甘蔗ScNRAMP基因家族的鉴定与生物信息学分析

NRAMP蛋白(natural resistance-associated macrophage proteins)家族在植物响应重金属胁迫时具有重要作用,能够转运Fe²⁺、Mn²⁺、Zn²⁺和Cd²⁺等重金属离子。为探究甘蔗ScNRAMP基因家族的特征,该文基于甘蔗割手密基因组鉴定了ScNRAMP基因家族,并进行了理化特性、基因结构、顺式作用元件、保守基序、结构域和进化关系等分析。结果表明:甘蔗ScNRAMP基因家族含有29个成员,不均匀分布在19条染色体上; 编码蛋白均为不稳定蛋白,无信号肽,亚细胞均定位在质膜上; 各成员保守基序有6～10个不等,跨膜数有6～12个不等,二级结构主要构成元件为α-螺旋和无规则卷曲; 顺式作用元件分析表明甘蔗ScNRAMP基因家族可能通过激素代谢参与逆境胁迫和生长发育等生物过程; 利用割手密的RNA-seq转录组表达数据进行的组织特异性分析发现,ScNRAMP在甘蔗不同发育阶段的叶和茎中具有时空表达特性; 进化树分析将甘蔗ScNRAMP家族成员分为3个亚家族(I、Ⅱ和Ⅲ)。该研究在全基因组水平上系统地鉴定了现代栽培甘蔗祖先种之一割手密NRAMP基因家族,既为进一步了解甘蔗NRAMP基因家族提供了基础,也为后续甘蔗重金属研究提供了重要候选基因。     

茶树TIFY基因家族鉴定及非生物胁迫下表达分析

TIFY基因家族在茶树激素信号转导及其逆境胁迫具有十分重要的作用。该文利用生物信息学方法对茶树基因组中的TIFY家族进行了鉴定,分析其理化性质、系统进化、基因结构、染色体定位、启动子区顺式作用元件、组织表达模式,并通过RT-qPCR对部分TIFY家族成员进行了非生物胁迫。结果表明:(1)茶树TIFY基因家族成员19个(CsTIFY1～CsTIFY19),分属于4个蛋白亚家族TIFY、JAZ、ZML 和 PPD,且不均匀地分布在8 条染色体上,按照进化关系及结构特点可分为 7 组,每组内具有相似的基因结构与保守基序组成。(2)CsTIFYs基因的启动子区具有多种包含激素和非生物胁迫响应的顺式作用元件,通过实时荧光定量(RT-qPCR)分析其家族成员对在茉莉酸甲酯、盐(20%氯化钠)、冷(4 ℃)以及干旱(20% PEG-6000)处理下反应强烈,部分基因在根与顶芽中有较高的表达量。由此推测,TIFY基因家族可能在茶树激素信号调控、胁迫响应、生长和发育等多方面发挥功能作用。     

甜菜BvHAK基因家族全基因组鉴定及其在盐处理下的表达分析

高亲和性K⁺转运蛋白（high-affinity K⁺ transporter,HAK）是植物中最重要的K⁺转运蛋白家族之一,在植物K⁺吸收和转运过程中发挥重要功能。为探究甜菜BvHAK基因家族成员生物学功能及基因表达模式,采用生物信息学手段,预测了蛋白质的理化性质、基因结构、染色体定位、系统进化、保守基序、三维结构、互作网络、启动子顺式作用元件,并通过qRT-PCR分析了盐胁迫下BvHAKs基因在甜菜不同组织中的表达水平。共鉴定出10个甜菜BvHAK基因家族成员,含有8-10个外显子、7-9个内含子;平均氨基酸个数为778.30,平均分子量为88.31 kDa,等电点为5.38-9.41,跨膜区为11-14个。BvHAK4、-5、-7和-13定位在质膜,而其余定位在液泡膜。系统进化分析发现,高等植物HAK可分为5个簇,分别为Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ簇,其中Ⅱ簇成员可进一步分为Ⅱa、Ⅱb和Ⅱc等3个亚簇;BvHAK家族成员则分布在前4簇,分别含有1、6、1和2个成员。甜菜BvHAK基因家族主要含有胁迫响应元件、激素响应元件和生长发育响应元件。进一步对BvHAK基因在盐处理下甜菜不同组织中的表达模式分析发现,50和100 mmol/L NaCl不同程度地诱导甜菜地上部和根部BvHAK基因家族成员的表达;高盐（150 mmol/L）则下调了其在地上部的表达水平。这些结果表明,BvHAK基因家族在响应盐胁迫过程中起重要作用。     

