Effects of Phosphorus Nutrient on the Hydraulic Conductivity of Sorghum (Sorghum vulgare Pers.) Seedling Roots Under Water Deficiency

Hydroponic experiments were conducted in a growth chamber and changes in the hydraulic conductivity of sorghum (Sorghum vulgare Pers.) roots (Lpr) at the three-leaf stage were measured using the pressure chamber method. Water deficiency was imposed with polyethylene glycol (PEG) 6000 and the phosphorus (P) levels were controlled by complete Hoagland solution with and without P nutrient. The objective of this study was to investigate the effect of P nutrition on root Lpr under water deficiency. The results showed that the Lpr in P deficiency treatments decreased markedly, but the Lpr recovered to the same value as that of control when sufficient P was supplied for 4-24 h. Water deficiency decreased Lpr, but the hydraulic conductivity of the roots with sufficient P supply was still higher than that of plants without P supply. When resuming water supply, the Lpr of the water-deficient plants under P supply recovered faster than that of plants without P supply, which indicates that plants with sufficient P nutrient are more drought tolerant and have a greater ability to recover after drought. The treatment of HgCl2 indicated that P nutrient could regulate the Lpr by affecting the activity and the expression levels of aquaporins.     

Effects of Phosphorus Nutrient on the Hydraulic Conductivity of Sorghum （Sorghum vulgare Pers.） Seedling Roots Under Water Deficiency

Hydroponic experiments were conducted in a growth chamber and changes in the hydraulic conductivity of sorghum (Sorghum vulgare Pers.) roots (Lpr) at the three-leaf stage were measured using the pressure chamber method. Water deficiency was imposed with polyethylene glycol (PEG) 6000 and the phosphorus (P) levels were controlled by complete Hoagland solution with and without P nutrient. The objective of this study was to investigate the effect of P nutrition on root Lpr under water deficiency. The results showed that the Lpr in P deficiency treatments decreased markedly, but the Lpr recovered to the same value as that of control when sufficient P was supplied for 4-24 h. Water deficiency decreased Lpr, but the hydraulic conductivity of the roots with sufficient P supply was still higher than that of plants without P supply. When resuming water supply, the Lpr of the water-deficient plants under P supply recovered faster than that of plants without P supply, which indicates that plants with sufficient P nutrient are more drought tolerant and have a greater ability to recover after drought. The treatment of HgCl2 indicated that P nutrient could regulate the Lpr by affecting the activity and the expression levels of aquaporins.     

Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.     

Aerenchyma formation in the wetland plant Juncus effusus is independent of ethylene

Flooded plant roots commonly form aerenchyma, which allows gas diffusion between shoots and roots. The programmed cell death involved in this induced aerenchyma formation is controlled by the plant hormone ethylene, as has been shown for maize (Zea mays). However, the role of ethylene is uncertain in wetland species that form constitutive aerenchyma (also under nonflooded conditions). The aim of this study is to shed light on the involvement of ethylene in constitutive aerenchyma formation in Juncus effusus. Plants of J. effusus and maize were treated with ethylene and inhibitors of ethylene action to determine the consequences for aerenchyma formation. Neither treatment with high ethylene concentrations nor with ethylene inhibitors resulted in changes in root aerenchyma in J. effusus. By contrast, ethylene increased aerenchyma development in maize unless ethylene action inhibitors were applied simultaneously. Similarly, root elongation was insensitive to ethylene treatment in J. effusus, but was affected negatively in maize. The data show that aerenchyma in J. effusus is highly constitutive and, in contrast to the inducible aerenchyma in maize, is not obviously controlled by ethylene.     

Dynamics of Aerenchyma distribution in the cortex of sulfate-deprived adventitious roots of maize

BACKGROUND AND AIMS: Aerenchyma formation in maize adventitious roots is induced in nutrient solution by the deprivation of sulfate (S) under well-oxygenated conditions. The aim of this research was to examine the extent of aerenchyma formation in the cortex of sulfate-deprived adventitious roots along the root axis, in correlation with the presence of reactive oxygen species (ROS), calcium levels and pH of cortex cells and root lignification. METHODS: The morphometry of the second whorl of adventitious (W2) roots, subject to S-deprivation conditions throughout development, was recorded in terms of root length and lateral root length and distribution. W2 roots divided into sectors according to the mean length of lateral roots, and cross-sections of each were examined for aerenchyma. In-situ detection of alterations in ROS presence, calcium levels and pH were performed by means of fluorescence microscopy using H(2)DCF-DA, fluo-3AM and BCECF, respectively. Lignification was detected using the Wiesner test. KEY RESULTS: S-deprivation reduced shoot growth and enhanced root proliferation. Aerenchyma was found in the cortex of 77 % of the root length, particularly in the region of emerging or developing lateral roots. The basal and apical sectors had no aerenchyma and no aerenchyma connection was found with the shoot. S-deprivation resulted in alterations of ROS, calcium levels and pH in aerenchymatous sectors compared with the basal non-aerenchymatous region. Lignified epidermal layers were located at the basal and the proximal sectors. S-deprivation resulted in shorter lateral roots in the upper sectors and in a limited extension of the lignified layers towards the next lateral root carrying sector. CONCLUSIONS: Lateral root proliferation is accompanied by spatially localized induced cell death in the cortex of developing young maize adventitious roots during S-deprivation.     

