14基因,可增加可溶性糖和脯氨酸的含量,过表达NtSODNtPODNtCAT基因,可增强SOD、POD和CAT的活性,分别从渗透调节和抗氧化系统上共同促进植物耐盐碱性。Sun等(2021)发现,在耐盐棉花(Gossypium hirsutum)品种中GhSOS1、GhNHX1和GhAKT1基因的相对表达量显著高于盐敏感品种,这些基因可通过激活Na+反转运蛋白来促进Na+的外排和区隔化,进而维持植物Na+/K+的动态平衡。此外,编码反向转运蛋白/通道离子的耐盐碱基因还包括PutAKT1、PutCAX1、PutNHA1、AtHKT1和NHX,这些基因在调控离子稳态中均具有重要作用(Ardie et al., 2010; Zhang et al., 2016; An et al., 2017)。
1.3.2 诱导转录因子表达
作为刺激信号和相关基因的桥梁,诱导转录因子在接收上游信号后通过结合相应的顺式调控序列,调控与抗盐碱相关基因的表达。对转录因子根据DNA结构域的独特结构进行分类,与盐碱适应相关的转录因子有MYB、NAC、bZIP、WRKY等,这些转录因子在水稻、小麦、玉米(Zea mays)、苜蓿(Medicago sativa)等植物中均被证明在植物耐盐碱机制中发挥重要作用。MYB转录因子家族成员的变化最为显著,大部分MYB转录因子的表达均有所增加,参与植物次生代谢、激素信号传导、防御和应激反应等多种生理过程(Shah et al., 2021)。GmMYB68基因的过表达可增强大豆(Glycine max)的抗盐碱能力,其渗透调节能力和光合速率均高于野生型大豆(He et al., 2020)。此外,其他转录因子在植物响应盐碱胁迫中也具有重要作用。Zhou W H等(2018)研究发现,与普通小麦相比,过表达SNAC3基因的小麦在盐胁迫下,H2O2和MDA含量较低。GsbZIP67基因的过表达降低苜蓿MDA含量,提高苜蓿POD活性和叶绿素含量,并且盐碱胁迫促使MsWRKY11基因表达上调,苜蓿可溶性蛋白和脯氨酸的含量增加,从而增强苜蓿的耐盐碱能力(Wu et al., 2018; Wang et al., 2018)。因此,筛选盐生植物耐受性的基因仍是未来研究的重点。例如,在盐碱胁迫下,GmMYB68基因的过表达可增加大豆的籽粒数和百粒重,在提高作物产量方面具有实际应用价值(Shah et al., 2021)。
1.3.3 激活信号转导通路
植物盐碱胁迫的信号通路主要包括盐过度敏感(salt overly sensitive, SOS)通路、蛋白激酶通路和ABA通路(Zhu, 2016)。SOS通路作为植物的首条盐碱胁迫离子信号转导通路,负责细胞Na+的外排。植物在盐碱胁迫下产生Ca2+信号,Ca2+信号可激活SOS通路以调控Na+/H+反向转运蛋白,将细胞质过量积累的Na+排出,从而维持内部离子稳态以适应盐碱胁迫(Zhu, 2016; Zhou Y et al., 2018)。蛋白激酶主要包括丝裂原活化蛋白激酶(mitogen-activated protein kinase, MAPK)和Ca2+依赖性蛋白激酶(calcium-dependent protein kinase, CDPK)(Shah et al., 2021)。盐碱胁迫可激活植物MAPK,活化后的MAPK可以磷酸化转录因子和其他信号因子,从而将细胞外的胁迫信息传递到细胞中,激活植物的防御系统(Lee et al., 2015)。与MAPK作用类似,CDPK可以直接将上游Ca2+信号转化为下游磷酸化信号,启动下游信号转导,调节植物代谢以响应盐碱胁迫(Kudla et al., 2018)。ABA通路主要通过调节几个耐盐性基因的表达参与植物抗盐碱反应(Yu et al., 2020)。当植物受到盐碱胁迫时,ABA在细胞内合成并积累,受体RCAR与ABA结合以释放SnRK2,SnRK2可磷酸化并激活下游转录因子AREB/ABF并启动ABA反应,以调节植物在盐碱环境下的生长和发育(Fujita et al., 2013)。