多胺对盐胁迫下黄瓜SOS2基因家族表达的影响

该研究基于黄瓜基因组数据库,利用生物信息学和荧光定量PCR等方法,对黄瓜SOS2基因家族进行全基因组鉴定,并对其表达特性进行系统分析。结果表明：(1)在黄瓜基因组中,共鉴定到 15个SOS2基因(CsSOS2 1~CsSOS2 15),且不均匀分布在5条染色体上;亚细胞定位显示SOS2蛋白主要位于细胞质膜。(2)系统进化分析显示,CsSOS2 2和CsSOS2 6与AtSOS2亲缘关系更近。(3)顺式作用元件预测显示,SOS2基因启动子序列主要含有干旱诱导和防御应激响应元件。(4)结构分析显示,SOS2蛋白所含保守基序的排列顺序完全一致,且主要含有STKc和NAF保守结构域,这些结构可能在基因响应盐胁迫过程中起调控作用。(5)荧光定量试验表明,SOS2基因家族主要在黄瓜的根和叶片响应盐胁迫时上调表达,其中,盐胁迫处理1 d时有5个基因上调表达,并随处理时间延长盐胁迫下的基因表达有所下调;增施多胺显著上调了CsSOS2 1~CsSOS2 5、CsSOS2 8、CsSOS2 9、CsSOS2 12和CsSOS2 15在不同组织中的表达,说明盐胁迫下多胺能诱导黄瓜SOS2基因家族的表达,进而参与植物耐盐分子网络的调控。     

小麦苗期盐胁迫下DREB基因的表达

为了从全基因组和转录组水平鉴定响应盐胁迫的小麦DREB (dehydration responsive element binding,DREB) 基因,该研究对小麦耐盐材料CH7034苗期施加盐胁迫后的根部样本进行Illumina转录组测序,从中分离TaDREB家族成员的表达数据和可变剪接信息,并对其下游靶基因进行预测;利用 qRT PCR对盐胁迫响应TaDREB成员和预测靶基因进行验证。结果显示：(1)从小麦中共鉴定出48个DREB成员(204个拷贝序列),命名为TaDREB1~TaDREB48,分布于21条染色体。(2)TaDREB家族分为14组(G1~G14),位于G2、G5、G10和G14的TaDREB成员受NaCl胁迫后转录水平均无显著变化,其余组中共有25个(52%) TaDREB成员表现出对盐胁迫不同程度的响应;其中有9个成员在盐胁迫后持续上调(含5个新报道基因),有2个成员表现为持续下调;蛋白互作预测结果显示,下调成员TaDREB35的编码蛋白可能会受到1个小麦RING型E3泛素连接酶作用而降解。(3)盐胁迫后有9个成员TaDREB3、TaDREB6、TaDREB16、TaDREB19、TaDREB21、TaDREB24、TaDREB25.12、TaDREB43和TaDREB47发生了可变剪切变化。(4)从转录组差异表达基因中进一步鉴定出3个起始密码子上游2 000 bp序列,包含DRE/CRT元件且在A/B/D组间表达趋势一致的候选靶基因TaRD29、TaGLOS和TaCKX。(5)qRT PCR验证结果显示,上调成员中,除TaDREB19外,其余成员以及TaDREB16均表现出持续上升的趋势;下调成员中只有TaDREB25和TaDREB35的表达量呈持续下降的趋势;3个预测靶基因的表达量均持续上升,验证结果与转录组测序结果一致。该研究鉴定出的11个盐胁迫响应TaDREB成员以及预测的3个下游靶基因为小麦耐盐机制解析和分子育种奠定了基础。     

谷子PAL基因家族全基因组的鉴定和表达分析

苯丙氨酸解氢酶(phenylalanine ammonia-lyase,PAL)基因家族参与苯丙烷类代谢过程,通过调控植物抗病次生物质的合成在植物抗逆反应中发挥重要作用。为明确谷子PAL基因家族在逆境胁迫下的表达规律,该研究利用生物信息学方法对谷子PAL基因家族进行鉴定和表达分析。结果表明:谷子具有11个PAL基因,在进化树中可分为3个亚家族,SiPAL7独自进化为一支。通过构建蛋白结构域发现PAL基因家族成员均含有保守的PAL结构域。启动子分析显示,PAL基因含有应答激素、逆境胁迫等多种因子的顺式作用元件,说明PAL基因广泛参与不同生物学调控过程。RT-qPCR结果显示,谷子PAL基因家族多为诱导型表达,不同光照条件下PAL基因表达量变化明显,不同基因具有不同响应模式,说明谷子PAL基因家族在参与光调节反应中发挥重要作用。谷子PAL基因高度保守,广泛响应不同非生物胁迫,具有表达特异性。该研究结果为揭示PAL基因家族在调节谷子抗性及胁迫应答过程中的作用提供了参考。     

