热带地区不同水稻品种对土壤镉胁迫的生理生化响应

以热带地区主栽品种博优225和矮糯为材料,采用盆栽试验研究了不同水稻品种对土壤外源重金属Cd胁迫的耐性机理。结果表明:在各浓度镉胁迫下,2个品种水稻都是根累积的镉含量要高于茎叶和稻壳、糙米,即根>茎叶>稻壳>糙米;随着镉胁迫浓度的增加,细胞膜受损的伤害程度越大,矮糯质膜透性略高于博优225;随着Cd浓度的升高,水稻叶片丙二醛(MDA)、脯氨酸含量逐渐增加,且博优225的增加幅度明显大于矮糯,超氧化物歧化酶(SOD)和过氧化物酶(POD)活性均呈上升趋势,且博优225明显高于矮糯;过氧化氢酶(CAT)活性呈下降趋势;表明同样条件下博优225受Cd毒害更严重。     

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.     

Differential cadmium stress tolerance in five indian mustard (Brassica juncea L.) cultivars: An evaluation of the role of antioxidant machinery

The presence of Cadmium (Cd) in the agricultural soils affects horticultural cultivars and constrains the crop productivity. A pot experiment was performed using five cultivars of mustard (Brassica juncea L.) to evaluate the difference in their response to Cd toxicity under greenhouse conditions. The pots containing reconstituted soil were supplied with different concentration of CdCl₂ (0, 25, 50, 100 or 150 mg Cd kg⁻¹ soil). Increasing concentration of Cd in the soil resulted in decreased growth, photosynthesis and yield. Maximum significant reduction in growth, photosynthesis and yield were observed with 150 mg Cd kg⁻¹ soil in all the cultivars. Our results indicate that the cultivar Alankar is found to be more tolerant to Cd stress, recording higher plant dry mass, net photosynthesis rate, associated with high antioxidant activity and low Cd content in the plant leaves and thus less oxidative damage. Cultivar RH30 experienced maximum damage in terms of reduction in growth, photosynthesis, yield characteristics and oxidative damage and emerged as sensitive cultivar. The data of tolerance index of Alankar were found to be higher among all tested mustard cultivars which indicate its higher tolerance to Cd. Better coordination of antioxidants protected Alankar from Cd toxicity, whereas lesser antioxidant activity in RH30 resulted in maximum damage. Cultivars of mustard were ranked with respect to their tolerance to Cd: Alankar > Varuna > Pusa Bold > Sakha > RH30, respectively.Key words: antioxidants, cadmium, growth, mustard cultivars, photosynthesis, stress, yield     

Arsenic mitigates cadmium toxicity in rice seedlings

Two contrasting rice (Oryza sativa L.) cultivars, i.e. Wuyujing 3 (WYJ3, Cd-tolerant) and Shanyou 63 (SY63, Cd-sensitive), were grown on a red soil (Ultisol) to study both individual and combined phytotoxicity of arsenic (As) and cadmium (Cd) in terms of Cd and As availability, their uptake and accumulation, antioxidant defense activity and oxidative damage. The antioxidant defense system examined in this study included enzymatic and non-enzymatic molecular antioxidants such as superoxide dismutase (SOD), peroxidase (POD), glutathione (GSH) and ascorbic acid (AsA). Results showed that As or Cd treatment decreased root and shoot biomass in both cultivars compared with their corresponding control (no Cd or As treatment), although less severe inhibition of plant growth was observed in WYJ3 than in SY63. Moreover, rice growth was inhibited more severely by Cd treatment than by As treatment, which could be explained by the higher amount of available Cd (60%) (0.1 M HCl-extractable Cd) compared to the lower amount of available As (15%) (0.5 M NaH₂PO₄-extractable As) in their postharvest soils. However, shoot biomass in cultivar SY63, and root and shoot biomass in cultivar WYJ3 were significantly higher in the As plus Cd treatment than in the Cd treatment alone, showing that the combined toxicity of these two heavy metals was not additive and on the contrary, As mitigated Cd-induced growth inhibition. The As plus Cd treatment also significantly decreased As or Cd concentrations both in roots and in shoots of the two rice cultivars compared with the As or Cd treatment alone, respectively. On the other hand, treatment with As or Cd alone significantly decreased the SOD and POD activities, and GSH and AsA concentrations, while the activities of these enzymes and the concentrations of GSH and AsA were significantly higher in the As plus Cd treatment than in the Cd treatment alone, resulting in less severe oxidative damage as indicated by the lower concentration of MDA in the As plus Cd treatment (P < 0.05). However, no significant difference was observed in the antioxidant defense activity between the As plus Cd treatment and the As treatment alone. These results suggest that the combined toxicity of As and Cd in rice is lower than that of individual Cd or As, which might be attributed to the decreased uptake and accumulation of Cd and As, and the less oxidative stress caused by the interactive effects of As with Cd both in rhizosphere and in plants.     

