Response of barley seedlings to water deficit and enhanced UV-B irradiation acting alone and in combination

Responses of barley seedlings to water deficit (WD) induced by polyethylene glycol (PEG 6000) and ultraviolet (UV-B; 280–320 nm) radiation and their interaction (UV-B + WD) were examined. A decrease in dry matter yield and water content of leaves and roots was observed following application of WD and UV-B + WD, while no changes were found after treating barley plants with UV-B. Proline content was increased in leaves under WD conditions and UV-B + WD. In contrast, UV-B treatment had no effect on the accumulation of proline in leaves of barley plants. Changes in root proline content showed a varied response: WD induced an increase in the level of this amino acid, while UV-B as well as UV-B + WD suppressed root proline content. The lipid peroxidation product malondialdehyde (MDA) was increased in leaves under WD and UV-B + WD stresses. Root MDA content increased in WD-stressed plants, but it decreased in the case of combined application of both stresses. The applied stress factors operated in a variable manner on phenylpropanoid metabolism. Phenylalanine ammonia-lyase (PAL) activity in leaves and roots was stimulated after exposure to WD and application of UV-B + WD stresses, while UV-B stress did not affect its activity. On the other hand, UV-B treatment enhanced the activity of 4:coumarate-CoA ligase (4CL) in leaves and this enhancement was positively correlated with the accumulation of anthocyanins and flavonols. However, the combined application of WD and UV-B reduced the positive effect of UV-B on the accumulation of these compounds and the activity of 4CL. Surprisingly, anthocyanins and flavonols were not detected in roots of examined barley seedlings despite increased 4CL activity. The results suggest that UV-B-induced activation of 4CL as well as accumulation of anthocyanin and flavonols in leaves is beneficial for the response to this stress factor. On the other hand, WD-induced reduction of the effect of UV-B on 4CL activity and the contents of anthocyanin and flavonol might be a cause of membrane damage in UV-B- and WD-stressed plants. In addition, conversely to what could be expected, the UV-B effect was perceived by the water-stressed roots, which exhibited reduced lipid peroxidation (MDA) and proline accumulation in WD-stressed plants exposed to UV-B.     

Compensatory function for water transport by adventitious roots of <Emphasis Type="Italic">Ipomoea pes-caprae</Emphasis>

To determine the role of adventitious roots in supplying water to Ipomoea pes-caprae (L.) Sweet (Convolvulaceae), we examined the effects of water deficit on water uptake and the growth patterns of leaves and shoots. After stopping the water supply from the primary root or adventitious roots, the water-uptake rate of the other root system increased steeply within 90–100 min to a level of 90% of the pretreatment water-uptake rate of the whole plant. Thus, the primary and adventitious roots can compensate for a decrease in the water-uptake rate of the whole plant caused by dehydration. The continuous growth of leaves and shoots after dehydration suggests that an increase in the water-uptake rate by either root system can support plant growth, although the growth rates of immature leaves in plants with no water supply from the primary or adventitious roots were lower than in controls. We conclude that the water supply from adventitious roots contributes to the survival and growth of plants, and will be important for vegetative propagation.     

单列毛壳菌通过促进秸秆降解并调控激素响应基因表达促进玉米生长

为探究生防真菌单列毛壳菌(Chaetomium uniseriatum)对秸秆降解和玉米(Zea mays)生长的影响,将单列毛壳菌接种到玉米盆栽土壤中,其它条件不变,以保证单一变量。于拔节期和抽雄期进行采样,通过测定土壤有机碳、土壤可溶性碳/氮、微生物量碳/氮以及酶活性,探究接种单列毛壳菌对土壤生物化学指标的影响。在抽雄期对秸秆降解率、地上部生物量、叶片SPAD值、玉米根系激素含量及根系转录组进行分析,探究接种单列毛壳菌对秸秆降解和玉米植株生长发育的影响。结果表明,接种单列毛壳菌后,土壤养分含量未出现显著性变化, β-葡萄糖苷酶(β-GC)活性显著降低;抽雄期玉米地上部生物量、叶片SPAD值以及秸秆降解率均显著高于对照组;玉米根系生长激素(IAA)和玉米素(ZR)含量均显著低于对照组。不同处理下玉米根系转录组分析筛选得到990个差异表达基因(383个基因表达上调,607个基因表达下调);对差异基因进行GO富集分析,得到5个植物激素相关的条目;KEGG富集分析得到1个与植物激素相关的通路(Pvalue<0.05,Q value<0.05)。综上,单列毛壳菌通过促进秸秆降解以...     