蓖麻RcMsc2基因功能分析

G2/有丝分裂特异性细胞周期蛋白 2(G2/mitotic-specific cyclin-2,Msc2)作为高等植物应对逆境胁迫的关键调控蛋白,参与多个抗逆境胁迫的应答。为探究RcMsc2基因的功能,该研究从蓖麻叶片组织中成功克隆了RcMsc2,并利用生物信息学分析RcMsc2蛋白的结构和潜在功能,同时借助qRT-PCR方法分析RcMsc2基因的组织表达特性和非生物胁迫表达特性。结果表明:(1)RcMsc2基因位于蓖麻第5号染色体长臂,该基因的CDS(coding sequence)区是1 299 bp,编码432个氨基酸。(2)RcMsc2蛋白拥有细胞周期(cyclin)家族特征结构域,是一个不稳定酸性亲水蛋白,无跨膜域和信号肽,相对分子量为49.38 kD。(3)RcMsc2蛋白质的二级、三级结构以α-螺旋和无规则卷曲为主。(4)RcMsc2蛋白与麻风树和巴西橡胶树的CYCB2蛋白的序列同源性最高,且同被聚为Group Ⅱ。(5)35S-RcMsc2-GFP融合蛋白定位于细胞核。(6)RcMsc2基因在蓖麻的所有组织中均有表达且主要在根和茎中发挥作用; 非生物胁迫分析表明RcMsc2基因可以被脱落酸(abscisic acid, ABA)、盐、干旱和低温处理诱导表达,并且RcMsc2基因对低温胁迫的响应最敏感。综上表明,该研究较全面地分析了RcMsc2基因的结构特征、系统进化和表达模式,为揭示RcMsc2基因在蓖麻的生长发育和应答冷胁迫过程中的功能提供了理论参考。     

中粒咖啡WRKY基因家族的生物信息学分析

WRKY转录因子是植物信号网络中不可缺少的部分,作为植物中最大的转录因子家族之一,在植物的多种应激反应中发挥着重要作用。该文利用生物信息学方法对中粒咖啡WRKY蛋白家族的理化特性及其分子进化进行了详细分析。结果表明:(1)CcWRKY蛋白氨基酸数量在103～994个之间,均无信号肽,推测其为非分泌性蛋白; 其二级结构以无规则卷曲为最主要的结构元件,三级结构主要分为6类,其中以CcWRKY15、CcWRKY25、CcWRKY37和CcWRKY42为主要成员的D类结构最稳定。(2)保守结构域及进化树分析结果显示,中粒咖啡WRKY基因家族含有49个成员,其中的10个成员归为WRKY第Ⅰ家族,34个成员归为WRKY第Ⅱ家族,5个成员归为WRKY第Ⅲ家族。(3)中粒咖啡 WRKY47基因与其他物种的系统进化分析结果显示,WRKY47与烟草亲缘关系最近,与非洲油棕(Elaeis guineensis)亲缘关系最远,说明WRKY47蛋白在生物进化过程中比较保守。该研究结果可为中粒咖啡WRKY基因家族分子功能的深层次研究提供一定的借鉴作用,对进一步探究中粒咖啡WRKY基因的功能、进化以及分子育种具有重要意义。     

