桑树响应盐碱胁迫的研究进展

曾玉理1,3,史建国1,2,3*,常风云1,3,宋怡颖1,3,付广军4,柴乖强1,2,3,蒋晋豫4,王海燕4,薛忠民1,2,3

1榆林学院陕西省陕北矿区生态修复重点实验室,陕西 榆林 719000;2国家林业和草原局桑树产业工程技术研究中心,西安 710000;3陕西榆阳桑树科技小院,陕西 榆林 719000;4陕西省林业科学院治沙研究所,陕西 榆林 719000)

摘 要:开展盐碱地综合利用对保障国家粮食安全,端牢中国饭碗具有重要战略意义。盐碱胁迫是影响植物生长发育的主要非生物逆境因素之一。桑树是具有广泛抗逆性的药食同源作物,不论是在生态还是经济建设中都具有举足轻重的作用。提高桑树对盐碱胁迫的耐受性,筛选耐盐碱桑树品种,为生物改良盐碱地,提供以种适地的途径,成为目前盐碱区域农业生态修复的一个重要研究方向。综述了盐碱胁迫对桑树生长发育、光合特性和生理性状的影响以及桑树响应盐碱胁迫的分子机制和缓解盐碱胁迫的应对措施的研究进展。在此基础上,还讨论了桑树植物在耐盐性研究中存在的问题,并展望了桑树耐受盐胁迫研究的方向。旨在进一步桑树耐盐机制的解析及耐盐品种的选育提供理论依据,进而为桑树栽培广泛应用于盐碱地治理提供新思路。

关键词:桑树;盐碱胁迫;耐盐机制

通过对比全国第二次土壤普查数据与2011年的调查数据,在过去的30年间,中国盐碱耕地面积从5.79×106 hm2增加到了7.60×106 hm2[1]。随着盐碱耕地面积不断增长以及盐碱地优质可垦资源的减少,农业生产面临着更加严峻的挑战。盐碱化不仅直接影响作物生长,导致农业减产,还会造成土壤退化,严重降低土地利用率[2],因此,盐碱地的改良利用对确保中国耕地“红线”不被突破、实现农业可持续发展具有极其重要的现实意义[3]

种植耐盐碱植物是提高盐碱地农业利用效率的有效生物措施之一[4]。通过选择适宜的耐盐植物进行种植,不仅可以改善盐碱土的理化性质[5],提高土壤的生物活性[6-7],还能带来一定的经济效益[8],从而为中国耕地农业生产能力的提升、国家粮食安全的保障以及耕地红线的坚守提供重要支撑[9]

桑树(Morus alba L.)根系发达[10],生物产量高[11],耐干旱、抗风沙[12],蒸腾系数小,使得其在防风固沙、保持水土、涵养水源等方面具有得天独厚的优势[13-14]。在干旱、盐碱、重金属污染等环境胁迫下,桑树更是展现出了独特的抗性机制和生态修复功效[15],成为了生态修复和环境治理领域的研究热点[16-18]。桑树不仅是一种重要的生态型树种[19],还具有很好的经济价值[20-21],更是一种“食药两用”型作物[22],然而,盐碱化土壤对桑树的生长发育和产量造成了严重的影响。因此,为了促进植物在盐胁迫下的生长和提高产量,研究植物对盐胁迫的反应机制和抗盐机制显得尤为重要。本文旨在综述桑树耐盐碱性研究的最新进展,为进一步改善桑树的生长环境和提高产量提供理论基础,并为农业发展、粮食安全以及生态环境保护方面提供参考依据。

1 盐胁迫对桑树生长性状的影响

盐碱胁迫会抑制植物细胞的分化,影响种子萌发、阻滞根系吸收、减弱植物体光合作用、抑制植株生长,对植物体生长发育中诸多过程造成不同程度的影响[23]。盐胁迫会抑制桑树种子的萌发和幼苗的生长[24],且盐分浓度越高,对桑树抑制作用越明显,严重时导致桑树死亡[25]

