Aerenchyma Formed Under Phosphorus Deficiency Contributes to the Reduced Root Hydraulic Conductivity in Maize Roots

Root hydraulic conductivity has been shown to decrease under phosphorus （P） deficiency. This study Investigated how the formation of aerenchyma is related to this change. Root anatomy, as well as root hydraulic conductivity was studied In maize （Zea mays L.） roots under different phosphorus nutrition conditions. Plant roots under P stress showed enhanced degradation of cortical cells and the aerenchyma formation was associated with their reduced root hydraulic conductivity, supporting our hypothesis that air spaces that form in the cortex of phosphorusstressed roots Impede the radial transport of water in a root cylinder. Further evidence came from the variation In aerenchyma formation due to genotypic differences. Five maize inbred lines with different porosity in their root cortex showed a significant negative correlation with their root hydraulic conductivity. Shoot relative water content was also found lower In P-deficient maize plants than that in P-sufficient ones when such treatment was prolonged enough, suggesting a limitation of water transport due to lowered root hydraulic conductivity of P-deficient plants.     

Root Cap Mucilage and Extracellular Calcium as Modulators of Cellular Growth in Postmitotic Growth Zones of the Maize Root Apex*

Abstract: The control of maize root growth by root cap mucilage and extracellular calcium (Ca) was examined. Special attention was paid to the influence of these factors on cellular aspects of root growth, such as cell shape and organization of the microtubular (MT) cytoskeleton. Externally supplied Ca impaired the transition of early post-mitotic cells from a more-or-less apolar mode of expansion to a strictly anisotropic mode of elongation accompanied by their more rapid growth. However, this inhibitory effect of Ca was not associated with any re-arrangement of the cortical MTs, their transverse arrays, with respect to the root axis, being maintained under these conditions. Root mucilage, collected from donor root caps and placed around root tips, exerted a similar effect on cell shapes as did externally supplied Ca. In contrast, roots grown in a medium of low Ca content, or from which the root cap mucilage was continually removed, had more elongated cell shapes in their post-mitotic growth regions when compared to the control roots. These findings are consistent with a notion that Ca is present in the root cap mucilage in physiologically relevant amounts and can mediate growth responses in both the PIG region and the apical part of the elongation zone. Integrating several known effects of Ca ions on growth at the root apex, a hypothesis is proposed that a Ca-mediated and MT-independent control of cell growth in the PIG region might be involved in morphogenetic root movements (e.g. gravitropism), and that root growth responses could be initiated by an asymmetric distribution of extracellular calcium, or root cap slime, around the growing root tip.     

Cellular Dimorphism in the Maize Root Cortex: Involvement of Microtubules,Ethylene and Gibberellin in the Differentiation of Cellular Behaviour in Postmitotic Growth Zones

Detailed morphometric analysis of cell shapes and an immunofluorescent study of microtubules were carried out on primary roots of Zea mays L. Two types of cells were found to be formed within the postmitotic isodiametric growth (PIG) region of the root cortex that were differentially responsive to low level of exogenous ethylene. The innermost and central cell rows of the cortex were sensitive to ethylene treatment and showed a disturbed distribution of cortical microtubules (CMTs) as well as changed polarity of cell growth, whereas the 2–3 outermost cell rows were less sensitive in this respect. This suggests that post-mitotic cells of the inner cortex are specific targets for ethylene action. These properties of the inner cortex are compatible with its cells being involved in the formation of aerenchyma; they may also favour root growth in compacted soil. By contrast, the specific properties of the outer cortex indicate that this tissue domain is necessary for the gaseous impermeability and the mechanical strengthening of subjacent aerenchymatous cortex, especially in the mature region of the root. Ethylene affected neither the pattern of cortical cell expansion in the meristem nor the position of the PIG region with respect to the root tip. This contrasts with gibberellin-deficiency which affected these parameters in both parts of the cortex. These observations indicate a fundamental difference between the role of these two phytohormones in the morphogenesis and development of maize roots.     