2 植物响应盐碱胁迫的根际微生态机制
2.1 根际微生物群落多样性及组成介导的植物响应盐碱胁迫的机制
在盐碱胁迫下,植物通过分泌化合物以招募特定的微生物或产生触发微生物变化的信号分子,最终影响微生物群落多样性和组成(Li H et al., 2021; Xiong et al., 2021)。例如,盐生植物西洋梨(Pyrus communis)可使根际土壤积累更多盐离子,引发耐盐或嗜盐细菌在根际中显著富集。其中,节杆菌属(Arthrobacter)的显著富集可为植物提供更多的氮素,促进植物在盐碱胁迫下的生长(Li M Y et al., 2021)。雷进田(2024)发现,耐盐碱水稻长白9号在盐碱胁迫下增加了变形菌门中的假单胞菌和盐单胞菌(Halomonas)、拟杆菌门中的肠杆菌(Enterobacter)和伯克霍尔德菌(Burkholderia)的相对丰度,这些细菌可产生胞外多糖以协助植物抵御盐碱胁迫。此外,抗盐碱水稻品种的根际细菌网络节点数量和边数量显著低于敏感水稻,但其网络稳定性更高且细菌间的联系也更加紧密,表明抗盐碱品种不仅能募集到与生长和抗逆相关的有益菌,还可影响细菌之间的相互关系,充分发挥微生物群落的相互作用以帮助植物抵御盐碱胁迫,同时在盐生植物花花柴(Karelinia caspia)和梭梭树的根际土壤中,也存在类似现象(Ding et al., 2024)。
2.2 根际微生物群落功能介导的植物响应盐碱胁迫的机制
虽然不同耐盐碱植物的根际具有不同的微生物群落,但它们都富集了Na+运输、固氮和磷酸盐溶解等基因,因此微生物的生态功能对植物响应盐碱胁迫具有重要作用(Qiu et al., 2022)。例如,对盐生植物碱蓬(Suaeda salsa)和芦苇(Phragmites australis)的根际土壤进行宏基因组分析发现,根际土壤中与糖酵解、ABC转运蛋白、磷酸盐溶解、盐适应途径有关的代谢途径显著富集,可减轻盐碱胁迫对植物的危害(Li et al., 2026)。Zheng等(2021)发现,在耐盐植物根际微生物中,Na+/H+逆向转运蛋白基因的丰度较低,使它们在细胞中保留Na+。由于微生物的耐盐性明显高于植物细胞,积累的Na+不会影响其生长,因此根际微生物可通过这种方式保护其宿主免受高钠浓度的影响。随着对微生物功能的深入研究,在微生物群落研究的基础上筛选关键的抗逆单菌微生物,并对其进行相应的功能研究,对进一步阐明微生物增强植物抗逆性具有重要作用。例如,范文强(2024)从抗盐紫花苜蓿品种根际土中筛选出18种关键耐盐碱细菌,发现它们均具有分泌IAA、胞外多糖和嗜铁素的能力,还有13种可溶解有机磷,14种可溶解无机磷,16种具有解钾能力。Wu等(2022)在耐盐甜高粱品种的根际中发现,鞘氨醇单胞菌可参与植物CO2固定,促进植物光合作用,同时β-变形菌、假单胞菌和黄杆菌能够分泌IAA和ACC脱氨酶,放线菌可分泌多种代谢物共同促进植物耐盐碱性。综上,微生物群落是植物抗盐碱胁迫的关键因素,现已有研究证明植物相关微生物的组成具有植物物种和环境特异性(Li M Y et al., 2021)。因此,挖掘强耐盐碱植物的微生物群落或关键微生物并探索其相关功能仍是当前的研究热点。
3 PGPM增强植物耐盐碱胁迫的机制
PGPM通常在植物耐受盐碱胁迫和促进生长方面具有关键作用,并且已被证明是一项可持续利用的技术(Choudhary et al., 2022)。PGPM具有以下作用:(1)调节植物激素和ACC脱氨酶控制植物防御系统;(2)产生抗氧化剂并诱导植物产生渗透物质抵御氧化损伤;(3)促进营养物质吸收利用进而改善植物生长;(4)调节植物代谢途径促进植物抵御盐碱胁迫。此外,PGPM还能够重塑根际微生物群落组成和生态功能,增强植物的耐盐碱能力(邵美琪,2021)。PGPM增强植物耐盐碱胁迫的机制见图2