1.1 种子萌发

种子萌发时期是整个植物生命过程中最脆弱的时期,盐碱胁迫处理会降低桑树种子的发芽率和发芽势,延缓种子发芽并抑制幼苗生长[26]。闫晶秋子等[27]选取碳酸盐渍土中含量最多的NaCl 为处理试剂,共设置6 个NaCl 浓度梯度,对蒙桑种子进行胁迫处理,与张国英等[28]对丰驰桑种子在盐胁迫下的发芽影响研究结果一致,桑种子的萌发率、发芽率随NaCl浓度的不断增加而呈现下降趋势,且各处理与对照均存在显著差异。此外,NaCl胁迫还显著抑制了桑种子胚根和胚芽的生长,班月圆等[29]对3 个地域来源桑种子进行耐盐机制的研究,在盐分胁迫条件下,根、芽伸长抑制率随NaCl 浓度的增大逐渐升高,而且在高浓度盐分胁迫下,3种来源桑种子还会出现子叶不能长出或生长不舒展、胚根反向生长以及抵抗力下降易霉变的现象,与祝娟娟等[30]在4 个桑品种种子萌发特性研究中得到的胚根长和胚芽长会随着NaCl浓度的增加而减小的结论相同。

综上所述,桑树种子的萌发受盐胁迫的抑制,影响幼苗胚根、胚芽生长,进一步影响桑种子发育成苗,因此,可以看出盐胁迫对桑树种子萌发期的影响较大。

1.2 苗期生长形态

植株苗期的形态会因为盐胁迫而受到较大影响,而且不同部位受影响的程度也存在差异[31]

张利琴等[32]选用5 个广泛种植的杂交桑种质,通过NaCl胁迫处理分析各种质桑树幼苗的耐盐性,结果表明,各种质幼苗的生长发育、株高和生物量以及植株地上地下部分鲜质量和干质量等在6%NaCl胁迫处理下均显著降低。孙景波等[33]研究发现,盐碱胁迫会显著降低两个桑树幼苗的株高和植株干物质量,并且两种桑树的叶片水势、渗透势、压力势和相对含水量会随着胁迫浓度的增加而明显下降。以上研究结论与胡博[34]开展的4个不同种源桑苗耐盐性研究试验结果一致。但董亚茹等[35]在研究盐胁迫对两种杂交桑种幼苗生长指标的影响中发现,两种桑树幼苗鲜干质量均在50 mmol·L-1时到达最大值且随着NaCl 浓度的增加均呈现先升高后降低的趋势,这与朱光书等[36]对6 个桑树品种的实生苗耐盐能力评价的研究结果一致,6个品种桑苗的株高、叶片数、最大叶长和最大叶宽在NaCl质量浓度≤4 g·L-1时与对照组相比均有所增长,但随着盐胁迫浓度的增加,大部分生长性状的相对增加量以及整体生物量逐渐降低,有的供试品种的桑苗在≥4 g·L-1 NaCl胁迫处理时则出现比较严重的生长停滞甚至枯死现象,NaCl胁迫时表现较好的桑品种生长速度也受到一定的抑制作用。说明低浓度的盐胁迫有利于植株干物质量的积累,对桑树的生长有一定促进作用,但达到一定的盐碱耐受值则会产生抑制作用,且因品种、种子以及苗期差异产生不同影响。

虽然盐胁迫对种子的萌发整体表现为抑制作用,但在某些情况下低浓度的盐胁迫对种子萌发或幼苗生长具有一定的促进作用,直至浓度逐渐升高则转为抑制作用[37-38]

2 盐碱胁迫对桑树光合特性的影响

光合作用是植物体获得营养物质和能量的主要来源。盐碱胁迫会造成植物体固定CO2能力下降、光合色素降解、光合电子传递过程受阻和光合器官损伤等,严重影响植物的光合作用[39]

2.1 不同盐浓度对桑树光合特性的影响

在低盐浓度(w,后同)下(0.1%),桑树幼苗的光合作用并未受到明显影响[40]。这一结论与其对1年生桑树幼苗光合作用和叶绿素荧光特性在NaCl含量(w,后同)为1、3 g·kg-1时同样没有明显影响的结果一致[41],表明在较低含量的盐胁迫下,桑树幼苗能够通过一定的生理调节来维持光合能力。此外,研究还发现,在NaCl胁迫下,当Na+浓度<150 mmol·L-1时,桑树幼苗的光合能力和生长受到的抑制程度较小,可以通过增加根冠比等形态特性来进一步适应盐胁迫[42]。盐碱胁迫抑制了PSⅡ受体侧QA 向QB的电子传递,导致电子传递链上QA向QB的电子传递速率下降和热耗散比例增大,从而降低了光化学效率[43-44]。张培宁[45]采用1年生实生苗分析了盐胁迫对桑树叶片光系统的损伤位点,认为轻度盐胁迫增加了叶片的热耗散,明显抑制了NDH依赖型环式电子传递。