芥菜型油菜WRKY71转录因子基因家族的鉴定及表达分析

WRKY转录因子是植物中最大的转录因子家族之一,在植物生长发育及逆境胁迫中起重要作用。该研究在芥菜型油菜基因组概览测序的基础上,采用RT PCR技术克隆了4个芥菜型油菜WRKY71s基因,分别命名为BjuWRKY71 1 4。生物信息学分析表明,BjuWRKY71 1 4基因开放阅读框（ORF）分别为822、840、834和855 bp,分别编码273、279、277和284个氨基酸,预测分子量和等电点分别为30.80、27.11、31.58、32.18 kD和7.75、9.08、8.35、7.76。系统进化树分析表明,BjuWRKY71 1 4与白菜WRKY71转录因子相似性最高。亚细胞定位预测分析结果显示,BjuWRKY71s蛋白均定位于细胞核。这4个基因分别定位到芥菜型油菜的A09、B04、A07和scaffold144上。qRT PCR结果表明,4个BjuWRKY71s基因在高盐、脱落酸和低温胁迫下都有响应。在盐胁迫下, BjuWRKY71 3的表达量呈持续上升趋势,而其他3个基因在6 h时表达量达到最大,然后降低;在ABA处理下,BjuWRKY71s基因于6 h时表达量最大,随后逐步降低;低温处理后,BjuWRKY71s基因受低温诱导都显著上调表达。BjuWRKY71 4在该研究的3个胁迫条件下响应最为敏感。研究结果为进一步探讨该基因家族对芥菜型油菜逆境调控机制奠定了基础。     

白菜CesA基因家族鉴定及表达模式分析

为探究CesA基因家族在白菜生长发育及纤维素合成过程中的作用机制,该文通过生物信息学的方法,以白菜的全基因组序列为研究区域,进行理化特征、基因结构、进化特征、保守基序及结构域、顺式作用元件和组织表达等鉴定分析。结果表明:(1)共鉴定出16个编码纤维素合成酶亚基的CesA基因,该家族成员所编码蛋白的理论等电点为4.76～9.12,相对分子量为17.76～122.67 kD,长度为153～1 089 aa。(2)15个基因不均匀地分布于白菜的7条染色体上,Bra036008定位于scaffold上。(3)大部分成员包含4～14个外显子,1～11个保守基序。(4)该家族具有保守的DDD-QXXRW 保守功能域。(5)该家族编码蛋白主要分布在质膜上,二级结构以无规则卷曲与α-螺旋为主,多数成员都含有CesA蛋白典型的N端、C端和跨膜区。(6)CesA基因在茎中表达量相对较高,其中Bra011865、Bra023952和Bra029874在茎、叶、花中显著表达。该研究结果为后续深入研究CesA基因功能以及白菜生长发育研究奠定了基础。     

Effect of Combined Salt and Heat Treatments on Germination and Heat-Shock Protein Synthesis in Lentil Seeds

The germination of lentil seeds was gradually reduced when seeds were exposed to temperature of 30 or 40 °C, either alone or combined with 0.1, 0.2 or 0.3 M NaCl or 34.1 % (m/v) PEG 8000, during 6 –12 h imbibition. [³⁵S]-methionine incorporation in 12 h imbibed lentil axes also decreased with increasing NaCl concentration at 20 and 40 °C, whereas at 30 °C only 0.3 M NaCl treatment partially inhibited protein synthesis. An analysis of newly synthesized proteins by 1-D SDS PAGE, showed that the expression of most polypeptides decreased following increasing stress. Among these, low molecular mass heat-shock proteins declining, higher in 40 °C treated axes than those treated at 30 °C, supports the hypothesis that at this temperature maximal level of expression of these proteins was achieved.     

Effect of dehydration and high light on photosynthesis of two C3 plants (Phaseolus vulgaris L. and Elatostema repens (Lour.) Hall f.)

The effect of drought on the photosynthetic functioning of two C₃ plants, Phaseolus vulgaris and Elatostema repens, has been examined. Leaf net CO₂ uptake measured in normal air was negligible at a leaf water deficit of about 30% while the calculated leaf intercellular CO₂ concentration (Ci) was unchanged. However, both the maximal photosynthetic capacity (CO₂-dependent O₂ evolution) and apparent quantum yield, measured in the presence of saturating CO₂ levels (5 to 14%), only started to decrease within the range of 25 to 30% leaf water deficit. This shows that the drought-induced inhibition seen in normal air is not caused by an inhibition of the photosynthetic mechanism, and that in this case Ci values can be misleading. Both 77 K and room-temperature fluorescence measurements indicate that the functioning of the photosystem-II reaction centre is hardly modified by water shortage. Furthermore, an analysis of photochemical chlorophyll fluorescence quenching shows, in the absence of CO₂, that O₂ can be an efficient acceptor of photosynthetic energy, even in severly dehydrated plants which do not show net CO₂ uptake in normal air. In these plants, O₂ is probably reduced mainly via Mehler-type reactions. High-light treatment given at low O₂ increases photoinhibition as measured by the decrease of apparent quantum yield in dehydrated P. vulgaris, whereas, interestingly, 1% O₂ protects dehydrated E. repens against high-light damage. The two plants could have different protective mechanisms depending upon the O₂ level or different photoinhibitory sites or mechanisms.Abbreviations and symbols Ca, Ci ambient and calculated intercellular CO₂ concentration - Fm, Fo, Fv maximum, initial and variable fluorescence emission - LWD leaf water deficit - PPFD photosynthetic photon flux density - PSII photosystem II - qQ photochemical quenching of chlorophyll fluorescence     