Arsenic mitigates cadmium toxicity in rice seedlings

Two contrasting rice (Oryza sativa L.) cultivars, i.e. Wuyujing 3 (WYJ3, Cd-tolerant) and Shanyou 63 (SY63, Cd-sensitive), were grown on a red soil (Ultisol) to study both individual and combined phytotoxicity of arsenic (As) and cadmium (Cd) in terms of Cd and As availability, their uptake and accumulation, antioxidant defense activity and oxidative damage. The antioxidant defense system examined in this study included enzymatic and non-enzymatic molecular antioxidants such as superoxide dismutase (SOD), peroxidase (POD), glutathione (GSH) and ascorbic acid (AsA). Results showed that As or Cd treatment decreased root and shoot biomass in both cultivars compared with their corresponding control (no Cd or As treatment), although less severe inhibition of plant growth was observed in WYJ3 than in SY63. Moreover, rice growth was inhibited more severely by Cd treatment than by As treatment, which could be explained by the higher amount of available Cd (60%) (0.1 M HCl-extractable Cd) compared to the lower amount of available As (15%) (0.5 M NaH₂PO₄-extractable As) in their postharvest soils. However, shoot biomass in cultivar SY63, and root and shoot biomass in cultivar WYJ3 were significantly higher in the As plus Cd treatment than in the Cd treatment alone, showing that the combined toxicity of these two heavy metals was not additive and on the contrary, As mitigated Cd-induced growth inhibition. The As plus Cd treatment also significantly decreased As or Cd concentrations both in roots and in shoots of the two rice cultivars compared with the As or Cd treatment alone, respectively. On the other hand, treatment with As or Cd alone significantly decreased the SOD and POD activities, and GSH and AsA concentrations, while the activities of these enzymes and the concentrations of GSH and AsA were significantly higher in the As plus Cd treatment than in the Cd treatment alone, resulting in less severe oxidative damage as indicated by the lower concentration of MDA in the As plus Cd treatment (P < 0.05). However, no significant difference was observed in the antioxidant defense activity between the As plus Cd treatment and the As treatment alone. These results suggest that the combined toxicity of As and Cd in rice is lower than that of individual Cd or As, which might be attributed to the decreased uptake and accumulation of Cd and As, and the less oxidative stress caused by the interactive effects of As with Cd both in rhizosphere and in plants.     

镉胁迫下水稻OsPT1的表达及功能分析

OsPT1编码的水稻磷酸盐（Pi）转运蛋白在水稻生长发育、非生物胁迫应答等方面发挥重要的调控作用。前期研究表明OsPT1为镉（Cd）响应基因,但其在Cd胁迫下的功能及作用机制仍然未知。阐明OsPT1在Cd胁迫下的作用,并为低Cd水稻品种的选育奠定基础。通过生物信息学方法对该基因的序列特征、结构和功能进行分析和预测,利用实时荧光定量PCR（RT-qPCR）方法检测Cd胁迫下水稻不同组织、不同时间点OsPT1的相对表达量。此外,利用PCR的方法克隆OsPT1的编码序列,构建pGADT7-OsPT1重组质粒载体,并将其转入Δycf1 BY4741酵母菌株（Cd敏感酵母菌株）用以验证OsPT1对酵母Cd耐受性的影响。结果表明,OsPT1编码序列全长为1 584 bp,编码分子量为57.46 kD,由527个氨基酸构成的蛋白。在水稻基因组中该基因上游启动子区含有与光、厌氧、茉莉酸甲酯等环境和激素响应相关的调控元件。系统进化分析表明,水稻OsPT1与高粱SbPT1亲缘关系最近。基因的镉响应表达分析结果表明,与对照相比,经100 μmol/L Cd处理的水稻在1、6和12 h后,地上部分OsPT1的转录水平分别上调1.31、1.34和2.46倍;水稻根部OsPT1在处理1和6 h后分别上调1.28和1.14倍,但在Cd处理12 h后,其表达水平下调至处理前的0.62倍。转基因酵母Cd耐受性结果表明,与对照（0 μmol/L Cd）相比,经25 μmol/L Cd处理后,转OsPT1的酵母对Cd的耐受性有一定的下降。OsPT1可能在水稻应对Cd胁迫过程中发挥一定的作用。     