随着Na+浓度的增加,到中高盐胁迫下,桑树叶片的净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)和水分利用率(WUE)均显著降低[40-42],说明中高盐浓度显著抑制了桑树幼苗的光合作用。此外,柯裕州等[41]在对1年生桑树苗光合作用和叶绿素荧光特性的研究中发现,当NaCl含量在5 g·kg-1以上时,1年生桑树苗叶片的最大荧光(Fm)、最大光化学效率(Fv/Fm)、潜在光化学效率(Fv/Fo)和PSⅡ有效光化学量子效率(ΦPSⅡ)均显著下降。这些变化反映了盐碱胁迫对桑树叶片PSⅡ反应中心活性和反应中心开放程度的影响,进而影响了光能的吸收、转化和热耗散[46-47]

重度盐胁迫(200 mmol·L-1)处理会严重破坏桑树光合系统,造成不可逆损伤。中度和重度盐胁迫均会降低叶片的热耗散,且重度盐胁迫会严重损伤桑树光合系统[45]。这表明在高盐浓度下,桑树幼苗的光合系统受到严重的损伤,导致光合效率大幅下降。

在低盐胁迫下,桑树幼苗主要依赖植株形态和光合代谢双重途径适应逆境[42]。而在中高盐胁迫下,尤其是碱性盐胁迫下,桑树幼苗的主要适应机制转向依赖光合代谢来适应逆境[42,48]。此外,可以通过施用农家肥改善盐碱地桑树的生长状况,来提高叶片的光合能力,从而减轻盐碱胁迫对桑树幼苗的伤害[49-50]。同时,嫁接桑树也可以在盐胁迫下表现出更高的PSⅡ光化学活性和更强的耐盐能力,张书博[51]的研究表明,嫁接可以降低盐胁迫下桑树嫁接苗根系对Na+的吸收和向叶片的运输,促进对氮磷钾等养分的吸收,从而提高植株生物量积累。

综上所述,盐浓度对桑树光合特性的影响是多方面的,从低盐胁迫下的适应机制到中高盐胁迫下的抑制作用,再到特定条件下如碱性盐胁迫下的特殊响应,以及外部因素如微生物接种的作用,共同决定了桑树幼苗在不同盐胁迫条件下的光合表现。

2.2 不同种类盐胁迫对桑树光合特性的影响

中性盐(NaCl)胁迫明显降低了桑树幼苗的株高、叶片数、生物量和叶片的光合能力[42]。当中性盐(NaCl)浓度≥0.3%时,盐胁迫显著降低桑树幼苗的净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)、水分利用率(WUE)、光能利用效率(SUE)和羧化效率(CUE),增加胞间CO2浓度(Ci[40]

碱性盐胁迫不仅降低了桑树幼苗叶片的叶绿素含量,还改变了叶片PSⅡ对光能的吸收、转化和热耗散的状况[43,46,48]。这表明碱性盐胁迫对桑树叶片PSⅡ的伤害程度大于中性盐(NaCl)[43]。李仕洪等[52]认为在HCO3-胁迫下,桑树的光合速率(Pn)和胞间CO2浓度(Ci)同时减小,而气孔导度(Gs)增大,这表明气孔限制是造成桑树光合速率降低的主要因素。当高浓度的HCO3-添加到根际时,桑树受到强烈的碱胁迫,这种高渗胁迫导致气孔进一步关闭,并降低了植物根系吸收HCO3-的能力,从而使植物的光合系统和膜系统受到更严重的破坏[53]

无论是中性盐还是碱性盐胁迫,都会明显降低桑树幼苗的株高、叶片数、生物量和叶片的光合能力[42]。特别是当Na+浓度超过50 mmol·L-1时,碱性盐(Na2CO3)胁迫对桑树的生长和光合能力表现出较强的抑制作用,并且这种抑制程度随着Na+浓度的增加而增大[42,48]

在碱性盐(Na2CO3)胁迫下,增加NO3--N可以降低Na2CO3胁迫下桑树叶片的气孔限制,改善叶肉细胞对CO2的利用,提高光合碳同化能力,明显减轻Na2CO3胁迫对桑树幼苗的盐害,促进地上部和根系生物量的积累[54]

在相同Na+浓度下,碱性盐(Na2CO3)胁迫通过较高的pH降低了类囊体膜两侧存在的ΔpH而抑制或伤害桑树叶片PSⅡ,导致PSⅡ反应中心活性和反应中心开放程度降低,叶片的电子传递受阻[43,46]。相比之下,中性盐(NaCl)胁迫下,桑树幼苗可以通过增大根冠比等形态特性来增强对盐胁迫的适应性[50]