3.1 调节植物激素水平
植物适应盐碱胁迫的能力与微生物能否产生植物激素密切相关(Bhise & Dandge, 2019; Kumawat et al., 2022)。此外,PGPM还可提高ACC脱氨酶活性,调节乙烯水平,减缓因胁迫引起的乙烯含量过高对植物造成的抑制作用。
3.1.1 ABA
调节ABA水平对植物应对盐碱胁迫具有积极作用。有研究发现,枯草芽孢杆菌(Bacillus subtilis)可分泌ABA,增加质膜H+-ATP酶的活性使根际酸化,以降低土壤pH值,并且枯草芽孢杆菌还可诱导NO合成,NO介导的信号通路进一步增强植物抗氧化酶活性、Fe积累,并降低Na+积累,有利于植物抵御盐碱胁迫(Zhou et al., 2017; Zou et al., 2024)。此外,枯草芽孢杆菌IB22(Bacillus subtilis IB22)通过影响小麦中与ABA分解代谢相关的基因(上调HvNCED2基因和下调HvCYP707A1基因),曼氏假单胞菌IB-Ki14(P. mandelii IB-Ki14)通过影响ABA从芽到根的运输,均可增加ABA在植物根中的积累,从而激活根系生长,促进植物对水分的吸收,降低盐碱胁迫对植物的损伤(Arkhipova et al., 2020; Krishnamoorthy et al.,2022)。综上,PGPM可通过影响植物ABA的合成与代谢增强植物对盐碱胁迫的耐受性。
3.1.2 IAA
IAA作为植物生长素,目前已发现许多共生细菌,如根瘤菌(Rhizobium)、慢生根瘤菌(Bradyrhizobium)、念珠藻(Nostoc)、固氮菌(Azotobacter)、假单胞菌和节杆菌(Arthrobacter)等在盐胁迫条件下均可分泌IAA(Abd_Allah et al., 2018; Egamberdieva et al., 2018; Kumar et al., 2020)。其中,大部分细菌可利用植物根部代谢释放的色氨酸合成IAA,改善根系结构,增加水分吸收,并通过合成渗透物质调节植物代谢稳态、ROS解毒、诱导大量胁迫相关基因的表达及特异性蛋白质合成(Etesami & Maheshwari, 2018; Duca & Glick, 2020)。例如,Gang等(2018)利用LC-MS技术鉴定了克雷伯氏菌SGM 81(Klebsiella SGM 81)中的ipdC基因,证明其可产生并分泌IAA,并在田间条件下显著增加了石竹(Dianthus caryophyllus)的根毛和根部表面积,从而加大了植物对水分的吸收。Kang等(2019)研究也发现,脱羧勒克菌MO1(Leclercia adecarboxylata MO1)产生的IAA参与番茄碳水化合物、有机酸和叶绿素的合成,并上调植物ipdc基因的表达,从而增强植物对盐胁迫的耐受性。
2 PGPM增强植物耐盐碱胁迫的机制
Fig. 2 Mechanism of PGPM to enhance plant tolerance to salinity stress
3.1.3 ACC脱氨酶
在盐碱等非生物胁迫中,PGPM产生的ACC脱氨酶可将ACC转化为氨和α-酮丁酸酯以降低植物中ACC和乙烯的水平,使植物内源乙烯含量不能达到抑制浓度(Glick, 2014)。此外,产ACC脱氨酶的PGPM在盐碱胁迫条件下还积极参与植物细胞质中渗透物质的形成和细胞膜稳定性的维持。例如,Shahid等(2021)发现,耐盐细菌蔗糖小迫氏菌(Kosakonia sacchari)显著增加了绿豆(Vigna radiata)ACC脱氨酶活性,并降低了细胞膜损伤和Na+/K+离子比;Sapre等(2018)研究还发现,克雷伯氏菌IG3(Klebsiella IG3)在100 mmol·L-1 NaCl胁迫下可显著提高编码ACC脱氨酶的acds和编码IAA的ipdc基因的表达,并增加燕麦(Avena sativa)的茎长和根长。这说明PGPM对植物的作用具有多面性,通过影响各种代谢途径来提高植物耐盐碱性。