Hemoglobin and Hypoxic Acclimation in Maize Root Tips

Class 1 hemoglobins (Hbs) have a wide distribution in the plant kingdom and have been demonstrated in root, seed, stem, and leaf tissues. They are present at low concentrations in aerobic tissue, but their synthesis is rapidly induced by hypoxic stress. The pattern of expression of the maize Hb gene in roots of young maize plants exposed to hypoxia has been examined. Root Hb gene expression increased rapidly to a maximum within first two hours of hypoxia, then declining to prehypoxia levels within 48-h hypoxic exposure. Limiting oxygen supply to the roots by total plant immersion and darkness did not alter the time course of hemoglobin expression. Hb gene expression was about 20-fold higher in the stele than in the cortex of control, aerobically grown roots. Stele Hb expression increased about fourfold under hypoxic conditions, whereas its expression in the cortex increased about 60-fold. In these samples, alcohol dehydrogenase (Adh) gene expression increased about four- and ten fold in the stele and cortex, respectively. The effect of the state of the Hb on anoxic survival of maize root tips was assessed by exposing root tips to a carbon monoxide atmosphere to maximize the proportion of hemoglobin in the carbonmonoxy form. Carbon monoxide had no significant effect on the survival or the ATP levels in anoxic maize roots, regardless of whether they had been acclimated by exposure to a hypoxic pretreatment. This would suggest that the presence of oxyhemoglobin is not essential for the survival of anoxic root tips.     

Changes in Root Hydraulic Conductivity During Wheat Evolution

A better understanding of the mechanisms of water uptake by plant roots should be vital for improving drought resistance and water use efficiency (WUE). In the present study, we have demonstrated correlations between root system hydraulic conductivity and root characteristics during evolution using six wheat evolution genotypes (solution culture) with different ploidy chromosome sets (Triticum boeoticum Bioss., T. monococcum L.: 2n=2x=14;T. dicoccides Koern., T. dicoccon (Schrank) Schuebl.:2n=4x=28;T. vulgare Vill., T. aestivum L. cv. Xiaoyan No. 6:2n=6x=42). The experimental results showed that significant correlations were found between root system hydraulic conductivity and root characteristics of the materials with the increase in ploidy chromosomes (2x→6x) during wheat evolution. Hydraulic conductivity of the wheat root system at the whole-plant level was increased with chromosome ploidy during evolution, which was positively correlated with hydraulic conductivity of single roots, whole plant biomass,root average diameter, and root growth (length, area), whereas the root/shoot ratio had an inverse correlation with the hydraulic conductivity of root system with increasing chromosome ploidy during wheat evolution. Therefore, it is concluded that that the water uptake ability of wheat roots was strengthened from wild to modern cultivated species during evolution, which will provide scientific evidence for genetic breeding to improve the WUE of wheat by genetic engineering.     

Changes in Root Hydraulic Conductivity During Wheat Evolution

A better understanding of the mechanisms of water uptake by plant roots should be vital for improving drought resistance and water use efficiency (WUE). In the present study, we have demonstrated correlations between root system hydraulic conductivity and root characteristics during evolution using six wheat evolution genotypes (solution culture) with different ploidy chromosome sets (Triticum boeoticum Bioss., T. monococcum L.: 2n = 2x = 14; T. dicoccides Koern., T. dicoccon (Schrank) Schuebl.: 2n = 4x = 28;T. vulgare Vill., T. aestivum L. cv. Xiaoyan No. 6: 2n = 6x = 42). The experimental results showed that significant correlations were found between root system hydraulic conductivity and root characteristics of the materials with the increase in ploidy chromosomes (2x→6x) during wheat evolution. Hydraulic conductivity of the wheat root system at the whole-plant level was increased with chromosome ploidy during evolution, which was positively correlated with hydraulic conductivity of single roots, whole plant biomass,root average diameter, and root growth (length, area), whereas the root/shoot ratio had an inverse correlation with the hydraulic conductivity of root system with increasing chromosome ploidy during wheat evolution. Therefore, it is concluded that that the water uptake ability of wheat roots was strengthened from wild to modern cultivated species during evolution, which will provide scientific evidence for genetic breeding to improve the WUE of wheat by genetic engineering.     