综上所述,与中性盐(NaCl)胁迫相比,碱性盐(Na2CO3)胁迫对桑树幼苗的生长和光合能力表现出更强的抑制作用。这种抑制作用主要通过降低叶绿素含量、改变PSⅡ对光能的吸收、转化和热耗散的状况以及提高pH值来实现。此外,硝态氮(NO3--N)的应用可以在一定程度上缓解Na2CO3胁迫对桑树幼苗的影响。

3 盐碱胁迫对桑树生理生化特性的影响

在盐胁迫条件下,植物体通过调节自身的生理生化反应来适应环境并减轻盐害的影响[55]

3.1 盐胁迫对桑树抗氧化酶系活性的影响

盐胁迫会破坏植物体内渗透平衡,植物细胞可以通过积累无机离子和合成有机溶剂来降低细胞内渗透势[56]。盐胁迫会造成桑树体内产生大量的活性氧(ROS),过多的ROS会改变植物体内的膜系统的质膜透性,破坏质膜的完整性,进而影响质膜上的酶活性[57]

胡博[58]研究认为,3 种桑超氧化物歧化酶(SOD)与过氧化物酶(POD)活性在NaCl 浓度200~300 mmol·L-1时出现峰值,随后降低,且鸡桑SOD与POD酶活性均显著高于蒙桑与龙桑,说明在低盐浓度下,鸡桑SOD 对盐胁迫产生的超氧阴离子自由基、POD对盐胁迫产生的H2O2发挥了有效的清除作用,但在高盐浓度下受到抑制,导致酶的活性降低,活性氧受到伤害。马娟[59]研究认为1年生黑桑移栽苗在NaCl胁迫下,其SOD、POD、过氧化氢酶(CAT)活性均存在不同程度的下降,这与苏国兴[54]研究结果相一致。但随着浓度的逐渐增加,SOD的活性呈现出先升高后下降的趋势,这与盖英萍等[60]和殷朝瑞等[61]的结论一致。

综上所述,当植物受到一定浓度的盐胁迫时会打破自身活性氧代谢的动态平衡,引起SOD和POD活性的变化。盐胁迫下,桑树植株能提高体内抗坏血酸过氧化物酶(APX)、过氧化氢酶(CAT)和超氧化物歧化酶(SOD)等的活性来清除植物体内过多的ROS,提高桑树对胁迫的适应能力[25]

3.2 盐胁迫对桑树渗透调节物质含量的影响

盐碱胁迫能够对植物细胞产生显著的渗透压力,进而影响细胞内离子的区域化分布,并破坏细胞内离子的平衡状态。植物遭受盐碱胁迫可通过调整形态特征来适应,或通过合成及积累特定的生理生化物质(如脯氨酸、可溶性糖等)来维持细胞的膨胀状态和降低盐碱胁迫对细胞膜的伤害[62]

张利琴等[32]在对5 个杂交桑的耐盐性评价中发现,各桑树种质叶片中的游离脯氨酸含量和丙二醛(MDA)含量都有不同程度的增加,可溶性糖和可溶性蛋白含量变化也存在明显差异,与清水处理相比,桂桑优12 号和桂桑优62 号可溶性糖和可溶性蛋白的含量降低最少,而鲁杂1 号可溶性糖和可溶性蛋白的含量降低最多。

李卫国等[63]在盐胁迫下4 个桑树品种的嫁接苗叶片内渗透调节物质含量的研究中,发现4 个桑树品种叶片中的脯氨酸含量在各种浓度盐分胁迫初期都迅速增加,与苏国兴等[57]在对桑树的耐盐性研究中发现盐胁迫处理的桑树叶片中脯氨酸含量随着盐浓度和处理时间的增加呈上升趋势的结论一致。但高浓度盐分胁迫后期叶片中的脯氨酸含量在品种间表现出显著差异;可溶性糖含量的变化在各种浓度盐分胁迫期间都表现出先升高后降低的趋势,但各品种下降的时期不同,与胡博[34]的研究结果一致。

综上所述,桑树品种叶片中的脯氨酸及可溶性糖和可溶性蛋白含量的变化呈现一定规律性,且品种间存在显著差异。盐胁迫下,可溶性糖、可溶性蛋白和脯氨酸等渗透调节物质含量的增加,有助于提高桑树对盐胁迫环境的适应性[64-65]。在桑树对盐分胁迫的响应过程中,其叶片内含有的渗透调节物质可能发挥了关键的调节作用,但这种作用在不同的桑树品种之间可能存在差异,且与各自品种的耐盐性能相关。