水稻不同品种对Cd吸收累积的差异和机理研究

采用盆栽和水培试验研究了华南地区水稻的主要品种对Ｃｄ吸收累积的差异和引起差异的原因。盆栽试验结果表明,供试的２０多个品种生长在同一污染土壤上,汕优６３,汕优６４等杂交稻,产量较高,但糙米Ｃｄ含量也较高,野奥丝苗,增城丝苗,黑糯等优质稻糙米重金属含量较低;常规稻则变幅较大,作物品种间差异可达１倍以上,在同一Ｃｄ浓度和营养液配方条件下的水培试验显示,与汕优６３相比,糙米Ｃｄ含量较低的野奥丝苗其单位产量     

水稻籽粒镉积累QTL定位及候选基因分析

水稻(Oryza sativa)是全世界重要的经济作物之一, 稻田镉(Cd)污染和镉积累问题严重威胁世界水稻的产量和品质以及人类健康, 如何降低水稻中镉积累已成为热点问题。以籼稻品种华占(HZ)为父本、粳稻品种热研2号(Nekken2)为母本, 连续自交多代后得到120个重组自交系群体, 对其镉积累进行检测和分析, 同时利用遗传图谱进行QTL作图。结果共检测到7个QTLs, 分别位于水稻第2、3、9和12号染色体上, 其中1个LOD值高达4.97。对这些QTL区间内与耐金属离子胁迫相关的候选基因进行定量分析, 发现LOC_Os02g50240、LOC_Os02g52780、LOC_Os09g31200、LOC_Os09g35030和LOC_Os09g37949这5个基因在双亲间的表达量差异显著, 结合亲本对不同金属离子的浓度积累数据, 推测LOC_ Os02g50240、LOC_Os09g31200及LOC_Os09g35030的高表达可能极大地提高了水稻对镉离子的吸收和胁迫耐受能力。通过QTL挖掘和分析, 发现这些基因与水稻籽粒的镉积累有关, 可能影响水稻耐镉胁迫的能力。研究结果为进一步筛选和培育耐镉胁迫的水稻品种创造了条件, 为阐明水稻镉积累的分子调控机制奠定了基础。     

以回交重组自交系定位水稻萌芽期耐镉胁迫相关QTLs

水稻是世界上最主要的粮食作物之一,目前农用耕地存在土壤重金属污染的问题,而水稻对镉（Cd）等重金属的耐受性较低,进而使水稻产量和质量受到影响。定位稻种耐Cd胁迫相关数量性状基因座(quantitative trait loci, QTLs),对于指导水稻耐Cd育种具有重要意义。为发掘Cd胁迫相关基因,以粳稻02428和籼稻昌恢891衍生的124个回交重组自交系群体（backcross recombinant inbred ines,BILs）为材料,对水稻萌芽期的根长、芽长进行了分析,并对萌芽期与Cd胁迫相关的QTLs进行了定位分析。结果显示：Cd胁迫处理下,02428和昌恢891根长和芽长均受到显著抑制（P＜0.01）,其中Cd对根长的抑制强于芽长;QTL分析共检测到5个萌芽期与Cd胁迫相关的QTLs：qCdBL3、qCdRL7、qCdBL8.1、qCdBL8.2和qCdBL9分别位于水稻第3、7、8、8和9号染色体上,贡献率为6.45%~19.46%。其中,qCdBL3、qCdBL8.1、qCdBL8.2和qCdBL9与芽长相关,qCdRL7与根长相关。同时,检测到2个在对照条件下（水溶液）影响根长和芽长的QTLs：qCKBL8、qCKRL4,分别位于第8和4号染色体上,贡献率为10.53%和10.89%。比较显示,对照和Cd处理条件下控制水稻萌芽期根长或芽长的QTLs均不相同,说明Cd胁迫条件下,控制水稻根长和芽长的遗传机制可能不同于非Cd胁迫条件。研究结果为耐Cd基因的克隆和耐Cd水稻新品种的选育提供了参考。     

