Predawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions

Predawn leaf water potential (O_w) and xylem pressure potential (O_p) are expected to be in equilibrium with the soil water potential (soil O_w) around roots of well-watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil O_w, leaf O_w, and xylem O_p in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil-root hydraulic continuity. Most species failed to equilibrate fully with soil O_w, depending on whether leaf O_w or xylem O_p was used as the measure of predawn plant water potential. The contribution of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem O_p for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil O_w. Plant xylem O_p or leaf O_w were significantly more negative than soil O_w for 15 of 15 species where soil O_w was measured. Predawn disequilibrium based on leaf O_w was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil O_w is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf O_w and xylem O_p may not reflect soil O_w, particularly for woody plants and halophytes, even under well-watered conditions.     

Automorphogenesis of maize roots under simulated microgravity conditions

Plant seedlings show exaggerated growth responses on a three-dimensional clinostat. Such an automorphogenesis appears to be one of major factors which govern the life cycle of higher plants under a microgravity environment. On the three-dimensional clinostat, maize roots exhibited curvatures in three different portions; 1) the basal region just protruding from the coleorhiza, 2) the region between the mature and the elongation zone, and 3) the elongation zone, several mm from the tip. Even non-clinostatted control roots showed some degree of curvature. The curvature occurred at random without any dorsiventrality. There was no difference in the osmotic concentration of the cell sap between the convex and the concave halves of any region. However, the convex, rapidly expanding side exhibited a higher extensibility of the cell wall in some regions, which appears to be a cause of the curvature. In order to understand the role of gravity in regulation of plant growth and development, we should clarify a series of events by which an automorphogenesis is induced under simulated microgravity conditions.     

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions

Background and Aims

The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants.

Methods

We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured.

Key Results

Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways.

Conclusions

The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.     

Cloning and characterization of miRNAs from maize seedling roots under low phosphorus stress

MicroRNAs (miRNAs) are a class of small, non-coding regulatory RNAs that regulate gene expression by guiding target mRNA cleavage or translational inhibition in plants and animals. In this study, a small RNA library was constructed to identify conserved miRNAs as well as novel miRNAs in maize seedling roots under low level phosphorus stress. Twelve miRNAs were identified by high throughput sequencing of the library and subsequent analysis, two belong to conserved miRNA families (miRNA399b and miRNA156), and the remaining ten are novel and one of latter is conserved in gramineous species. Based on sequence homology, we predicted 125 potential target genes of these miRNAs and then expression patterns of 7 miRNAs were validated by semi-RT-PCR analysis. MiRNA399b, Zma-miR3, and their target genes (Zmpt1 and Zmpt2) were analyzed by real-time PCR. It is shown that both miRNA399b and Zma-miR3 are induced by low phosphorus stress and regulated by their target genes (Zmpt1 and Zmpt2). Moreover, Zma-miR3, regulated by two maize inorganic phosphate transporters as a newly identified miRNAs, would likely be directly involved in phosphate homeostasis, so was miRNA399b in Arabidopsis and rice. These results indicate that both conserved and maize-specific miRNAs play important roles in stress responses and other physiological processes correlated with phosphate starvation, regulated by their target genes. Identification of these differentially expressed miRNAs will facilitate us to uncover the molecular mechanisms underlying the progression of maize seedling roots development under low level phosphorus stress.     

Effects of low water potential on cortical cell length in growing regions of maize roots

Roots growing under low water potential commonly exhibit a marked decrease in growth rate and in diameter. Using median longitudinal sections of fixed maize (Zea mays L. cv WF9 × Mo 17) seedling roots, we investigated the cellular basis for these effects. Cortical cells in the shortened elongation zone of water stressed roots were longer than cortical cells in the comparable location of well-watered roots. Nearly twofold differences in cell length were seen in the region 2 to 4 millimeters behind the root apex. The shortened growth zone, however, leads to a final mean cortical cell length approximately 30% shorter in the stressed roots. These differences were present regardless of the age of the control roots. These data, and the slower growth rate seen in water-stressed roots, suggest that the water deficit causes a significant reduction in the rate of cell supply to the cortical cell files.     