盐旱交叉胁迫对灰胡杨(Populus pruinosa)幼苗生长和生理生化特性的影响

以一年生灰胡杨幼苗为试验材料,利用田间控盐控水的方法,进行干旱和盐胁迫试验,通过测定生长和生理生化指标探讨幼苗在盐旱交叉胁迫下的生长发育及适应规律,旨在阐明干旱及盐交叉胁迫下植物抗旱抗盐机理。研究结果表明:在盐、旱及交叉胁迫下,灰胡杨幼苗抗氧化酶活性、MDA和脯氨酸含量与对照存在显著差异(P0.05)。(1)在8、11 g/L和15 g/L盐处理下,灰胡杨幼苗相对高生长、相对枝长和冠幅增量均受到抑制,且差异显著(P0.05),而干旱胁迫和盐旱交互胁迫下差异不显著。(2)在盐胁迫、盐旱交叉胁迫下,随着胁迫程度的加重,抗氧化酶SOD、POD、CAT活性表现出先增加后降低的趋势,三者协调一致;仅干旱胁迫时,抗氧化酶SOD、POD、CAT活性显著增加;(3)在盐、旱及其盐旱交叉胁迫下,脯氨酸含量呈上升趋势,MDA含量则表现出先降低后升高趋势,这与抗氧化酶活性先升高后降低的趋势相对应。因此,抗氧化酶活性对缓解脂膜过氧化的伤害具有一定限度,MDA含量与抗氧化酶活性呈负相关,灰叶胡杨幼苗在盐旱交叉胁迫下表现出一定的耐性。     

Osmotic adjustment and ion balance traits of an alkali resistant halophyte Kochia sieversiana during adaptation to salt and alkali conditions

Kochia sieversiana (Pall.) C. A. M., a naturally alkali-resistant halophyte, was chosen as the test organism for our research. The seedlings of K. sieversiana were treated with varying (0–400 mM) salt stress (1:1 molar ratio of NaCl to Na₂SO₄) and alkali stress (1:1 molar ratio of NaHCO₃ to Na₂CO₃). The concentrations of various solutes in fresh shoots, including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, NO₃⁻, H₂PO₃⁻, betaine, proline, soluble sugar (SS), and organic acid (OA), were determined. The water content (WC) of the shoots was calculated and the OA components were analyzed. Finally, the osmotic adjustment and ion balance traits in the shoots of K. sieversiana were explored. The results showed that the WC of K. sieversiana remained higher than 6 [g g⁻¹ Dry weight (DW)] even under the highest salt or alkali stress. At salinity levels >240 mM, proline concentrations increased dramatically, with rising salinity. We proposed that this was not a simple response to osmotic stress. The concentrations of Na⁺ and K⁺ all increased with increasing salinity, which implies that there was no competitive inhibition for absorption of either in K. sieversiana. Based on our results, the osmotic adjustment feature of salt stress was similar to that of alkali stress in the shoots of K. sieversiana. The shared essential features were that the shoots maintained a state of high WC, OA, Na⁺, K⁺ and other inorganic ions, accumulated largely in the vacuoles, and betaine, accumulated in cytoplasm. On the other hand, the ionic balance mechanisms under both stresses were different. Under salt stress, K. sieversiana accumulated OA and inorganic ions to maintain the intracellular ionic equilibrium, with close to equal contributions of OA and inorganic ions to anion. However, under alkali stress, OA was the dominant factor in maintaining ionic equilibrium. The contribution of OA to anion was as high as 84.2%, and the contribution of inorganic anions to anion was only 15.8%. We found that the physiological responses of K. sieversiana to salt and alkali stresses were unique, and that mechanisms existed in it that were different from other naturally alkali-resistant gramineous plants, such as Aneurolepidium chinense, Puccinellia tenuiflora. Responsible Editor: John McPherson Cheeseman.     