Growth, Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen (N) uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root: shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Growth,Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen （N） uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root ： shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Aluminum Exclusion Mechanism in Root Tips of Maize (Zea mays L.): Lysigeny of Aluminum Hyperaccumulator Cells

Abstract: Ultrastructural studies, together with X-ray microanalytical, immunocytochemical and cytochemical analysis performed in root tips of Al-resistant (C-525 M) and Al-sensitive (Adour 250, HS 7777 and BR 201 F) maize plants ( Zea mays L.), after 96 h exposure to 20 μM Al, showed qualitatively similar results in the four cultivars.
Al was identified in electron-opaque precipitates, which were insoluble even in EDTA chelate. They also contained an elevated proportion of P and also of K and Ca, some traces of Mn, Fe and Zn and sometimes of Mg. This elemental composition is similar to that described for phytin (Al-phytin), and the precipitates were localized in the two principal extraplasmatic compartments: cell walls and vacuoles. Al-phytin was detected in swollen areas of cell walls in membraneous concentric configurations, resembling myelin figures, probably rich in phosphatidyl inositol, which also intervene in the vacuolar internalization of Al-phytin and are similar to a peculiar form of endocytosis (not previously described). Abnormal apoplastic protuberances containing abundant electron-opaque Al-phytin deposits, agglutinated by callose (immunocytochemically identified), were shown in cortex cells with high mitotic activity (around 1 - 1.5 mm from cap root). Al-hyperaccumulator cells parallel to the root axis were correlated with longitudinal lysigenous intercellular spaces after cell death and dissolution (lysigeny). Indicators of activated lysigeny, as low levels of Al and callose (in agreement with other authors) and high levels of phosphoinositides, can mark Al-resistant genotypes, contrary to Al-sensitive genotypes, probably derived from a partially activated or even inactivated lysigeny.
The lysigeny of Al hyperaccumulator cells constitutes new ultrastructural evidence of an Al exclusion mechanism, supporting biochemical results reported by other investigators.     

玉米幼苗地上部/根间氮的循环及其基因型差异

以两个玉米（ZeamaysL.）自交系原引1号（YY1）和综31（Z31）为研究材料,采用盆栽土培的培养方法,在正常供氮（HN,0.15gN/kg干土）和低氮量供应（LN,0.038gN/kg干土）培养条件下对玉米幼苗植株体内氮的循环量及其在地上部/根间的分配量进行了定量地测定、计算。结果表明,在玉米幼苗地上部/根间氮的循环量很高。低氮量供应使玉米幼苗植株吸氮量下降,根中氮的分配比例增加,同时地上部/根间氮的循环量也随之减少。与氮低效自交系Z31相比,氮高效自交系YY1幼苗中地上部/根间的氮循环量大、氮向根的分配量高,因而有利于其根系的生长,表现为根/地上部之比和总根长较高。这可能有利于其中后期对氮素的高效吸收与利用。     

Root hair phenotypes influence nitrogen acquisition in maize

Background and AimsThe utility of root hairs for nitrogen (N) acquisition is poorly understood.MethodsWe explored the utility of root hairs for N acquisition in the functional–structural model SimRoot and with maize genotypes with variable root hair length (RHL) in greenhouse and field environments.Key ResultsSimulation results indicate that long, dense root hairs can improve N acquisition under varying N availability. In the greenhouse, ammonium availability had no effect on RHL and low nitrate availability increased RHL, while in the field low N reduced RHL. Longer RHL was associated with 216 % increase in biomass and 237 % increase in plant N content under low-N conditions in the greenhouse and a 250 % increase in biomass and 200 % increase in plant N content in the field compared with short-RHL phenotypes. In a low-N field environment, genotypes with long RHL had 267 % greater yield than those with short RHL. We speculate that long root hairs improve N capture by increased root surface area and expanded soil exploration beyond the N depletion zone surrounding the root surface.ConclusionsWe conclude that root hairs play an important role in N acquisition. We suggest that root hairs merit consideration as a breeding target for improved N acquisition in maize and other crops.     