Effect of water stress on lupin stem protein analysed by two-dimensional gel electrophoresis

Lupinus albus plants can withstand severe drought stress and show signs of recovery 24 h after rewatering (RW). Two-dimensional gel electrophoresis was used to evaluate the effect of water deficit (WD) on the protein composition of the two components of the lupin stem (stele and cortex). This was performed at three distinct stress levels: an early stage, a severe WD, and early recovery. Protein characterisation was performed through mass spectrometric partial sequencing. Modifications in the protein expression were first noticed at 3 days of withholding water, when the plant water status was still unaffected but some decrease in the relative soil water content had already occurred. An increase in serine proteases, possibly associated with WD sensing, was an early alteration induced by WD. When the stress severity increased, a larger number of stem proteins were affected. Immunophilin, serine protease and cysteine protease (well-known components of animal sensing pathways) were some of these proteins. The simultaneous expression of proteases and protease inhibitors that reacted differently to the stress level and to RW was found. Although the level of protease inhibitors was significantly raised, RW did not cause de novo expression of proteins. Many amino acid sequences did not match known sequences of either protein or expressed sequence tag databases. This emphasises the largely unknown nature of stem proteins. Nevertheless, some important clues regarding the way the lupin plant copes with WD were revealed.     

Influence of arbuscular mycorrhiza on lipid peroxidation and antioxidant enzyme activity of maize plants under temperature stress

The influence of the arbuscular mycorrhizal (AM) fungus, Glomus etunicatum, on characteristics of growth, membrane lipid peroxidation, osmotic adjustment, and activity of antioxidant enzymes in leaves and roots of maize (Zea mays L.) plants was studied in pot culture under temperature stress. The maize plants were placed in a sand and soil mixture under normal temperature for 6 weeks and then exposed to five different temperature treatments (5oC, 15oC, 25oC, 35oC, and 40oC) for 1 week. AM symbiosis decreased membrane relative permeability and malondialdehyde content in leaves and roots. The contents of soluble sugar content and proline in roots were higher, but leaf proline content was lower in mycorrhizal than nonmycorrhizal plants. AM colonization increased the activities of superoxide dismutase, catalase, and peroxidase in leaves and roots. The results indicate that the AM fungus is capable of alleviating the damage caused by temperature stress on maize plants by reducing membrane lipid peroxidation and membrane permeability and increasing the accumulation of osmotic adjustment compounds and antioxidant enzyme activity. Consequently, arbuscular mycorrhiza formation highly enhanced the extreme temperature tolerance of maize plant, which increased host biomass and promoted plant growth.     

The interactive effect of water deficit and UV-B radiation on salicylic acid accumulation in barley roots and leaves

The aim of the paper was to determine the effect of water deficit (WD) and UV-B radiation acting individually and in combination on salicylic acid (SA) accumulation as well as on the activity of phenylalanine ammonia-lyase (PAL) and benzoic acid hydroxylase (BA2H) that control its biosynthetic route from phenylalanine. An additional aim was to test whether the interaction of these stresses limits the negative effect of a single stress on tissue hydration and membrane injury. Two-week-old seedlings were subjected to water deficit (WD), UV-B irradiation (UV-B) and three different combinations of WD and UV-B: (I) WD and UV-B applied at the same time, (II) UV-B applied before WD, and (III) WD applied before UV-B. Water deficit was imposed by immersing the root system in aerated nutrient solution with polyethylene glycol (PEG 6000) of water potential – 0.5 MPa. UV-B dosage was 24 kJ m⁻² day⁻¹ (0.84 W m⁻²) at the canopy level. UV-B and WD imposed individually and jointly, caused, in a time-dependent manner, an increase in SA content in both organs. Increased levels of SA in WD stressed plants were accompanied by an increase in the activity of PAL and BA2H. However, in plants exposed to UV-B were accompanied only by an increase in the activity of BA2H. Under WD conditions, an earlier increase of SA content was observed in roots than in leaves, which may indicate the involvement of SA in the signal transduction between roots and leaves. In plants exposed to sequential action of WD and UV-B, regardless of the order of its imposition, the effect of each single factor on SA accumulation in leaves was strengthened. WD had a greater effect on water loss and membrane injury than UV-B radiation. In plants exposed to WD after pre-treatment with UV-B radiation, a cross-tolerance mechanism was observed. Leaves of these plants did not show increased lipid peroxidation, measured in terms of malondialdehyde content, and a decrease in water content. This protective action was probably caused by the increase of the SA level in leaves of the UV-B treated plants prior to WD imposition.     