3.3 盐胁迫对桑树离子平衡的影响

盐胁迫会对植物造成离子毒害,抑制植物对N、P、K、Ca、S等营养元素的吸收,植物营养缺乏,细胞内离子平衡失调,破坏植物营养平衡,抑制植物生长[66]。在盐胁迫下,当植物体内Na+浓度过高时,植物细胞膜会选择性地吸收Na+,通过质膜上的Na+/H+逆向转运蛋白将Na+从细胞中排出,另外还可以通过位于液泡上的Na+/H+逆向转运蛋白来区隔Na+,从而维持细胞内的离子平衡[67]

苏国兴[68]认为在盐胁迫下,桑树体内的Na+主要积累于桑树主茎、主根中,以保持叶片组织的低Na+水平;通过改变体内K+的分配来平衡Na+的大量积累来维持组织细胞内的离子平衡,从而降低盐碱胁迫对桑树的毒害,提高桑树的耐盐能力。宋尚文[23]在对盐胁迫下6个桑树品种反应特性研究中认为桑树幼苗产生的盐害可能主要是由单盐离子毒害引起的,6个桑树品种中Na+含量最多,Na+的大量吸收积累,在一定程度上抑制了CI-、K+等矿质元素的吸收,从而导致桑树幼苗营养失衡[69]。这与盐胁迫下植物对Na+的高敏感性相一致,因为Na+的积累会导致其他必需离子如K+和Ca2+的相对缺乏,进而影响植物的正常生长和发育[33]

综上所述,盐胁迫通过破坏桑树体内的离子平衡,特别是Na+的过量积累,抑制桑树对N、P、K、Ca、S 等营养元素的吸收,导致桑树营养缺乏和细胞内离子平衡失调,最终破坏桑树营养平衡,抑制桑树生长。桑树通过一系列生理和分子机制来适应盐胁迫,包括Na+的外排或胞内区隔化、改变体内离子分配以及调整光合作用等途径,以维持细胞内的离子平衡和促进桑树的耐盐性增强。

4 外源调节物质对桑树耐盐碱性的影响

植物中还存在其他盐分胁迫适应机制,植物激素调节在植物适应盐胁迫过程中也发挥着重要作用。

已有研究表明,施加K+、Ca2+、N素肥料,叶片喷洒脱落酸(ABA)、赤霉素(GA3)等,以及使用外源水杨酸(SA)、脯氨酸、甜菜碱、多胺等,是提高植物耐盐碱能力、缓解盐碱胁迫伤害的有效措施[70-72]

K+和Ca2+作为植物生长发育的重要营养元素,对维持细胞膜的稳定性和提高植物的抗逆性具有重要作用[33]。N 素肥料的施用可以促进桑树生长,增强其对盐碱胁迫的抵抗力。此外,N 素还能参与合成多种与抗逆性相关的物质,如蛋白质、核酸等[73]。

ABA 和GA3是植物激素,在调节植物生长发育和应对环境胁迫中起着关键作用。叶片喷洒ABA能够提高桑树的抗旱性和抗盐碱性,而GA3则有助于促进植物生长,增强其对逆境的恢复能力[74]

SA是一种重要的植物内源信号分子,参与调控植物的防御反应,对抗病原菌和环境胁迫[25]。脯氨酸是一种渗透调节物质,能够在盐胁迫下积累,帮助维持细胞内的渗透平衡[75]

甜菜碱是一种天然的渗透保护剂,能够提高植物的抗盐碱能力,通过调节离子平衡和抗氧化系统来减轻盐胁迫的影响[76-77]。外源甜菜碱(glycine betaine,GB)通过提高桑种子在盐胁迫条件下的发芽率、增加幼苗叶片的叶绿素含量、保持适当的含水量,并促进游离脯氨酸的积累,从而减轻盐胁迫对细胞膜的伤害,增强桑树的抗盐性[78]。此外,甜菜碱还能在一定程度上限制桑树幼苗对外界Na+的吸收,增加K+的吸收,提高K+/Na+比值,缓解因盐离子大量进入桑树体内造成的离子失衡和伤害[79]

