RNA-Seq Analysis Reveals MAPKKK Family Members Related to Drought Tolerance in Maize

The mitogen-activated protein kinase (MAPK) cascade is an evolutionarily conserved signal transduction pathway that is involved in plant development and stress responses. As the first component of this phosphorelay cascade, mitogen-activated protein kinase kinase kinases (MAPKKKs) act as adaptors linking upstream signaling steps to the core MAPK cascade to promote the appropriate cellular responses; however, the functions of MAPKKKs in maize are unclear. Here, we identified 71 MAPKKK genes, of which 14 were novel, based on a computational analysis of the maize (Zea mays L.) genome. Using an RNA-seq analysis in the leaf, stem and root of maize under well-watered and drought-stress conditions, we identified 5,866 differentially expressed genes (DEGs), including 8 MAPKKK genes responsive to drought stress. Many of the DEGs were enriched in processes such as drought stress, abiotic stimulus, oxidation-reduction, and metabolic processes. The other way round, DEGs involved in processes such as oxidation, photosynthesis, and starch, proline, ethylene, and salicylic acid metabolism were clearly co-expressed with the MAPKKK genes. Furthermore, a quantitative real-time PCR (qRT-PCR) analysis was performed to assess the relative expression levels of MAPKKKs. Correlation analysis revealed that there was a significant correlation between expression levels of two MAPKKKs and relative biomass responsive to drought in 8 inbred lines. Our results indicate that MAPKKKs may have important regulatory functions in drought tolerance in maize.     

Genome-wide association mapping of tan spot resistance (Pyrenophora tritici-repentis) in European winter wheat

The effect of low temperature on the physiology of maize has been well studied, but the genetics behind cold tolerance is poorly understood. To better understand the genetics of cold tolerance we conducted a quantitative trait locus (QTL) analysis on a segregating population from the cross of a cold-tolerant (EP42) and a cold-susceptible (A661) inbred line. The experiments were carried under cold (15 °C) and control (25 °C) conditions in a phytotron. Cold temperature reduced the shoot dry weight, number of survival plants and quantum yield of electron transport at photosystem II (ΦPSII) and increased the anthocyanin content in maize seedlings. Low correlations were found between characteristics under low and optimum temperature. Ten QTLs were identified, six of them at control temperatures and four under cold temperatures. Through a meta-QTL analysis we identified three genomic regions in chromosomes 2, 4 and 8 that regulate the development of maize seedlings under cold conditions and are the most promising regions to be the target of future marker-assisted selection breeding programs or to perform fine mapping to identify genes involved in cold tolerance in maize.     

Maize leaf temperature responses to drought: Thermal imaging and quantitative trait loci (QTL) mapping

Leaf temperature has been shown to vary when plants are subjected to water stress conditions. Recent advances in infrared thermography have increased the probability of recording drought tolerant responses more accurately. The aims of this study were to identify the effects of drought on leaf temperature using infrared thermography. Furthermore, the genomic regions responsible for the expression of leaf temperature variation in maize seedlings (Zea mays L.) were explored. The maize inbred lines Zong3 and 87-1 were evaluated using infrared thermography and exhibited notable differences in leaf temperature response to water stress. Correlation analysis indicated that leaf temperature response to water stress played an integral role in maize biomass accumulation. Additionally, a mapping population of 187 recombinant inbred lines (RILs) derived from a cross between Zong3 and 87-1 was constructed to identify quantitative trait loci (QTL) responsible for physiological traits associated with seedling water stress. Leaf temperature differences (LTD) and the drought tolerance index (DTI) of shoot fresh weight (SFW) and shoot dry weight (SDW) were the traits evaluated for QTL analysis in maize seedlings. A total of nine QTL were detected by composite interval mapping (CIM) for the three traits (LTD, RSFW and RSDW). Two co-locations responsible for both RSFW and RSDW were detected on chromosomes 1 and 2, respectively, which showed common signs with their trait correlations. Another co-location was detected on chromosome 9 between LTD and shoot biomass, which provided genetic evidence that leaf temperature affects biomass accumulation. Additionally, the utility of a thermography system for drought tolerance breeding in maize was discussed.     