QTLs for the elongation of axile and lateral roots of maize in response to low water potential

Changes in root architecture and the maintenance of root growth in drying soil are key traits for the adaptation of maize (Zea mays L.) to drought environments. The goal of this study was to map quantitative trait loci (QTLs) for root growth and its response to dehydration in a population of 208 recombinant inbred lines from the International Maize and Wheat Improvement Center (CIMMYT). The parents, Ac7643 and Ac7729/TZSRW, are known to be drought-tolerant and drought-sensitive, respectively. Roots were grown in pouches under well-watered conditions or at low water potential induced by the osmolyte polyethylene glycol (PEG 8000). Axile root length (L _Ax) increased linearly, while lateral root length (L _Lat) increased exponentially over time. Thirteen QTLs were identified for six seedling traits: elongation rates of axile roots (ER_Ax), the rate constant of lateral root elongation (k _Lat), the final respective lengths (L _Ax and L _Lat), and the ratios k _Lat/ER_Ax and L _Lat/L _Ax. While QTLs for lateral root traits were constitutively expressed, most QTLs for axile root traits responded to water stress. For axile roots, common QTLs existed for ER_Ax and L _Ax. Quantitative trait loci for the elongation rates of axile roots responded more clearly to water stress compared to root length. Two major QTLs were detected: a QTL for general vigor in bin 2.02, affecting most of the traits, and a QTL for the constitutive increase in k _Lat and k _Lat/ER_Ax in bins 6.04–6.05. The latter co-located with a major QTL for the anthesis-silking interval (ASI) reported in published field experiments, suggesting an involvement of root morphology in drought tolerance. Rapid seedling tests are feasible for elucidating the genetic response of root growth to low water potential. Some loci may even have pleiotropic effects on yield-related traits under drought stress.     

Zeta potential of protoplasts from gravireacting maize roots

Protoplasts were isolated from cortical cells of the elongating zone of maize (Zea mays L. cv. LG 11) roots and submitted to microelectrophoresis. Significant and transient differences in zeta potential between protoplasts from upper and lower root sides were compared with the gravireaction and the differential elongation of these roots. The maximum difference in the zeta potential was obtained between protoplasts from the upper and lower cortical cells after 90 min, exactly the time of gravipresentation for which the maximum rate of gravireaction was observed. In addition, this almost corresponded to the time for which the difference between the elongation rates of upper and lower sides of the extending zone began to increase. Consequently, the changes in the charges of the plasmalemma of the cortical cells from the growing part of roots could be more or less directly related to the root graviresponse.     

Gene expression profiles in rice roots under low phosphorus stress

Phosphorus (P), an important plant macronutrient, is a component of key molecules such as nucleic acids, phospholipids and ATP. P is often the limiting nutrient for crop yield potential because of the low concentration of soluble P that can be absorbed directly by plant. Plants have evolved a series of molecular and morphological adaptations to cope with P limitation. However, the molecular bases of these responses to P deficiency have not been thoroughly elucidated. In this report, the gene expression profiles of low-P-tolerant rice Zhongzao 18 (Oryza sativa ssp. Indica) and not-low-P-tolerant rice Lagrue (Oryza sativa ssp. Indica) roots at 6 h, 24 h and 72 h under low P stress were investigated and compared with a control (normal P conditions) profile, using a DNA chip of 60,000 oligos (70 mer) that represented all putative genes of the rice genome. A total of 1,518 and 2,358 genes exhibited alterations in expression in response to low P stress in at least one of the three time points in rice Zhongzao 18 and rice Lagrue, respectively. The differentially expressed genes included those involved in phosphate (Pi) transportation, transportations except for Pi transportation, phosphatase, enzymes other than phosphatase, primary metabolism, secondary metabolism and so on. Several genes involved in glycolysis and TCA cycle were up-regulated during the early stages of low P treatment in rice Zhongzao 18 roots, but not in rice Lagrue roots. The results may provide useful information to further studies of the molecular mechanism of plant adaptation to low P and thus facilitate research in improving P utilization in crop species.     