外源γ-氨基丁酸(GABA)能显著缓解NaCl对桑苗生长的抑制作用,减少O2-和H2O2在桑树幼苗根尖和叶片中的积累,从而抑制活性氧的爆发[80]。这表明GABA 能够有效降低桑树体内的氧化应激水平,保护细胞膜免受损伤。

综上所述,通过施加K+、Ca2+、N素肥料、叶片喷洒ABA、GA3等植物激素,以及使用外源SA、脯氨酸、甜菜碱等物质,可以有效提高桑树的耐盐碱能力,缓解盐碱胁迫伤害。这些措施通过不同的机制作用于桑树,包括改善离子平衡、增强抗氧化能力、调节渗透压等,共同提高了桑树对盐碱胁迫的适应能力。

5 桑树响应盐碱胁迫的分子机制研究

在探索桑树盐胁迫状态下的分子调控机制方面,近年来的研究进展表明,转录组测序技术的应用为揭示桑树耐盐机制提供了新的视角和方法[81]。在分子层面,盐胁迫诱导了多种基因的表达变化,这些基因涉及到了离子运输、渗透调节、信号转导等多个方面[82]。例如,SOS途径是植物响应Na+毒性的一个重要信号通路,它通过调节Na+/H+反向转运蛋白(如SOS1)的活性来维持细胞内Na+浓度的稳定[83]。瞬时过表达MnERF2 基因的研究结果也表明,该基因通过提高植株的保护酶活性和抗氧化物质含量、降低活性氧氧化及细胞损伤程度,从而增强植物对盐胁迫的耐受性[84]。此外,苯丙素代谢途径和MAPK信号途径也与桑树对盐胁迫的响应密切相关,这些途径的激活有助于提高植物的抗氧化能力和增强细胞壁的稳定性[85]

蛋白质组学分析进一步揭示了盐胁迫下桑树叶片中差异表达蛋白的功能和分布,其中一些蛋白参与了硫代葡萄糖苷的生物合成、氨基酸合成和降解等过程,这些过程可能与桑树适应盐胁迫的能力有关[85]。这表明,桑树在盐胁迫下不仅可以通过调节基因表达来应对,还可能通过改变蛋白质的种类和功能来实现其适应策略。

迄今为止,在桑树中已经发现了多种响应盐胁迫的转录因子和基因,例如,对Na+/H+逆向转运蛋白基因(MnNHX1)的研究表明,这种基因在桑树中的表达受到NaCl胁迫的诱导,且过量表达该基因可以显著提高拟南芥的耐盐能力[86-87]。通过比较盐敏感和盐耐受桑树品种的转录组数据,可以发现差异表达基因,并进一步分析这些基因参与的生物学途径[88]

一些特定基因在桑树响应盐碱胁迫中也发挥着重要作用,胡景涛等[89]在桑树HD-ZipⅠ亚家族基因的鉴定及表达分析的研究中认为:HD-ZipⅠ亚家族基因在桑树逆境胁迫响应中具有潜在重要功能,特别是在盐和脱水胁迫下显著诱导表达。这些研究结果不仅揭示了桑树对盐胁迫的分子响应机制,而且为培育耐盐桑树品种提供了重要的分子标记和候选基因。

进一步的研究还发现,Jacalin 类凝集素基因(MnJRL8MnJRL11/12)在桑树对盐胁迫的响应中也发挥了重要作用。这些基因的过表达能够增强转基因烟草的耐盐性,通过调节活性氧产生和清除系统及渗透调节物质脯氨酸的积累,从而提高植物的抗盐性[90]。此外,Mul-NHX5基因及其调控的转录因子MYC2 在桑树耐盐性中的作用也被证实,表明这些基因在桑树应对盐胁迫的过程中起着关键作用[91]。除了上述基因和转录因子外,WRKY33EIL1 等转录因子也在桑树的盐胁迫响应中发挥作用。WRKY33 基因的沉默会导致桑树叶片中Mm WRKY33 及其下游基因CYP94B1 的表达水平下降,而EIL1 基因的沉默则会降低其下游基因ERF1B 的表达水平,这表明这些转录因子是桑树盐胁迫条件下的正向调控因子[92]

综上所述,通过转录组和蛋白组联合分析,在桑树中已经挖掘出许多响应盐胁迫的转录因子和基因,包括Na+/H+逆向转运蛋白基因、Jacalin类凝集素基因、Mul-NHX5 基因及其调控的转录因子MYC2WRKY33EIL1等。这些研究成果不仅为揭示桑树耐盐机制提供了坚实的理论依据,也为耐盐品种的选育提供了重要的候选基因和分子标记。未来的研究需要进一步探索这些分子机制的具体作用机制,以及它们如何相互作用共同调控桑树的耐盐性。