Effect of Chilling on Photosynthesis in Flag Leaves at Primary Heading Stage of F1 Hybrid Rice Cultivars and Their Parental Lines (Three Lines)

With the purpose of selecting and breeding cold stress tolerance of hybrid rice cultivars, the effect of chilling on five hybrid rice combinations was investigated. The results indicated that the cold tolerance of different hybrid rice varieties were different. The order of the cold tolerance as the degrees of inhibition of maximum photosynthetic rate and of apparent quantum yield of flag leaves at primary heading stage by chilling was as follows: The cold tolerance of Japanica type Xiuyou-57 was the highest then lndica type Qing-Youzao followed by Shangyou-63, Shangyou-64 and Weiyou-64. The cold tolerance of Fx hybrid rice was similar to that of the maternal lines and not similar to the paternal lines. The cold tolerance of maintainer lines was similar to that of the male sterile lines. The tests on the survival rate of hybrid rice seedlings after chilling treatment also showed a similar re gular patterns. These indicated that the responses by the chilling temperature in these five hybrid combinations were exactly the same in the Seedling stage as well as in the late growing stage. The results of these experiments on the impact of selecting and breeding cold tolerance in hybrid rice were discussed.     

不同水稻品系幼苗对砷（As）的耐性、吸收及转运

利用琼脂培养基模拟水稻生长的厌氧环境,研究了As对不同水稻品系幼苗生长的影响以及As在其体内的积累及转运特性.结果表明,不同浓度(0～4.0mg/L)的As对供试水稻品系根部干物质积累无显著影响(P>0.05).杂交稻与糯稻的地上部干物质积累随基质中As浓度的增加呈减小趋势,但低剂量的As(0.5mg/L)促进常规稻的生长.水稻地上部的As积累量随基质中As浓度的升高总体均呈增加趋势.水稻根系对As具有较强的吸收与累积能力.水稻根部As的积累量为156.31～504 03mg/kg,占总As含量的63.40～81.90%,远远高于其地上部As的积累量.相比于其它两个品系,糯稻的生物量积累高,耐性指数较大,根部及地上部对As的积累量较低,因此更适合种植在As污染土壤.     

Drought-induced root aerenchyma formation restricts water uptake in rice seedlings supplied with nitrate

Previous studies demonstrated that ammonium nutrition results in higher water uptake rate than does nitrate nutrition under water stress, and thus enhances the tolerance of rice plants to water stress. However, the process by which water uptake is related to nitrogen form under water stress remains unknown. A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG6000) was conducted in a greenhouse to study the relationship between root aerenchyma formation and water uptake rate, such as xylem sap flow rate and hydraulic conductance, in two different rice cultivars (cv. 'Shanyou 63' hybrid indica and cv. 'Yangdao 6' indica, China). The results showed that root aerenchyma tissue increased in water-stressed plants of both cultivars fed by nitrate. No significant difference was found in root hydraulic conductivity and/or xylem sap flow rate between the two rice cultivars fed by ammonium regardless of water status, whereas these parameters decreased significantly in water-stressed plants fed by nitrate. It was concluded that aerenchyma that formed in the root cortex impeded the radial transport of water in the root cylinder and decreased water uptake in water-stressed rice plants fed by nitrate. Water transport occurred mainly through Hg-sensitive water channels in rice roots supplied with ammonium.     