3.2 控制渗透平衡
PGPM能够通过多种方式协助植物调节细胞内的渗透压和离子平衡。例如,一些耐盐微生物能够合成并分泌渗透保护物(如脯氨酸、海藻糖、谷氨酸等类似物),通过吸收和利用这些物质降低其细胞内的渗透势,增强其对盐碱胁迫的耐受性(Madhu et al., 2022)。这些物质中,脯氨酸是一种水溶性氨基酸,是重要的渗透调节物质,在最佳生长条件和盐碱条件下,其在植物中的含量从游离氨基酸总数的20%增加到80%(Kumar et al., 2021)。Niu等(2022)接种的黄白色链霉菌OsiLf-2(Streptomyces albidoflavus OsiLf-2)在水稻根际中,能提高水稻脯氨酸含量,增强水稻的耐盐碱性。海藻糖和甜菜碱等物质也是植物响应逆境胁迫中重要的渗透物质。例如,蜡样芽孢杆菌G2(Bacillus cereus G2)可增加甘氨酸、甜菜碱和脯氨酸的生物合成底物,加快这些渗透物质的生物合成速度,从而减少电解质的泄漏,增加相对含水量,最终保护甘草(Glycyrrhiza uralensis)细胞膜系统,使其免受盐胁迫的伤害(Peng et al., 2023)。除积累渗透物质以外,水通道蛋白作为质膜内在蛋白(PIP)和细胞质内在蛋白(TIP)家族的成员,在维持植物渗透压中也发挥重要作用(Grondin et al., 2020)。有研究发现,PGPM可通过上调水通道蛋白基因的表达,增强植物对盐胁迫的耐受性。例如,巨大芽孢杆菌(Bacillus megaterium)通过上调玉米水通道蛋白基因ZmPIPPIP2-1的表达,提高玉米根系的导水性和耐盐性(Gond et al., 2015);氮螺旋菌AZ39(Azospirillum brasilense AZ39)通过上调大麦(Hordeum vulgare)水通道蛋白基因HvPIP2;1的表达,增加大麦幼苗根毛的长度和密度,促进水分吸收,从而减轻盐胁迫对大麦的损伤(Zawoznik et al., 2011)。
3.3 增强抗氧化防御系统
盐碱胁迫诱导植物产生大量的ROS,从而引发氧化胁迫。PGPM通过抗氧化防御系统清除过量的ROS,该系统包括APX、CAT、SOD等酶的分泌,以及非酶的低分子量抗氧化化合物,包括类胡萝卜素、抗坏血酸、谷胱甘肽等。例如,Ali等(2022)研究发现,阴沟肠杆菌PM23(Enterobacter cloacaePM23)可显著上调玉米编码抗氧化酶基因的表达,增强植物SOD、APX和POD的活性,提高玉米对盐碱胁迫的耐受性;Waller等(2005)研究发现,定殖于大麦根部的印度拟枝孢菌(Piriformospora indic)可通过谷胱甘肽-抗坏血酸循环激活抗氧化能力,增强大麦对盐胁迫的耐受性;Andrés-Barrao等(2021)的研究也发现,肠杆菌SA187(Enterobacter SA187)可促进拟南芥硫代谢,从而提高谷胱甘肽水平以减轻ROS诱导的损伤;Gul等(2023)将枯草芽孢杆菌NA2(Bacillus subtilis NA2)接种到小麦后,小麦叶片中的类胡萝卜素和花青素含量均有所增加,从而改善小麦对盐碱胁迫的耐受性。综上,逆境环境条件下,PGPM可通过调节植物多种抗氧化途径提高植物对盐碱胁迫的耐受性。
3.4 维持离子平衡