Responses to winter dormancy,temperature, and plant hormones share gene networks

Gene networks modulated in winter dormancy (WD) in relation to temperature and hormone responses were analyzed in tea [Camellia sinensis (L.) O. Kuntze]. Analysis of subtracted cDNA libraries prepared using the RNA isolated from the apical bud and the associated two leaves (two and a bud, TAB) of actively growing (AG) and winter dormant plant showed the downregulation of genes involved in cell cycle/cell division and upregulation of stress-inducible genes including those encoding chaperons during WD. Low temperature (4°C) modulated gene expression in AG cut-shoots in similar fashion as observed in TAB during WD. In tissue harvested during WD, growth temperature (25°C) modulated gene expression in the similar way as observed during the period of active growth (PAG). Abscisic acid (ABA) and gibberellic acid (GA₃) modulated expression of selected genes, depending upon if the tissue was harvested during PAG or WD. Tissue preparedness was critical for ABA- and GA₃-mediated response, particularly for stress-responsive genes/chaperons. Data identified the common gene networks for winter dormancy, temperature, and plant hormone responses.     

Sorbitol and NaCl stresses affect free, microsome-associated and thylakoid-associated polyamine content in Zea mays and Phaseolus vulgaris

Most of commercially important crops, including maize and common bean, are sensitive to water deficit and salinity. Polyamines are considered to be osmotic and salt tolerance modulators and biochemical indicators of these stresses. In the present study, we measured organ-specific changes in levels of free, microsome- and thylakoid-associated polyamines in leaves and roots of maize and common bean plants exposed for 24 h to osmotic and saline stresses. Putrescine levels were generally higher in the studied organs of both species and under both stresses; only in the roots of salt-treated bean it considerably decreased. In both species, salt stress (200 mM NaCl) induced a significant decrease in free spermidine in roots. We observed a significant decrease in the contents of all polyamines associated with the microsomes isolated from the roots of maize and bean growing in sorbitol and salt conditions. Also the microsomes isolated from the leaves of stressed plants were characterized by the lower contents of polyamines. Our data showed a reduction of putrescine content, with significantly decreased spermidine and spermine levels in thylakoids isolated from the chloroplasts of maize and bean plants growing under both stresses. The results indicate that the studied maize and bean cultivars are rather drought-sensitive. Additionally, microsome- and thylakoid-associated polyamines seem to be good markers of plant stress tolerance.     

Nature and nurture: the importance of seed phosphorus content

Background

Low phytoavailability of phosphorus (P) limits crop production worldwide. Increasing seed P content can improve plant establishment and increase yields. This is thought to be a consequence of faster initial root growth, which gives seedlings earlier access to growth-limiting resources, such as water and mineral elements. It can be calculated that seed P reserves can sustain maximal growth of cereal seedlings for several weeks after germination, until the plant has three or more leaves and an extensive root system.

Case study

In this issue of Plant and Soil, Muhammad Nadeem and colleagues report (1) that measurable P uptake by roots of maize seedlings begins about 5 d after germination, (2) that the commencement of root P uptake is coincident with the transition from carbon heterotrophy to carbon autotrophy, and (3) that neither the timing nor the rate of uptake of exogenous P by the developing root system is influenced by initial seed P content.

Hypothesis

Here it is hypothesised that the delay in P acquisition by roots of maize seedlings might be explained if the expression of genes encoding phosphate transporters is not upregulated either (1) because the plant has sufficient P for growth or (2) because a systemic signal from the shoot, which relies on photosynthesis or phloem development, is not produced, translocated or perceived.     