Association of polyamines in governing the chilling sensitivity of maize genotypes

Chilling stress is an important constraint of global production of maize. This study was undertaken to compare the chilling responses of different maize seedling tissues and to analyze changes in polyamines as a result of chilling stress. Reponses to chilling were characterized in two maize (Zea mays L.) inbred lines, ‘HuangC’ and ‘Mo17’, that putatively differ in chilling sensitivity. Seedlings were exposed to low temperature (5°C) and chilling injury was estimated by electrical conductivity (EC), malonaldehyde (MDA) concentration, and by changes in putrescine (Put), spermidine (Spd) and spermine (Spm) concentrations in root, mesocotyl, and coleoptile tissues. Membrane permeability (as measured by EC), MDA concentrations and Put concentrations in the three tissue of maize seedlings increased after chilling stress, except for the Put concentration in roots. Spd and Spm concentrations in the three tissues of seedlings decreased after chilling stress. The EC for cold stressed tissues were lower in HuangC than Mo17. Also, the EC of coleoptile tissues were lower than for mesocotyl in both inbred lines. We suggest that mesocotyl tissue can be used to evaluate cold tolerance in maize. Stepwise regression analyses showed that chilling injury in roots was generally correlated with Spd concentration while in the mesocotyl injury was mainly correlated with Put and Spd concentrations. Spermidine and Spm concentrations in the coleoptile were correlated with chilling injury. Characteristics changes of polyamines in chill-tolerant maize seedling combined with regression analysis are a reliable method for evaluating chill tolerance in maize lines.     

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO₂/H₂O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO₂, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m² s)) at normal and half atmospheric CO₂ concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO₂ assimilation against control material due to the elevation of stomatal conductance for CO₂ diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.     

Stomatal Opening Mechanism of CAM Plants

Stomata usually open when leaves are transferred from darkness to light. However, reverse-phase stomatal opening in succulent plants has been known. CAM plants such as cacti and Opuntia ficus–indica achieve their high water use efficiency by opening their stomata during the cool, desert nights and closing them during the hot, dry days. Signal transduction pathway for stomatal opening by blue light photoreceptors including phototropins and the carotenoid pigment zeaxanthin has been suggested. Blue light regulated signal transduction pathway on stomatal opening could not be applied to CAM plants, but the most possible theory for a nocturnal response of stomata in CAM plants is photoperiodic circadian rhythm.     

<Emphasis Type="Italic">Trichoderma asperellum</Emphasis> alleviates the effects of saline–alkaline stress on maize seedlings via the regulation of photosynthesis and nitrogen metabolism

This study aimed to investigate the influence of the fungus Trichoderma asperellum on photosynthesis and nitrogen metabolism in maize seedlings of different genotypes, subjected to saline–alkaline stress. Saline–alkaline tolerant and sensitive varieties, Jiangyu 417 and Xianyu 335 (XY335), respectively, were grown in naturally saline–alkaline soil (pH 9.30) in 5-inch pots. Root and leaf samples were collected when seedlings had three heart-shaped leaves and the fourth leaf developing. Meadow soil (pH 8.23) was used as a positive control. Saline–alkaline stress remarkably increased NH₄⁺ content and caused ammonia toxicity, weakened the ammonium assimilation process, and reduced photosynthesis in maize seedlings. Our results show that T. asperellum alleviated these effects to a certain degree, especially in XY335. The application of T. asperellum likely improved the content of photosynthetic pigments, enhanced the photochemical activity of the photosystem II reaction center, increased the activities of ATP enzymes in the chloroplasts, reduced the non-stomatal limitation of photosynthesis owing to saline–alkaline stress, and promoted photosynthesis to provide more raw materials and energy for nitrogen metabolism, thereby improving the activity of nitrogen metabolism and the capacity for material production in maize seedlings. By coordinating the synergistic effect of glutamate dehydrogenase, glutamine synthetase/glutamate synthase, and transamination, T. asperellum promoted the assimilation of excessively accumulated ammonia, maintained the balance of NH₄⁺ and the enzymes related to its metabolism, and subsequently alleviated ammonia toxicity and negative changes in nitrogen metabolism resulting from saline–alkaline stress. Thus, the application of T. asperellum alleviated damage to chloroplasts and thylakoid membranes, and improved nitrogen metabolism, thereby promoting seedling growth. The concentration of 1?×?10⁹ spores L^?1 was found to be the most effective and economical treatment.     