植物在盐胁迫下积累大量Na+,使细胞内K+/Na+比例失衡,影响多种蛋白质的生理过程和功能,对植物造成毒害(Assaha et al., 2017)。Abdel Latef等(2020)发现,接种生脂固氮螺菌(Azospirillum lipoferum)或褐球固氮菌(Azotobacter chroococcum)可减少玉米Na+的含量,同时增加K+的含量,说明PGPM有助于植物在盐碱胁迫下维持离子稳态。植物中的高亲和力K+转运蛋白1(HKT1)通过从木质部去除Na+并将Na+送回根部,从而有助于维持细胞内K+/Na+的比例平衡(Kaundal et al., 2019)。Na+/H+反转运蛋白(NHX1)和质子泵(AVP1)也被证明在不同植物的耐盐性中发挥重要作用(Bassil et al., 2011; Lian et al., 2024)。Bhattacharyya等(2015)研究发现,在盐胁迫下,由粪产碱菌JBCS129(Alcaligenes faecalis JBCS129)产生的挥发性化合物通过上调拟南芥HKT1、NHX1和AVP1基因的表达使植物在盐胁迫下维持离子稳态。谷氨酸棒状杆菌YD01(Glutamicibacter YD01)通过诱导水稻OsHKT1基因的表达使水稻幼苗在盐胁迫下维持离子稳态(Ji et al., 2020),该菌株还产生ACC脱氨酶和IAA,进一步改善了植物在逆境下的生长。
3.5 促进营养吸收
植物中Na+和Cl-水平的升高使植物对其他营养物质的吸收受到限制,PGPM可通过增加土壤中养分含量以增强植物耐盐碱性(Guo et al., 2020)。其中,固氮细菌利用一种高度保守的酶——由两种金属蛋白(FeMo-蛋白和Fe-蛋白)组成的固氮酶将大气中的氮转化为可利用的氮,以合成铵或硝酸盐的形式供植物吸收利用(Patel et al., 2022)。溶磷菌分泌的高分子量有机酸将不能被植物吸收和利用的磷转化为能被植物吸收和利用的磷形式。此外,这些酸有助于降低周围土壤的pH值,从而维持有效磷含量(Cherchali et al., 2019)。在盐碱胁迫下,溶钾菌对促进植物养分吸收具有重要作用,如假单胞菌、芽孢杆菌可通过溶解各种硅酸盐矿物,促进植物对钾的吸收和利用(Jaiswal et al., 2016; Vasanthi et al., 2018)。PGPM还会产生铁载体,将Fe3+还原为Fe2+,在Fe3+有限的条件下供自身和植物利用(Sultana et al., 2021; Timofeeva et al., 2022)。例如,Sultana等(2021)发现,阿耶波多氏芽孢杆菌(Bacillus aryabhattai MS3)分泌一种铁载体(2,3-二羟基苯甲酰甘氨酸),可增加盐碱土壤中水稻的叶绿素含量,并提高植物的耐盐碱性。此外,PGPM分泌的有机酸、酶等活性物质,还可改善土壤结构,提高土壤的保水能力和透气性(Xie et al., 2024)。由此可见,有益微生物可通过改良土壤的物理化学性质促进植物在盐碱胁迫下的生长和发育。
3.6 调节代谢途径
随着高通量技术和生物信息学的快速发展,微生物对植物盐碱胁迫耐受性的研究取得了显著进展。例如,Akbar等(2022)通过转录组分析发现,在盐胁迫下接种枯草芽孢杆菌会引发棉花的481个基因上调,75个基因下调,这些基因显著富集在戊糖与葡萄糖相互转化途径中,以及植物病原体相互作用和植物激素信号转导等多条代谢途径中,有助于植物响应盐胁迫;Prabhukarthikeyan等(2023)通过蛋白质组学分析证实,接种巨大芽孢杆菌BS11(Bacillus megateriumBS11)可上调水稻与植物代谢、转录、转运体、信号传导、防御和胁迫反应相关基因的表达;Zhao等(2022)通过蛋白质组和代谢组学分析发现,芽孢杆菌wp-6(Bacilluswp-6)显著影响了小麦幼苗中88个差异蛋白的表达,并对植物的缬氨酸、亮氨酸和异亮氨酸降解、核糖体、过氧化物酶体、乙醛酸盐和二羧酸盐代谢、半乳糖代谢、脂肪酸代谢、脂肪酸降解及α-亚麻酸代谢造成的影响,有效调控了小麦对盐胁迫的适应性。