6 桑树在耐盐性研究中存在的问题

综上可知,不同的学者利用不同的研究材料,得到的不同品种桑树的耐盐性结果并不相同。目前关于桑树耐盐性的相关研究多集中在种子萌发、幼苗生长发育、光合生理生化指标的测定以及综合评价、筛选耐盐品种方面,只有极少数的研究涉及到耐盐性相关基因和分子机制的研究,对于桑树耐盐性的生理和分子机制的理解尚且不足。虽然已有研究表明桑树对盐胁迫的响应包括细胞、器官和整体植株水平上的适应机制,如渗透调节物质的积累、抗氧化酶系统的激活等,但对于整个植物体在整体水平上如何控制Na+积累和耐受渗透胁迫的分子调控机制的研究仍不够深入。

不同品种的桑树对盐胁迫的响应也存在显著差异,这种差异可能与遗传背景、生理生化特性以及环境适应性有关。因此,如何准确评估和筛选出具有高耐盐性的桑树品种成为一个重要问题,这也就需要更细致的研究来揭示其背后的分子基础。

桑树耐盐性研究涉及植物生理学、分子生物学、遗传学等多个领域,需要跨学科的合作和综合研究方法。目前,这一领域的研究往往局限于某一特定方面,还缺乏系统性和综合性的研究。

此外,桑树的耐盐性研究还都缺乏大田大规模的生产性评价。如何有效利用桑树进行盐碱地治理,以及如何选择适合当地条件的桑树品种进行大规模推广,仍然是需要解决的问题。

7 展望

就目前的研究现状而言,虽然对桑树响应耐盐胁迫在生理生化方面取得了一定的进展,但是对耐盐基因资源分子调控机制以及对桑树耐盐机制的研究尚不充分,在耐盐基因挖掘方面的研究有待于进一步深入。

并且,目前桑树的盐碱胁迫研究多停留在盆栽试验阶段,加之试验材料和盐分选择单一,胁迫处理时间短,只注重胁迫下的短期变化等问题,很少在大田以及野外进行大规模推广试验,研究内容具有一定的局限性。此外,自然环境中盐碱胁迫往往与其他环境因子通过复杂的相互作用共同对桑树产生影响,如盐碱与干旱、高温、低温、水分胁迫、营养胁迫等的协同作用,都值得深入研究。

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Research progress in mulberry responding to saline-alkali stress

ZENG Yuli1,3, SHI Jianguo1,2,3*, CHANG Fengyun1,3, SONG Yiying1,3, FU Guangjun4, CHAI Guaiqiang1,2,3,JIANG Jinyu4,WANG Haiyan4,XUE Zhongmin1,2,3

(1Key Laboratory of Ecological Restoration of Mining Areas in Northern Shaanxi Province,Yulin University,Yulin 719000,Shaanxi,China;2Mulberry Industry Engineering Technology Research Center of State Forestry and Grassland Administration, Xian 710000,Shaanxi,China;3Mulberry Science and Technology Backyard in Yuyang,Shaanxi,Yulin 719000,Shaanxi,China;4Institute of Sand Control of Shaanxi Academy of Forestry,Yulin 719000,Shaanxi,China)