Starch Mobilization in Ultradried Seed of Maize (Zea mays L.) During Germination

The effects of ultradry storage on the starch mobilization in maize (Zea mays L.) seed after aging were investigated. The results indicated that there were no significant differences in the content of ATP,starch, and soluble sugar, as well as the activity of amylase, between ultradried seeds and seeds stored at -20 ℃ during germination. These results were consistent with the higher level of vigor of the ultradried seed. Sieve tube introduction of a fluorescence dye (carboxyl fluoresceindiacetate) and laser confocal microscopy were used to study the development of plasmodesmata in the ultradried seeds. The results indicated that plasmodesmata developed well in ultradried seeds. Fluorescence analysis also showed that the fluorescence intensity in the radicle of ultradried seeds was stronger than that in seeds with a higher moisture content. This suggests that ultradry treatment has no adverse effects on the seeds. After seed imbibition, cell orgaelles could be resumed. It is concluded that ultradry seed storage is beneficial for maintaining seed vigor and that starchy mobilization proceeds regularly during germination.     

Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.     

Aluminum-Induced Changes in Cell-Wall Glycoproteins in the Root Tips of Al-Tolerant and Al-Sensitive Wheat Lines

To investigate wheat (Triticum aestivumL.) responses to Al stress, KCl- and SDS-extracted glycoproteins (covalently bound proteins isolated by cell-wall digestion by cellulysine–pectolase mixture) and extensins (hydroxyproline-containing glycoproteins, HRGPs) were isolated from cell-wall preparations purified from the root apices of Al-sensitive and Al-tolerant near-isogenic lines ES8 and ET8. Under Al stress conditions, two lines differed mostly in their extensins. The untreated plants of two lines were low in covalently bound extensins, although the content of this protein fraction in ES8 was higher than in ET8. When the seedlings were treated with Al, the extensin content increased in both wheat lines and especially in the Al-tolerant ET8 plants. Using two-dimensional electrophoresis, the authors demonstrated the accumulation of polypeptides with mol wts of 22.2 kD (pI 5.5–6.5), 24.5 kD (pI 5.8–6.0), and 33.1 kD (pI 5.25) and polypeptides of 22.2 kD (pI 6.8–7.6) and 40.5 kD (pI 7.6) in the extensin fraction from the cell walls of the Al-sensitive plants. The regulation of cell responses to Al stress may involve extensin expression.     

Efficiency of the first steps of sulfate utilization by maize hybrids in relation to their productivity

The genetic variability of the efficiency of the first steps of sulfate utilization and its correlation with productivity were evaluated in nine maize hybrids. ³⁵SO₄²⁻ uptake by excised roots, uptake by intact plant roots, translocation to leaves, and ATP sulfurylase in leaves were taken into account. Uptake rate by roots of intact plants did not show any pulse within 7 to 12 days from emergence, in contrast with the previously observed behaviour of excised roots during root elongation. The uptake rate of intact plants was positively correlated with that of excised roots, but the variability within the nine genotypes tested was less. Productivity was positively correlated with sulfate uptake by both intact plant and excised roots, the level of significance being higher in the first case. Translocation to leaves and ATP sulfurylase activity were not correlated to productivity. Therefore, in the case of sulfate, the grain yield of commonly cultivated maize hybrids appeared to be controlled more by the root uptake step than by the activation and translocation steps.     

Starch Mobilization in Ultradried Seed of Maize （Zea mays L.） During Germination

The effects of ultradry storage on the starch mobilization in maize (Zea mays L.) seed after aging were investigated. The results indicated that there were no significant differences in the content of ATP, starch, and soluble sugar, as well as the activity of amylase, between ultradried seeds and seeds stored at -20℃ during germination. These results were consistent with the higher level of vigor of the ultradried seed. Sieve tube introduction of a fluorescence dye (carboxyl fluoresceindiacetate) and laser confocal microscopy were used to study the development of plasmodesmata in the ultradried seeds. The results indicated that plasmodesmata developed well in ultradried seeds. Fluorescence analysis also showed that the fluorescence intensity in the radicle of ultradried seeds was stronger than that in seeds with a higher moisture content. This suggests that ultradry treatment has no adverse effects on the seeds. After seed imbibition, cell orgaelles could be resumed. It is concluded that ultradry seed storage is beneficial for maintaining seed vigor and that starchy mobilization proceeds regularly during germination.     

Root hair formation and elongation of primary maize roots

In maize ( Zea mays L. cv. LG 11) roots cultured in humid air, the presence of hairs was not related to root growth. However, maximum hair length and length of the hair zone could be correlated to the elongation rate of the primary root. Under the growth conditions used, the emergence of root hairs always took place in the extending zone. In more basal regions, rhizodermal cells could not give rise to root hairs. Results were similar for roots preincubated in a buffer solution.     