胡杨种子萌发对温光条件和盐旱胁迫的响应特征

以胡杨(Populus euphratica)种子为材料,分别设置光照(连续光照、12h光照/12h黑暗、连续黑暗)温度(10/15℃、15/20℃、20/25℃、25/30℃、30/35℃和35/40℃)试验、PEG6000渗透胁迫(0、-0.1、-0.2、-0.4、-0.6、-0.8、-1.0、-1.2、-1.4和-1.6MPa)试验和NaCl胁迫(0、0.05、0.10、0.20、0.30、0.40、0.60和0.80mol/L)试验,考察室内种子萌发对温度、光照和盐旱胁迫的敏感性,揭示胡杨种子萌发阶段对生境资源的适应策略。结果显示:(1)胡杨种子在温度(10℃/15℃~35℃/40℃)与连续光照、连续黑暗和12h光照/12h黑暗组合处理条件下均能萌发,且最终种子萌发率均能达到77%以上;3种光照条件下,种子萌发的最适温度范围均为25℃/30℃~30℃/35℃,在该温度范围种子萌发表现出快速、集中的特点,且3种光照条件对种子萌发的影响无显著差异。(2)胡杨种子可以在-1.4~0 MPa渗透势溶液中萌发,而在-1.0~0 MPa间最终萌发率均达到90%以上,且相互之间无显著差异;但当渗透势低于-0.4 MPa时胡杨种子萌发进程和萌发速率受到显著影响,当溶液渗透势低于-1.2 MPa时种子萌发受到显著抑制。(3)胡杨种子可以在0~0.80mol/L NaCl溶液中萌发,而最终萌发率、萌发速率均随着NaCl溶液浓度的增高逐渐降低,但在0~0.20mol/L范围内无显著差异;当NaCl溶液浓度大于0.20mol/L时,种子最终萌发率、种子萌发进程和萌发速率均受到显著抑制,萌发高峰期逐渐向后推移。研究结果表明,胡杨种子萌发时温度比较宽泛,对光照无严格要求,适宜温度下萌发快速集中,且萌发时对盐旱胁迫具有一定程度的忍耐性。这些特性有助于胡杨种子充分利用有限的水分条件而快速完成萌发,是胡杨种子萌发对干旱荒漠地区干旱少雨环境的一种生态适应策略。     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Metabolite levels in the stroma of spinach chloroplasts exposed to osmotic stress: Effects of the pH of the medium and exogenous dihydroxyacetone phosphate

The levels of stromal photosynthetic intermediates were measured in isolated intact spinach (Spinacia oleracea L.) chloroplasts exposed to reduced osmotic potentials. Stressed chloroplasts showed slower rates of metabolite accumulation upon illumination than controls. Relative to other metabolites sedoheptulose-1,7-bisphosphate (SBP) and fructose-1,6-bisphosphate (FBP) accumulated in the stroma in the stressed treatments. Under these conditions 3-phosphoglycerate (3-PGA) efflux to the medium was restricted. Chloroplasts previously incubated with [³²P]KH₂PO₄ and [³²P]dihydroxyacetone phosphate ([³²P]DAP) in the dark were characterized by very high FBP and SBP levels prior to illumination. Metabolism of these pools upon illumination increased with increasing pH of the medium but was consistently inhibited in osmotically stressed chloroplasts. The responses of stromal FBP and SBP pools under hypertonic conditions are discussed in terms of both inhibited light activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) and sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37), and likely increases in stromal ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) active-site concentrations.Abbreviations and symbols DAP dihydroxyacetone phosphate - FBP fructose-1,6-bisphosphate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate - SBP sedoheptulose-1,7-bisphosphate - _s osmotic potential     

Induction of Heat Shock Proteins and Acquisition of Thermotolerance in Germinating Pigeonpea Seeds

Heat shock proteins (HSPs) ranging in molecular masses from 14 to 110 kDa were induced in embryonic axes of germinating Cajanus cajan (L.) Millspaugh seeds after exposure to 40 °C for 1 or 2 h. At 45 °C, there was a marked decline in synthesis of HSPs. A close relationship was observed between HSPs induced and the growth of the germinating seeds. Pretreatment of germinating seeds at 40 °C for 1 h or 45 °C for 10 min followed by incubation at 28 °C for 3 h led to considerable thermotolerance (45 °C, 2 h) and the recovery of protein synthesis. This revised version was published online in July 2006 with corrections to the Cover Date.