3.7 调节根际微生物群落结构和功能
PGPM通过调节植物代谢活动增强其适应盐碱胁迫的耐受性,同时也影响根系释放各种分泌物,进而重塑根际微生物群落的组成和功能,招募有益微生物在根际定殖,进而对植物产生可持续的有益作用(Roesti et al., 2006; 徐扬等,2020)。例如,解淀粉芽孢杆菌NBRISN13(Bacillus amyloliquefaciens NBRISN13)通过调节水稻渗透物质的分泌,招募更多有益微生物,并分泌更多渗透保护剂,从而对水稻产生可持续的保护作用(Nautiyal et al., 2013)。PGPM通过合成和感知信号分子调节自身代谢活动,与群体进行相互调控,从而维持整个根际生态系统的动态平衡和各元素(碳、氮、铁等)生物地球化学循环(宋凯等,2021)。例如,枯草芽孢杆菌NCD-2(Bacillus subtilis NCD-2)通过增加番茄根际节杆菌、芽孢杆菌、鞘氨醇单胞菌和微枝形杆菌属等有益菌的相对丰度,显著提高根际微生物对羧酸类碳源和碳水化合物碳源的利用率,从而促进土壤碳循环,进一步加强枯草芽孢杆菌NCD-2对番茄的持续促生作用(邵美琪,2021)。宋凯等(2021)通过根际共现网络分析发现,嗜根寡养单胞菌DSM14405T(Stenotrophomonas rhizophila DSM14405T)可重塑根际微生物群落,增加与氮素利用相关的微生物(如Cyanobacteria和Actinobacteria),进而提高植物总氮和可溶性蛋白的含量。同时,嗜根寡养单胞菌DSM14405T还阻碍了根际微生物群落多样性的降低,并通过氨化作用,提高了土壤养分,对增强油菜适应盐碱胁迫具有改善作用。因此,PGPM不仅通过自身代谢特性促进植物适应盐碱胁迫,还通过影响其他有益微生物的共线性网络关系,对宿主植物产生有益作用。同时,根际微生物群落功能也会随着其结构的变化而发生改变。Xiao等(2024)研究发现,接种枯草芽孢杆菌增强了甘草根际细菌群落的共线性网络,并且通过PICRUSt2预测细菌群落功能发现,与新陈代谢途径(主要包括碳水化合物代谢、氨基酸代谢、脂质代谢和能量代谢等)相关的微生物类群丰度显著增加,这对土壤中碳水化合物和铵态氮的转化至关重要,可促进植物在盐碱环境中的生长。Hou等(2024)研究也发现,接种萎缩芽胞杆菌WZYH01(Bacillus atrophaeus WZYH01)改变了玉米根际细菌群落的碳循环功能,主要包括化学异养、发酵和光养,从而促进玉米光合作用并改善根际土壤的生态系统功能。因此,PGPM在一定程度上影响土壤微生物群落结构,形成一个复杂的生态关系网络,并发挥微生物群落的生态功能,增强植物对盐碱胁迫的耐受性。
4 总结与展望
土壤盐碱化威胁着全球粮食安全和生态系统安全,在世界范围内造成严重的作物损失和草牧业损失,植物在盐碱胁迫下可通过自身的多种代谢活动(抗氧化系统、渗透系统、离子平衡、激素分泌和HCO\begin{equation}_{3}^{-}\end{equation}/CO\begin{equation}_{3}^{2-}\end{equation}应激)以响应盐碱胁迫。目前,已挖掘到多种抗盐碱基因、转录因子和信号转导通路参与这些代谢途径,为耐盐碱性作物或牧草育种工作提供了科学依据。随着生物分子技术的快速发展,植物育种仍是应对土壤盐碱化的主要研究内容,并且将育种研究与PGPM研究相融合更有利于植物抵御逆境胁迫,这是目前的研究热点。PGPM不仅可以调节植物代谢活动帮助植物响应盐碱胁迫,还可调节土壤养分和微生物结构促进植物抵御盐碱胁迫。这可有效缓解盐碱胁迫对植物造成的损伤,促进植物生长发育并维持其生产力,并且相关调控效应可在土壤环境中长期存续,能够对植物生长及生态系统产生持续稳定的正向作用。目前,科学研究主要集中在植物响应盐碱胁迫的生理分子机制和根际微生物机制,但更进一步的机制研究仍有欠缺,未来可从植物响应盐碱胁迫的基因、养分循环及人工混合菌群等方向做更深入的研究。