低温锻炼后桑树幼苗光合作用和抗氧化酶对冷胁迫的响应

以桑树品种“秋雨”为试验材料,研究了桑树幼苗在低温锻炼、冷胁迫和常温恢复期间的光合作用和抗氧化酶活性的变化.结果表明: 12 ℃3 d低温锻炼明显提高了桑树幼苗的抗冷性.3 ℃3 d冷胁迫下,12 ℃3 d低温锻炼后的桑树幼苗叶片净光合速率(P_n)、气孔导度(G_s)和PSⅡ 最大光化学效率(F_v/F_m)明显高于对照(未经低温锻炼)处理的桑树幼苗,而且其在常温下的恢复也较对照桑树幼苗迅速.在12 ℃ 3 d低温锻炼和3 ℃ 3 d冷胁迫期间,桑树幼苗叶片脯氨酸和可溶性糖含量明显增加,而经低温锻炼的桑树幼苗叶片丙二醛(MDA)含量明显低于未经低温锻炼的桑树幼苗,经低温锻炼的桑树幼苗叶片抗坏血酸过氧化物酶(APX)活性则明显高于未经低温锻炼的桑树幼苗.说明渗透调节物质含量增加和APX活性提高在低温锻炼诱导桑树幼苗的抗冷性上发挥着重要的作用.     

Drought Responses of Foliar Metabolites in Three Maize Hybrids Differing in Water Stress Tolerance

Maize (Zea mays L.) hybrids varying in drought tolerance were treated with water stress in controlled environments. Experiments were performed during vegetative growth and water was withheld for 19 days beginning 17 days after sowing. Genotypic comparisons used measured changes of leaf water potential or results were expressed by time of treatment. Total dry matter of the drought tolerant hybrid on the final harvest was 53% less than that of the intermediate and susceptible maize hybrids when plants were water sufficient. This showed that maize hybrids selected for extreme drought tolerance possessed a dwarf phenotype that affected soil water contents and leaf water potentials. Changes of shoot and root growth, leaf water potential, net photosynthesis and stomatal conductance in response to the time of water stress treatment were diminished when comparing the drought tolerant to the intermediate or susceptible maize hybrids. Genotypic differences were observed in 26 of 40 total foliar metabolites during water stress treatments. Hierarchical clustering revealed that the tolerant maize hybrid initiated the accumulation of stress related metabolites at higher leaf water potentials than either the susceptible or intermediate hybrids. Opposite results occurred when changes of metabolites in maize leaves were expressed temporally. The above results demonstrated that genotypic differences were readily observed by comparing maize hybrids differing in drought tolerance based on either time of treatment or measured leaf water potential. Current findings provided new and potentially important insights into the mechanisms of drought tolerance in maize.     

The influence of water stress on the photosynthetic performance and stomatal behaviour of tree seedlings subjected to variation in temperature and irradiance

Summary Seedlings of Betula pendula Roth. and Gmelina arborea L. were subjected to variation in temperature and irradiance. The influence of a mild water-stressing treatment on the photosynthetic performance and stomatal behaviour of these plants was assessed. For both species, the shape of the relationships between irradiance and photosynthesis and temperature and photosynthesis resembled those reported for other species. The effect of water stress was to reduce the rate of photosynthesis, particularly at high temperatures. This was largely a function of a reduction in mesophyll conductance under these conditions. The optimum temperature for stomatal opening was significantly lower than the optimum temperature for photosynthesis, which was in turn lowered by the water stress treatment. The stomata of birch seedlings showed maximum opening at an intermediate temperature while the stomata of Gmelina generally exhibited a closing movement when leaf temperatures increased from 15° C. Mesophyll conductances of both species increased with increasing temperature.The physiological basis for the variation in photosynthetic performance and stomatal behaviour and the ecological significance of this variation are discussed.     

Global gene expression responses to waterlogging in roots of sesame (Sesamum indicum L.)

Waterlogging stress lowers yields in sesame (Sesamum indicum L.). A major component of waterlogging stress is the lack of oxygen available to submerged tissues. Although the morphology and physiology of plants grown under anaerobic conditions have been studied in detail, limited work has been done to elucidate adaptations at the molecular level. To gain comprehensive insight into how sesame responds to hypoxia at the genome level, we performed gene expression profiling at two time points during a 36-h period following hypoxic treatment using a whole-genome RNA-Seq analysis. We identified sets of significantly positively and negatively expressed genes (induced and repressed, respectively) in response to hypoxia with distinct temporal profiles. The genes that were affected were associated with glycolysis, nitrogen metabolism, starch and sucrose metabolism and plant hormone signal transduction and indicated the upregulation of particular pathways (glycolysis/glycogenesis) in the Kyoto Encyclopedia of Genes and Genomes. Moreover, significant changes in the expression of genes were found for pathways, including flavone and flavonol biosynthesis, steroid biosynthesis, photosynthesis, cysteine and methionine metabolism, glutathione metabolism, as well as phenylpropanoid biosynthesis, spliceosome, circadian rhythm. This study helps in elucidating the molecular mechanisms of waterlogging tolerance and provides a basis for the genetic engineering of sesame.     