Abstract: With the development of economy and society,the improvement of people’s living standards and the continuous upgrading of grain consumption structure,China’s grain supply is in a tight balance as a whole, and the national food security pressure persists, Moreover, the acceleration of industrialization and urbanization has led to an irreversible decline in the quantity and quality of cultivated land in China,and the sustainable utilization and protection of cultivated land are also facing huge pressure and challenges, and the saline-alkali land has become the largest non-traditional cultivated land resource in China that can be transformed and utilized. Therefore, the comprehensive utilization of saline-alkali land is of great strategic significance for ensuring national food security and securing the rice bowl in China.It is an effective biological measure to cultivate saline-alkali tolerant plants and improve their saline-alkali tolerance in saline-alkali land development and technical improvement. It is an urgent need to cultivate and introduce excellent varieties that can adapt to saline-alkali environment, and to explore and cultivate more salt-tolerant plants that have both ecological restoration function and economic benefits. Mulberry is rich in nutrients and functional factors, which is also a medicinal and edible homologous crop and economic tree species with extensive stress resistance. At the same time, mulberry has many varieties, different characteristics, developed roots, lush foliage and strong resistance, which can resist the invasion of cold,drought and flood,and has many functions such as water conservation,wind and sand fixation, air purification and ecological environment improvement. It is one of the ecological tree species with great potential for ecological protection,saline-alkali land management and soil remediation. Mulberry plays an important role in both ecological and economic construction. Therefore, it has become an important research direction of agricultural ecological restoration in saline-alkali areas to improve the tolerance of mulberry to saline-alkali stress,screen mulberry varieties with saline-alkali tolerance, and provide a way for biological improvement of saline-alkali land. In this paper, the effects of saline-alkali stress on the growth and development, photosynthetic and physiological characteristics of mulberry, the molecular mechanism of mulberry responding to saline-alkali stress and the countermeasures to alleviate saline-alkali stress were reviewed. The main results are as follows: (1) The influence of saline-alkali stress on the growth and development of mulberry shows that the germination of mulberry seeds is inhibited by salt stress,which affects the growth of radicle and embryo of seedlings,and further affects the development of mulberry seeds into seedlings. In some cases, low-concentration salt stress can promote seed germination or seedling growth to some extent,and gradually turns into inhibition with the increase of concentration.(2)The effects of saline-alkali stress on the photosynthetic characteristics of mulberry trees are as follows: the effects of salt concentration on the photosynthetic characteristics of mulberry trees are various,from the adaptive mechanism under low salt stress to the inhibitory effect under moderate and high salt stress, and then to the special response under specific conditions,like alkaline salt stress,which jointly affect the photosynthetic performance of mulberry trees under salt stress.(3)The effects of saline-alkali stress on the photosynthetic characteristics of mulberry are as follows: under salt stress, mulberry plants can increase the activities of ascorbate peroxidase (APX),catalase (CAT) and superoxide dismutase (SOD) to eliminate excessive ROS in plants, improve the adaptability of mulberry to stress and increase the contents of osmotic adjustment substances such as soluble sugar, soluble protein and proline, which are helpful to improve the adaptability of mulberry to salt stress environment. Mulberry adapts to salt stress through a series of physiological and molecular mechanisms to maintain intracellular ion balance and promote the development of salt tolerance of mulberry.(4)Molecular mechanism of mulberry responding to saline-alkali stress:Through the joint analysis of transcriptome and proteome, many transcription factors and genes responding to salt stress have been discovered in mulberry, including Na+/H+ antiporter gene, Jacalin lectin gene, Mul-NHX5 gene and its regulated transcription factors MYC2,WRKY33 and EIL1.The research results provide not only solid theoretical basis for revealing the salt-tolerant mechanism of mulberry,but also important candidate genes and molecular markers for salt-tolerant variety breeding.(5)Countermeasures to alleviate saline-alkali stress:Applying K+,Ca2+and N fertilizers,spraying plant hormones such as ABA and GA3 on leaves,and using exogenous substances such as SA,proline and betaine can effectively improve the saline-alkali tolerance of mulberry trees and alleviate the damage caused by saline-alkali stress. On this basis, this paper also discusses the problems existing in the study on salt tolerance of mulberry plants.Only a few studies related to salt tolerance are involved in the study on salt tolerance-related genes and molecular mechanisms,and all of them lack large-scale productive evaluation in the field.Simultaniously, the research direction of mulberry tolerance to salt stress is also prospected, focusing on the period change under stress and the large-scale extension experiment in Ottawa.The purpose is to provide theoretical basis for further analysis of salt-tolerant mechanism of mulberry and breeding of salt-tolerant varieties, so as to provide new ideas for mulberry cultivation to be widely used in saline-alkali land management.

Key words: Mulberry;Saline-alkali stress;Salt tolerance mechanism

中图分类号:S663.2

文献标志码:A

文章编号:1009-9980(2024)09-1862-13

DOI: 10.13925/j.cnki.gsxb.20240212

收稿日期:2024-05-10

接受日期:2024-06-22

基金项目:陕西林业科技创新重点专项(SXLK2023-02-36,SXLK2023-02-13);陕西省教育厅重点科研计划项目(22JS044);榆林市科技局项目(2023-CXY-171);榆林学院高层次青年科学启动基金(18GK08);陕西省林业科学院创新团队建设“沙产业研究与开发创新团队”(SXLK2020-0309)

作者简介:曾玉理,女,在读硕士研究生,研究方向为农艺与种业。E-mail:1928565301@qq.com

*通信作者 Author for correspondence.E-mail:378279015@qq.com