Differential expression profiles of growth-related genes in the elongation zone of maize primary roots

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. Rates of root segmental elongation change from accelerating to decelerating as cell development proceeds from newly formed to fully elongated status. One of the primary variables regulating these changes in elongation rates is the extensibility of the elongating cell walls. To help decipher the complex molecular mechanisms involved in spatially variable root growth, we performed a gene identification study along primary root tips of maize (Zea mays) seedlings using suppression subtractive hybridization (SSH) and candidate gene approaches. Using SSH we isolated 150 non-redundant cDNA clones representing root growth-related genes (RGGs) that were preferentially expressed in the elongation zone. Differential expression patterns were revealed by Northern blot analysis for 41 of the identified genes and several candidate genes. Many of the genes have not been previously reported to be involved in root growth processes in maize. Genes were classified into groups based on the predicted function of the encoded proteins: cell wall metabolism, cytoskeleton, general metabolism, signaling and unknown. In-situ hybridization performed for two selected genes, confirmed the spatial distribution of expression shown by Northern blots and revealed subtle differences in tissue localization. Interestingly, spatial profiles of expression for some cell wall related genes appeared to correlate with the profile of accelerating root elongation and changed appropriately under growth-inhibitory water deficit.     

Arbuscular mycorrhizal fungi influence growth, osmotic adjustment and photosynthesis of citrus under well-watered and water stress conditions

The influence of arbuscular mycorrhizal (AM) fungus Glomus versiforme on plant growth, osmotic adjustment and photosynthesis of tangerine (Citrus tangerine) were studied in potted culture under well-watered and water stress conditions. Seven-day-old seedlings of tangerine were transferred to pots containing Glomus versiforme or non-AMF. After 97 days, half of the seedlings were subject to water stress and the rest were well-watered for 80 days. AM colonization significantly stimulated plant growth and biomass regardless of water status. The soluble sugar of leaves and roots, the soluble starch of leaves, the total non-structural carbohydrates (NSC) of leaves and roots, and the Mg(2+) of leaves were higher in AM seedlings than those in corresponding non-AM seedlings. The levels of K(+) and Ca(2+) in leaves and roots were higher in AM seedlings than those in non-AM seedlings, but differences were only significant under water stress conditions. Moreover, AM colonization increased the distributed proportions of soluble sugar and NSC to roots. However, the proline was lower in AM seedlings compared with that in non-AM seedlings. AM seedlings had higher leaf water potential (Psi), transpiration rates (E), photosynthetic rates (Pn), stomatal conductance (g(s)), relative water content (RWC), and lower leaf temperature (Lt) than corresponding non-AM seedlings. This research also suggested that AM colonization improved the osmotic adjustment originating not from proline but from NSC, K(+), Ca(2+) and Mg(2+), resulting in the enhancement of drought tolerance.     

不同施氮处理玉米根茬在土壤中矿化分解特性

以黄土高原南部地区7a定位试验不同氮肥处理玉米根茬为研究对象,通过室内培养试验研究了施氮量分别为0、120和240 kg N/hm2处理玉米根茬(分别用R0、R120、R240表示)在15-20 cm和45-50 cm土层土壤中有机碳矿化及其对土壤微生物量碳、可溶性有机碳和矿质态氮含量的影响.结果表明,不同处理玉米根茬C/N为R0＞R240 ＞R120.培养条件下,R120和R240根茬的碳矿化速率高于R0根茬,R120与R240根茬之间差异不显著.不同处理根茬C/N与其培养过程中碳素累积表观矿化量呈极显著负相关关系.3种施氮量处理的玉米根茬在培养过程中有机碳矿化率、潜在碳矿化量、土壤微生物量碳、可溶性有机碳含量均为添加R120根茬的处理最高,R240次之,R0最低.添加R120和R240根茬显著提高了培养起始时土壤矿质态氮含量.R0、R120和R240根茬在15-20 cm土层土壤中的碳矿化率分别比其在45-50 cm土层土壤中高51.70％、26.41％和27.84％.在评价根茬还田对农田生态系统碳、氮等养分循环的作用时,应同时考虑施肥对根茬分解和转化的影响.     