Water-deficit tolerance in citrus is mediated by the down regulation of PIP gene expression in the roots

Water deficit (WD) is a growing problem in agriculture. In citrus crops, genetically-determined rootstock characteristics are important factors influencing plant responses to WD. Aquaporins are involved in regulating the water supply to the plant by mediating water flow through the cell membranes. Recent studies support a direct role for aquaporins in plant water relations and demonstrate their involvement in WD tolerance. This study investigates the relationship between photosynthetic and water-balance parameters with aquaporin expression levels and hydraulic conductance of roots (Kr) in conditions of moderate WD in citrus rootstocks. The plant materials used were the rootstocks Poncirus trifoliata (L.) Raf. (PT), Cleopatra mandarin (Citrus reshni Hort ex Tan.) (CM) and 030115 (a hybrid of the two former rootstocks), all grafted with the citrus variety ??Valencia Late?? (C. sinensis (L.) Osb). Plants were irrigated with two differents irrigation doses (normal irrigation and moderate WD) during 70 days and leaf water potential (??s), net CO₂ assimilation (A_CO2), transpiration, stomatal conductance (gs) and substomatal CO₂ concentration (Ci) were measured periodically under both irrigation conditions. Kr and PIP1 and PIP2 gene expression levels in fine roots of control plants and plants subjected to WD on day 43 of the experiment were determined. Under WD conditions, the hybrid 030115 drastically reduced aquaporin expression and Kr, accompanied by a loss of plant vigour but without reducing the net CO₂ assimilation (A_CO2). PT maintained the same aquaporin expression level and similar Kr under WD as under normal irrigation conditions, but suffered a sharp reduction in A_CO2. CM, which has lower Kr and aquaporin expression than PT under both normal irrigation conditions and WD, responded better to water stress conditions than PT. Low aquaporin levels, or down-regulated aquaporin expression, accompanied by decreased plant vigour led to decreased plasma membrane permeability, thereby facilitating water retention in the cells under water stress conditions. This may induce water stress tolerance in citrus rootstocks.     

Spatial soil heterogeneity has a greater effect on symbiotic arbuscular mycorrhizal fungal communities and plant growth than genetic modification with Bacillus thuringiensis toxin genes

Maize, genetically modified with the insect toxin genes of Bacillus thuringiensis (Bt), is widely cultivated, yet its impacts on soil organisms are poorly understood. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with plant roots and may be uniquely sensitive to genetic changes within a plant host. In this field study, the effects of nine different lines of Bt maize and their corresponding non‐Bt parental isolines were evaluated on AMF colonization and community diversity in plant roots. Plants were harvested 60 days after sowing, and data were collected on plant growth and per cent AMF colonization of roots. AMF community composition in roots was assessed using 454 pyrosequencing of the 28S rRNA genes, and spatial variation in mycorrhizal communities within replicated experimental field plots was examined. Growth responses, per cent AMF colonization of roots and AMF community diversity in roots did not differ between Bt and non‐Bt maize, but root and shoot biomass and per cent colonization by arbuscules varied by maize cultivar. Plot identity had the most significant effect on plant growth, AMF colonization and AMF community composition in roots, indicating spatial heterogeneity in the field. Mycorrhizal fungal communities in maize roots were autocorrelated within approximately 1 m, but at greater distances, AMF community composition of roots differed between plants. Our findings indicate that spatial variation and heterogeneity in the field has a greater effect on the structure of AMF communities than host plant cultivar or modification by Bt toxin genes.     

Metabolic acclimation of source and sink tissues to salinity stress in bermudagrass (Cynodon dactylon)

Salinity is one of the major environmental factors affecting plant growth and survival by modifying source and sink relationships at physiological and metabolic levels. Individual metabolite levels and/or ratios in sink and source tissues may reflect the complex interplay of metabolic activities in sink and source tissues at the whole‐plant level. We used a non‐targeted gas chromatography–mass spectrometry (GC‐MS) approach to study sink and source tissue‐specific metabolite levels and ratios from bermudagrass under salinity stress. Shoot growth rate decreased while root growth rate increased which lead to an increased root/shoot growth rate ratio under salt stress. A clear shift in soluble sugars (sucrose, glucose and fructose) and metabolites linked to nitrogen metabolism (glutamate, aspartate and asparagine) in favor of sink roots was observed, when compared with sink and source leaves. The higher shifts in soluble sugars and metabolites linked to nitrogen metabolism in favor of sink roots may contribute to the root sink strength maintenance that facilitated the recovery of the functional equilibrium between shoot and root, allowing the roots to increase competitive ability for below‐ground resource capture. This trait could be considered in breeding programs for increasing salt tolerance, which would help maintain root functioning (i.e. water and nutrient absorption, Na⁺ exclusion) and adaptation to stress.