Comparative proteomic analysis of leaves between photoperiod-sensitive and photoperiod-insensitive maize inbred seedlings under long day treatments

Day length is an important environmental factor affecting the growth and development of maize (Zea mays), a short day (SD) plant grown in different latitudes. Leaf has been recognized as the light perceiving and signal producing organ. Under long day (LD) conditions, photoperiod-sensitive induction phase in maize begins at the fourth fully expanded leaf stage. However, the changes of maize leaf proteome in response to LD are largely unknown. To reveal maize proteome response to LD, proteins extracted from newly expanded fifth, sixth and seventh leaves from maize inbred line 496-10 (photoperiod sensitive) and Huangzao4 (HZ4, photoperiod insensitive) under LD treatments were compared via gel-based proteomic approach. As a result, eleven differentially expressed proteins were identified between 496-10 and HZ4 by mass spectrometry. This difference in protein accumulation was highly reproducible during the fifth to seventh leaf stages and most obvious at the seventh leaf stage. The identified proteins are mainly involved in circadian clock or iron metabolism, light harvesting and photosynthesis, nucleic acid metabolism and carbon fixation or energy metabolism. This study provides new insight into the influences of LD treatment on SD plants, such as maize, at proteome level.     

Developing a mathematical model for variation of physiological responses of Jatropha curcas leaves depending on leaf positions under soil flooding

The effect of soil flooding on photosynthesis, transpiration and stomatal conductance of Jatropha curcas seedlings were studied under natural environmental variables. Soil flooding reduced photosynthesis (P _N), transpiration (E) and stomatal conductance (gs) in response to leaf positions of Jatropha curcas plants. Based on the results, we conclude that decrease in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake of flooded seedlings. A mathematical relationship was successfully developed to describe photosynthesis, transpiration and stomatal response of Jatropha under soil flooding stress.     

光质对植物光合作用的调控及其机理

光合作用是植物生长发育的基础.光质对植物光合作用的调控主要包括可见光对植物气孔器运动、叶片生长、叶绿体结构、光合色素、D1蛋白及其编码基因和光合碳同化等的调节,以及紫外光对植物光系统Ⅱ的影响.蓝光和红光能促进气孔的开张,而绿光能够逆转这种作用.蓝光有利于叶绿体的发育,红、蓝、绿复合光有利于叶面积的扩展,而红光更有利于光合产物的积累;不同光质对不同植物、不同组织器官叶绿素积累的影响不同.蓝光和远红光可以促进psbA基因转录物质的积累.大多数高等植物和绿藻在橙、红光下光合速率最高,蓝紫光其次,绿光最低.紫外光可以导致光系统Ⅱ的电子传递活性下降.此外,针对光质与光合作用研究领域中存在的问题,对今后的研究方向进行了讨论.     

甘草叶片形态结构和光合作用对干旱胁迫的响应

叶片结构在植物防御生物和非生物胁迫方面起着重要的作用,可通过合成、储存和分泌次生代谢产物提高植物抗性。以甘草幼苗为试材,采用盆栽控水自然干旱法,探讨叶片光合作用、气孔微形态和腺体形态对干旱胁迫的响应。结果表明:①随着干旱胁迫程度的加剧,叶片净光合速率(Pn)、气孔导度(Gs)和蒸腾速率(Tr)均呈先升高后降低的趋势;其中胞间CO2浓度(Ci)在重度干旱胁迫(severe stress,SS)时迅速增高。②随着干旱胁迫程度的加剧,叶片总气孔密度和气孔开张比呈先增大后减小的趋势;而气孔开张宽度呈逐渐减小的趋势。③随着干旱胁迫程度的加剧,叶片上表皮和下表皮腺体密度总数整体上呈增大的趋势,腺体颜色随着干旱胁迫程度的加剧逐渐加深,形状出现不规则褶皱和内陷。总之,甘草叶片表面的腺体特征参与抗旱逆境调节,从而避免干旱胁迫对甘草植株的伤害;在SS下,胁迫程度加速了气孔细胞的程序性死亡(PCD),甘草幼苗失去抗旱能力。