在基因方面,植物中的抗逆基因可调控根系分泌物的组成,吸引PGPM定殖。与离子平衡、抗氧化和渗透调节相关的基因可增强PGPM的促生效应。信号传导基因还可调控PGPM诱导的信号网络,提升植物耐盐性。但是,现有研究大多集中于单一功能基因或单一菌株的互作机制,未能全面解析多基因、多菌种协同作用的复杂网络。对于功能基因如何精确调控PGPM的定殖和促生效应的分子机制了解不足,尤其是在不同植物种类或生长阶段中的动态变化尚未明确。此外,环境因素(如盐浓度、pH值的变化)对功能基因与PGPM互作的影响研究较少,限制了PGPM在实际盐碱土壤中的应用潜力。未来研究应致力于利用多组学技术(如转录组、代谢组、宏基因组)解析功能基因与PGPM互作的全局机制,聚焦植物-促生菌互作的基因网络,探究植物基因如何调节生理代谢活动,改变根系分泌物组分,从而招募有益微生物,促进植物抵御盐碱胁迫,并挖掘关键调控基因,利用基因编辑技术优化植物与促生菌的协同效应,为提升植物耐盐碱能力提供新策略。同时,亟须探索不同环境条件下功能基因与PGPM的互作动态,并开发多菌种协作的生物制剂。
在养分循环方面,PGPM在盐碱胁迫下可通过固氮、氨化、硝化和反硝化作用,优化土壤中氮的转化和利用效率。此外,PGPM还可分泌有机酸和铁载体,改善盐碱土壤结构,提升氮的可利用性。但是,现有研究大多集中在碳循环中,而对氮循环机制研究甚少。一方面,关于PGPM在盐碱环境中氮代谢功能的适应机制尚不明确,尤其是其在高盐、高碱条件下的活性和氮素转化效率的变化规律研究较少。另一方面,PGPM与植物氮代谢基因互作的分子机制缺乏系统性解析,具体如何调控植物根际氮吸收与代谢仍需深入探讨。因此,未来研究应结合多组学技术(如宏基因组、转录组和代谢组),解析PGPM促进氮循环的分子网络和环境适应机制。同时,需重点研究菌株对植物氮代谢基因的调控路径,并筛选高效、耐盐促生菌群体。开发适用于不同盐碱土壤环境的多功能微生物制剂,以及构建高效稳定的人工菌群体系,可为盐碱地农业提供理论支持和技术解决方案,推动其可持续发展。
在人工混合菌群方面,PGPM能够从多途径改善盐碱环境中的土壤养分利用效率和植物抗逆性。然而目前,人工混合菌群研究仍存在较多问题:(1)菌群的稳定性和功能协同效应难以长期维持。(2)在复杂盐碱土壤环境中,不同菌株之间可能存在竞争,导致某些功能菌失活或效能下降。(3)缺乏对菌群与植物互作的动态机制研究,目前多聚焦于单一菌株功能,而忽视菌群内部相互作用及其与植物信号网络的综合调控。因此,未来研究可通过组学技术解析菌群与植物互作的动态分子机制,构建功能稳定的高效菌群,还可利用合成生物学优化菌株特性,提升其耐盐碱能力和生态适应性。此外,我们还可探索菌群与植物信号通路的协同调控,制定精准化、区域化的菌群应用策略,为盐碱地农业可持续发展提供新思路。
1 植物响应盐碱胁迫的生理生化机制
Fig. 1 Physiological and biochemical mechanisms of plant response to saline-alkaline stress
2 PGPM增强植物耐盐碱胁迫的机制
Fig. 2 Mechanism of PGPM to enhance plant tolerance to salinity stress
1 盐生植物种类及其耐盐机制
Table 1 Halophytes with their salt tolerance mechanisms
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