稻纵卷叶螟幼虫驯化产生的热适应能力的继代效应

摘要: 【目的】在全球不断变暖背景下,昆虫受到热胁迫的频率不断增加。短期内反复受到热胁迫会使昆虫产生热适应性,但是昆虫热驯化所产生的耐热能力的传代效应还不完全清楚。稻纵卷叶螟Cnaphalocrocis medinalis是水稻上的重要害虫,对其幼虫在特定温度下进行几代热锻炼可提高其对高温的适应能力。本研究旨在摸清稻纵卷叶螟热适应的传代能力,为在全球变暖形势下以温度因子预测其种群发展趋势提供指导。【方法】将实验室内分别经39℃和41℃连续锻炼30代建立的稻纵卷叶螟热锻炼品系HA39和HA41以及非锻炼品系HA27的1-5龄期幼虫在不同温度(36℃和41℃)下进行不同时长(1~144 h)的暴露处理,调查幼虫的存活率,确定热锻炼品系幼虫的耐热能力;采用两品系间杂交实验测定HA39和HA27各交配组合的繁殖力及后代3龄幼虫的耐热能力;对HA39停止高温锻炼,并测定停止锻炼2代后3龄幼虫的耐热能力。【结果】稻纵卷叶螟3龄幼虫经历多代次短期热锻炼不仅可提高该龄幼虫的高温适应力,而且可提高其他龄期幼虫对特定高温的耐受能力,表现为HA39和HA41在36或41℃下处理特定时长的存活率显著高于HA27。锻炼高温的不同,幼虫获得的热耐受能力也有差异。39℃下锻炼可提高4龄幼虫在36℃下暴露2和4 d以及5龄幼虫在41℃下暴露5和6 h时的存活率,但41℃下锻炼则不可。HA39和HA27的自交及杂交后代的繁殖力之间均无显著差异,杂交后代3龄幼虫在41℃下处理5和6 h时的存活率与HA39自交后代的相当,并且显著高于HA27自交后代的,幼虫通过热锻炼获得的耐热能力可从亲本遗传给后代。停止热锻炼2代后,在39℃下处理4 h时HA39 3龄幼虫的存活率显著高于HA27的,但39℃下其余处理时间以及36和41℃下处理1~7 h HA39 3龄幼虫的存活率均与HA27的无显著差异,表明幼虫热锻炼产生的耐热能力在停止锻炼后2代仍可部分保持。【结论】稻纵卷叶螟幼虫的热适应能力具有继代效应。经过长期的气候变暖适应后,稻纵卷叶螟种群的热适应能力很可能在不断增强,从而夏季高温对其种群的抑制作用减弱,其种群暴发频率增加。     

Plant growth regulators improve growth,photosynthesis, mineral nutrient and antioxidant system under cadmium stress in menthol mint ( Mentha arvensis L.)

Menthol mint (Mentha arvensis L.) cultivation is significantly affected by the heavy metals like cadmium (Cd) which also imposes severe health hazards. Two menthol mint cultivars namely Kosi and Kushal were evaluated under Cd stress conditions. Impact of plant growth regulators (PGRs) like salicylic acid (SA), gibberellic acid (GA3) and triacontanol (Tria) on Cd stress tolerance was assessed. Reduced growth, photosynthetic parameters, mineral nutrient concentration, and increased oxidative stress biomarkers like electrolyte leakage, malondialdehyde, and hydrogen peroxide contents were observed under Cd stress. Differential upregulation of proline content and antioxidant activities under Cd stress was observed in both the cultivars. Interestingly, low electrolyte leakage, lipid peroxidation, hydrogen peroxide and Cd concentration in leaves were observed in Kushal compared to Kosi. Among all the PGRs tested, SA proved to be the best in improving Cd-stress tolerance in both the cultivars but Kushal responded better than Kosi.     

低温对杂交水稻及其亲本三系始穗期旗叶光合作用的影响

研究了低温对5套杂交水稻组合的影响。始穗期旗叶光合作用的最大速率和表观量子效率受低温抑制的程度表明:其耐冷顺序为粳稻型的“秀优57”>“釉稻型的青优早”>“汕优63”、“汕优64”与“威优64”。杂交水稻 F_1的耐冷水平与母本近似,与父本关系不大,保持系的耐冷性相似于不育系。杂交水稻幼苗经低温处理后的存活率亦表现出同样的规律。证明所试5套杂交水稻的生育后期与苗期对低温的反应是一致的。本文对该结果在杂交水稻抗冷性选育种中的意义进行了讨论。     

A single recessive gene controls cadmium translocation in the cadmium hyperaccumulating rice cultivar Cho-Ko-Koku