缺磷对黑麦草-内生真菌共生体生长和根中酚含量的影响

培养液中缺磷或内生真菌感染对黑麦草地上部生长的影响不显著,但二者的交互作用却有着显著影响。随着磷浓度的下降,感染内生真菌的黑麦草显示出较强的生长优势,其根长和根中酚类物质的含量显著增加。     

植物根系形态对低磷胁迫应答的研究进展

本文综述了近年来植物对磷营养高效吸收有关的根系形态方面的研究进展,总结了植物适应低磷胁迫的根系形态特征,以及植物适应低磷胁迫根系形态变化的激素调控的内在机制,着重阐述了植物适应低磷根系形态变化的分子生物学基础,并对开展此类工作的有效途径进行了探讨.     

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.     

Production of Polyamines Is Enhanced by Endogenous Abscisic Acid in Maize Seedlings Subjected to Salt Stress

It is known that salt stress and exogenously applied abscisic acid （ABA） can enhance the polyamine content in plants and that salt stress itself can lead to an increase in endogenous ABA production. In the present study, the relationships between salt-induced ABA and polyamine accumulation were inves- tigated using ABA-deficient mutant （vp5/vp5） maize （Zea mays L.） seedlings and ABA and polyamine biosynthesis inhibitors. The results show that reduced endogenous ABA levels, as a result of either the mutation or by using a chemical inhibitor （sodium tungstate）, also reduced the accumulation of polyamines in salt-stressed leaves of maize seedlings. The polyamine synthesis inhibitors D-arginine and α- difluoromethylornithine also reduced the polyamine content of the leaves of maize seedling under salt stress. Both ABA and polyamine enhanced the dry weight accumulation of salt-stressed seedlings and also increased the activities of the two dominant tonoplast membrane enzymes, H^＋-ATPase and H^＋-PPase, when plants were under salt stress. The results suggest that salt stress induces an increase in endogenous ABA levels, which then enhances polyamine synthesis. Such responses may increase a plant＇s tolerance to salt.     

玉米根系水流导度差异的生理形态原因分析

在人工气候室水培条件下,研究了水分胁迫对不同基因型玉米杂交种及其亲本根系水流导度（Lpr）变化的影响,并从生理和形态角度对其差异进行了分析。结果表明：表型抗旱的杂交种F1代（户单四号）整株根系水流导度最高,具有根系水流导度上的杂种优势现象。对其差异的生理和形态因素分析表明,F1代水流导度高与其高脯氨酸含量、低MDA含量和低质膜透忡有关。同时表明,根系的形态特征对根系的水流导度也存在一定的影响。     

桉树幼苗对难溶性磷的吸收及其根系对低磷胁迫的响应

以‘广林9号’桉树幼苗为试验材料,采用水培和土培试验方法研究了桉树幼苗对难溶性磷酸盐的吸收及其在低磷胁迫下的根构型和根系的生理反应,以揭示桉树高效吸收磷素的机制。结果显示:(1)桉树幼苗在含磷酸铝的缺磷培养液中吸收的磷达4.24mg/株,与供应水溶性磷和磷酸钙处理的相当。(2)土壤缺磷或仅在上土层(0~20cm)施磷肥处理均有利于桉树幼苗浅层根的分布,使根表面积及根数在上土层与下层(20~40cm)比值明显增高。(3)桉树幼苗根尖的H+-ATPase活性在缺磷处理15d后显著提高,其根尖周围的溴甲酚紫指示剂变黄,根基环境明显酸化;根尖分泌的酸性磷酸酶活性在低磷胁迫也显著提升,且随着处理时间(10、15、20d)的延长而进一步提高;铝和低磷胁迫能明显诱导桉树根系分泌草酸,其分泌量显著高于对照和缺磷处理。研究结果表明,桉树幼苗具有较强的难溶性磷吸收能力,而在缺磷及磷铝胁迫下根系的浅层化、根尖酸化及根分泌的酸性磷酸酶及草酸量增加可能是桉树幼苗适应酸性土壤铝毒和缺磷环境的重要机制。     