玉米离体根尖的多层滤纸床液体静止培养方法

设计建立了适于玉米根尖离体培养的多层滤纸床液体静止培养方法,培养的适宜体系为：1／4MS大量元素改良＋1／2MS微量元素＋IBA0.1-0.3mg/L,黑暗培养。该方法避免了传统液体培养通气状况不良的问题,玉米根的生长速度可达到1－2cm/d,分支和生长正常。该方法在控制条件下快速繁殖根系,成本低廉,简便易行,是根系发育和生理研究的理想实验体系。     

玉米胚发育过程中热激蛋白的合成

35S-Met标记玉米胚蛋白合成结果表明,热激处理(42℃)与对照(25℃)的蛋白合成趋势相近,热激抑制16 DAP的蛋白合成,增加22和34 DAP蛋白合成.SDS-PAGE自显影图谱表明,热激诱导16DAP的胚合成86.4、80.0、73.2 kD等3种分子量较高的热激蛋白,22DAP后热激诱导合成86.4、80.0、73.2、24.4、18.2、16.8和13.6 kD等7种分子量的热激蛋白.2D-PAGE自显影图谱进一步显示,热激诱导22和28 DAP的胚合成近20种热激蛋白,其中超过10种为小分子热激蛋白.特异热激蛋白BiP(HsP70)、PDI(HsP60)Western blot表明,这2种热激蛋白在玉米胚发育过程均有高水平的表达,热激对其合成影响不明显.     

Nickel Toxicity and Distribution in Maize Roots

A new histochemical method for Ni determination has been developed and employed to study the pattern of Ni distribution in plant tissues. Two-day-old seedlings of maize (Zea mays L.) were transferred onto 15, 20, 25, and 35 M Ni(NO₃)₂ solutions in the presence of 3 mM Ca(NO₃)₂, and Ni localization in shoot and root tissues was investigated at days 2 and 7 of the incubation. Following two days of incubation, Ni was found in all root tissues, and its content increased with the period of exposure and from the tip to the root base. Independent of root region and tissue, Ni content in the protoplasts exceeded that in the cell walls. Ni penetrated the endodermal barrier and accumulated in the endodermis and pericycle to the highest concentration. Ni accumulation in the pericycle restricted root branching. Ni did not affect the final cell length, and the inhibition of root growth resulted from suppressed cell division. In the shoots, Ni content was below the level discerned by the dimethylglyoximine method; we therefore conclude that maize belongs to excluder plants, with their root systems functioning as a barrier limiting heavy metal intake by aboveground organs. The pattern of Ni transport differs from that of Cd and Pb; this difference stands for specific toxic effects of Ni, including an arrest of root branching.     

Glycolytic Flux and Hexokinase Activities in Anoxic Maize Root Tips Acclimated by Hypoxic Pretreatment

In vitro cyclic electron transport around PSI was studied in thylakoids isolated from barley (Hordeum vulgare L.). Redox poising was obtained by using anaerobic conditions, preillumination, and the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Postillumination rates of P700+ re-reduction of 1 to 5 electrons s-1 were observed, depending on the conditions. The thylakoids supported two parallel paths of cyclic electron transport that were distinguishable by differences in antimycin sensitivity, saturation characteristics, and substrate specificity. The pathway most sensitive to antimycin was not saturated at ferredoxin concentrations up to 50 [mu]M, whereas the more insensitive pathway was saturated at 5 [mu]M ferredoxin. At the lower concentration of reduced ferredoxin, the antimycin-sensitive rate of P700+ re-reduction was lower than the antimycin-insensitive rate. The lower range of reduced ferredoxin concentrations are closer to in vivo conditions. Flavodoxin is shown to mediate cyclic electron transport. Flavodoxin was less efficient in mediating the antimycin-sensitive pathway but mediated the antimycin-insensitive pathway as efficiently as ferredoxin. Antibodies raised against ferredoxin:NADP+ oxidoreductase had no effect on either pathway for re-reduction of P700+. However, the ferredoxin: NADP+ oxidoreductase inhibitor 2[prime]-monophosphoadenosine-5[prime]-diphosphoribose was able to inhibit the antimycin-sensitive as well as the antimycin-insensitive pathway.