Development of the Casparian strip in primary roots of maize under salt stress

The Casparian strip in the endodermis of vascular plant roots appears to play an important role in preventing the influx of salts into the stele through the apoplast under salt stress. The effects of salinity on the development and morphology of the Casparian strip in primary roots of maize (Zea mays L.) were studied. Compared to the controls, the strip matured closer to the root tip with increase in the ambient concentration of NaCl. During growth in 200 mM NaCl, the number and the length of the endodermal cells in the region between the root tip and the lowest position of the endodermal strip decreased, as did the apparent rate of production of cells in single files of endodermal cells (the rate of cell formation being equal to the rate at which cells are lost from the meristem). The estimated time required for an individual cell to complete the formation of the strip after generation of the cell in the presence of 200 mM NaCl was not very different from that required in controls. Thus, salinity did not substantially affect the actual process of formation of the strip in individual cells. The radial width of the Casparian strip, a morphological parameter that should be related to the effectiveness of the strip as a barrier, increased in the presence of 200 mM NaCl. The mean width of the lignified region was 0.92 m in distilled water and 1.33 m in 200 mM NaCl at the lowest position of the strip. The mean width of the strip relative to that of the radial wall at this position was significantly greater after growth in the presence of 200 mM NaCl than in the controls, namely, 20.5% in distilled water and 33.9% in 200 mM NaCl. These observations suggest that the function of the strip is enhanced under salt stress.     

Early changes of water diffusional transfer in maize roots under the influence of water stress

The time dependent response of the hydrodynamic root system to PEG-induced water stress was studied in intact maize Zea mays L. seedlings at intervals varying from several seconds to 3 h by detecting diffusional water transfer with the use of pulsed NMR. In order to establish the contribution of water transfer through aquaporins in response to water stress, the transmembrane water transport in control roots and roots treated with aquaporin blocker was detected. Changes in diffusional water transfer under stress were shown to depend on the duration of osmotic treatment, and include the series of heterogeneous processes. A transient pulsed jump in diffusional water transfer detected several seconds after beginning the osmotic treatment is associated with the spread of the wave of hydraulic pressure along the root. It is proposed that early responses of the hydrodynamic system of maize roots to PEG-induced water stress lies in the unequal change in water permeability of the plasmalemma and tonoplast resulting from the changes in aquaporin activity and perhaps in the escalation of water transfer along the cell vacuome.     

Effects of low water potential on ion uptake and loss for excised roots

Summary In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.Lowering the water potential greatly increased the rate of loss of potassium and bromide from the cytoplasm, but the increases in loss from the vacuoles were much smaller. The results suggest that the mechanisms of ion uptake are not affected by low water potential and that the decrease in ion accumulation is caused by the increased leakage from the cells.     

Root-ABA1 QTL affects root lodging, grain yield, and other agronomic traits in maize grown under well-watered and water-stressed conditions

A major QTL affecting root traits and leaf ABA concentration was identified in maize (Zea mays L.) and named root-ABA1. For this QTL, back-cross-derived lines (BDLs) homozygous either for the (+) or for the (-) allele increasing or decreasing, respectively, root size and leaf ABA concentration, were developed. This study was conducted to evaluate the QTL effects in various genetic backgrounds and at different water regimes. The (+/+) and (-/-) BDLs were crossed with five or 13 inbred tester lines of different origin, thus producing two sets of test-crosses that were evaluated in Italy and China, respectively. Testing was conducted under both well-watered and water-stressed conditions. In Italy, the test-crosses derived from (+/+) BDLs, as compared with those derived from (-/-) BDLs, showed, across both water regimes, higher leaf ABA concentration (on average 384 versus 351 ng g(-1) DW) and lower root lodging (28.0 versus 52.5%), and lower grain yield under water-stressed conditions (4.88 versus 6.27 Mg ha(-1)). In China, where root lodging did not occur, the test-crosses derived from (+/+) BDLs were less productive at both water regimes (on average, 6.83 versus 7.49 Mg ha(-1)). The lower grain yield of the test-crosses derived from (+/+) BDLs was due to a lower number of ears per plant and to lower kernel weight. The results indicate that the (+) root-ABA1 allele confers not only a consistently lower susceptibility to root lodging but also a lower grain yield, especially when root lodging does not occur.