菜豆根构型对低磷胁迫的适应性变化及基因型差异

利用特殊设计的营养袋纸培养和分层式磷控释砂培等根系生长系统结合计算机图像分析技术,以基根根长在生长介质各层的相对分布和基根平均生长角度为指标,定量测定菜豆（Ｐｈａｓｅｏｌｕｓ ｖｕｌｇａｒｉｓ Ｌ．）根构型在低磷胁迫下的适应性变化及其与磷效率的关系。结果表明,菜豆根构型对低磷胁迫具有适应性反应,在缺磷条件下基根向地性减弱,基根在生长介质表层相对分布增多、基根平均生长角度（与水平线夹角）变小,从而导     

Adaptive Changes and Genotypic Variation for Root Architecture of Common Bean in Response to Phosphorus Deficiency

Root architectural responses to phosphorus (p) availability may be an important trait for P acquisition efficiency. In the present study, The authors examined the effects of P availability on root architectural responses of different common bean genotypes. Five common bean (Phaseolus vulgaris L.) genotypes representing different origins and ecotypic races were compared both in a specially designed paper pouch system and a stratified P buffer sand culture system with computer image analysis. The results showed that root architecture was regulated by P availability. P deficiency led to form a shallower root system, as indicated by increased relative distribution of basal root length in the upper layers and decreased the growth angle of basal roots. There was significant genetic variation in root architecture in response to P deficiency both in the paper pouch system and the stratified sand culture system. Under low P conditions some genotypes were more gravitropically sensitive to low P availability, resulting in producing a shallower root system and enhanced root exploration into the surface soil, where soil available P is more concentrated. G19833 and DOR364, which were most contrasting in P efficiency, were also very different in root architectural response to P availability. The results from this study suggest that P availability regulates root architecture and P deficiency leads to shallower root architecture in beans. The genetic potential of root architecture provides the possibility of selecting this trait for improving P acquisition efficiency in common bean.     

玉米CMS分子生物学研究进展

本文对玉米CMS研究已获得的、并为普遍接受的分子生物学研究结果进行了粗略总结;对近年来在玉米细胞质雄性不育育性相关核基因的分子标记定位、克隆及辅助选择,育性相关胞质基因的克隆与表达方面的研究进展进行了简要概述;我们认为在今后一段时期,玉米CMS研究将着重围绕核不育基因的克隆及表达模式、线粒体功能基因组、育性相关胞质基因与核育性基因相互作用等方向进行研究,以期阐述玉米CMS的形成机理。 Progress of Molecular Biology of CMS in Maize ZHANG Zu-xin,ZHANG Fang-dong,ZHENG Yong-lian National Key Laboratory of Crop Genetic Improvement,Huazhong Agricultural University,Wuhan 430070,China Abstract:In the paper,we have summarized the molecular biological accomplishment acquired and accepted by most of maize researchers on CMS of maize.A brief review of current molecular biological progress of CMS of maize are displayed in the paper.These progresses concern in the positioning,cloning and maker-assisted selection of nucleic genes associated with fertility,expression and cloning of cytoplasmic genes associated with male sterility,In order to elucidate the molecular mechanism of CMS of maize,the areas about cloning and expression profiling of male sterile nucleic genes,and functional genomics of mitochondria,and interaction cytoplasmic genes with nucleic genes will need to be researched in the future. Key words：maize(Zea mays L.);CMS;mtDNA;gene associated with fertility     

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.     

Genetic Analysis of Maize Root Characteristics in Response to Low Nitrogen Stress

Under low-input cropping systems, nitrogen (N) can be a limiting factor in plant growth and yield. Identifying genotypes that are more efficient at capturing limited N resources and the traits and mechanisms responsible for this ability is important. Root trait has a substantial influence on N acquisition from soils. Nevertheless, inconsistencies still exist as to the effect of low N on root length and its architecture in terms of lateral and axial roots. For maize, a crop utilizing heterosis, little is known about the relationship between parents and their crosses in the response of root architecture to N availability. Here 7 inbred maize lines and 21 of their crosses created by diallel mating were used to study the effect of N stress on root morphology as well as the relationship between the inbreds and their crosses. With large genotypic differences, low N generally suppresses shoot growth and increases the root to shoot ratio with or without increasing root biomass in maize. Maize plants responded to N deficiency by increasing total root length and altering root architecture by increasing the elongation of individual axial roots and enhancing lateral root growth, but with a reduction in the number of axial roots. Here, the inbreds showed weaker responses in root biomass and other root parameters than their crosses. Heterosis of root traits was significant at both N levels and was attributed to both the general combining ability (GCA) and special combining ability (SCA). Low N had substantial affects on the pattern of heterosis, GCA and SCA affects on root traits for each of the crosses suggesting that selection under N stress is necessary in generating low N-tolerant maize genotypes.