The heavy metal cadmium (Cd) is highly toxic to humans and can enter food chains from contaminated crop fields. Understanding the molecular mechanisms of Cd accumulation in crop species will aid production of safe Cd-free food. Here, we identified a single recessive gene that allowed higher Cd translocation in rice, and also determined the chromosomal location of the gene. The Cd hyperaccumulator rice variety Cho-Ko-Koku showed 3.5-fold greater Cd translocation than the no-accumulating variety Akita 63 under hydroponics. Analysis of an F₂ population derived from these cultivars gave a 1:3 segregation ratio for high:low Cd translocation. This indicates that a single recessive gene controls the high Cd translocation phenotype. A QTL analysis identified a single QTL, qCdT7, located on chromosome 7. On a Cd-contaminated field, Cd accumulation in the F₂ population showed continuous variation with considerable transgression. Three QTLs for Cd accumulation were identified and the peak of the most effective QTL mapped to the same region as qCdT7. Our data indicate that Cd translocation mediated by the gene on qCdT7 plays an important role in Cd accumulation on contaminated soil.     

Distinct physiological responses of two rice cultivars subjected to iron toxicity under field conditions

Iron toxicity is recognised as the most widely distributed nutritional disorder in lowland and irrigated rice, derived from the excessive amounts of ferrous ions generated by the reduction of iron oxides in flooded soils. Rice cultivars with variable degrees of tolerance to iron toxicity have been developed, and cultural practices such as water management and fertilisation can be used to reduce its negative impact. However, because of the complex nature of iron toxicity, few physiological data concerning tolerance mechanisms to excess iron in field conditions are available. To analyse the physiological responses of rice to iron excess in field conditions, two rice cultivars with distinct tolerance to iron toxicity [BR‐IRGA 409 (susceptible) and IRGA 420 (tolerant)] were grown in two areas, with a well‐established history of iron toxicity (in Camaquã, RS, Brazil) and without iron toxicity (in Cachoeirinha, RS, Brazil). Plants from the susceptible cultivar grown in the iron‐toxic site showed lower levels of chlorophylls and soluble proteins (together with higher carbonyl levels) indicating photooxidative and oxidative damage. The toxic effects observed were because of the accumulation of high levels of iron and not because of any indirectly induced shoot deficiency of other nutrients. Higher activities of antioxidative enzymes were also observed in leaves of plants from the susceptible cultivar only in the iron‐toxic site, probably as a result of oxidative stress rather than because of specific involvement in a tolerance mechanism. There was no difference between cultivars in iron accumulation in the symplastic and apoplastic space of leaves, with both cultivars accumulating 85–90% of total leaf iron in the symplast. However, susceptible plants accumulated higher levels of iron in low‐molecular‐mass fractions than tolerant plants. The accumulation of iron in the low‐molecular‐mass fraction probably has a direct influence on iron toxicity, and the adaptive strategy of tolerant plants may rely on their capacity to buffer the iron amounts present in the low mass fraction, a new parameter to be considered when evaluating tolerance to iron excess in field‐cultivated rice plants.     

Natural variation in the promoter of rice calcineurin B‐like protein10 (OsCBL10) affects flooding tolerance during seed germination among rice subspecies

Rice (Oryza sativa L.) has two ecotypes, upland and lowland rice, that have been observed to show different tolerance levels under flooding stress. In this study, two rice cultivars, upland (Up221, flooding‐intolerant) and lowland (Low88, flooding‐tolerant), were initially used to study their molecular mechanisms in response to flooding germination. We observed that variations in the OsCBL10 promoter sequences in these two cultivars might contribute to this divergence in flooding tolerance. Further analysis using another eight rice cultivars revealed that the OsCBL10 promoter could be classified as either a flooding‐tolerant type (T‐type) or a flooding‐intolerant type (I‐type). The OsCBL10 T‐type promoter only existed in japonica lowland cultivars, whereas the OsCBL10 I‐type promoter existed in japonica upland, indica upland and indica lowland cultivars. Flooding‐tolerant rice cultivars containing the OsCBL10 T‐type promoter have shown lower Ca²⁺ flow and higher α‐amylase activities in comparison to those in flooding‐intolerant cultivars. Furthermore, the OsCBL10 overexpression lines were sensitive to both flooding and hypoxic treatments during rice germination with enhanced Ca²⁺ flow in comparison to wild‐type. Subsequent findings also indicate that OsCBL10 may affect OsCIPK15 protein abundance and its downstream pathways. In summary, our results suggest that the adaptation to flooding stress during rice germination is associated with two different OsCBL10 promoters, which in turn affect OsCBL10 expression in different cultivars and negatively affect OsCIPK15 protein accumulation and its downstream cascade.