Root growth analysis: An underutilised approach to understanding aluminium rhizotoxicity

Despite numerous published studies, the mechanistic bases for Al rhizotoxicity and for intraspecific differences in Al tolerance remain elusive. Classical methods of growth analysis offer a means to describe quantitatively the onset of root growth inhibition in response to an Al challenge, and any subsequent recovery in growth rates (i.e. acclimation). Such information could help elucidate tolerance mechanisms (i.e. constitutive vs. inducible). Despite this potential utility, however, published data of high accuracy and precision are few, especially with wheat (Triticum aestivum L.). In this paper, a conceptual framework for analysing root growth responses to Al is presented. Specially designed tanks and a computerised video analysis system were used to monitor root elongation in a simple medium (1 mM CaCl₂, pH 4.3) containing varied Al concentrations. Root lengths of five wheat cultivars were measured every 2 to 5 h for 48 h after Al challenge. Growth rates typically exhibited a brief (1–2 h) lag phase prior to a sharp decline, followed by a recovery phase of 12 h duration. At low levels of Al (i.e. sufficient to cause a 10% reduction in net elongation) post-recovery growth rates generally equalled those of control roots, but were significantly reduced at Al concentrations yielding a 40 to 50% reduction in net elongation. All cultivars exhibited similar inhibition-acclimation response patterns and these responses did not explain differences in sensitivity to Al. Nonetheless, solution Al concentrations required to elicit the same growth responses varied by 20-fold across the five cultivars, consistent with the notion that root exclusion of Al is a significant component of differential Al tolerance in wheat.     

Role of phytohormones in insect-specific plant reactions

The capacity to perceive and respond is integral to biological immune systems, but to what extent can plants specifically recognize and respond to insects? Recent findings suggest that plants possess surveillance systems that are able to detect general patterns of cellular damage as well as highly specific herbivore-associated cues. The jasmonate (JA) pathway has emerged as the major signaling cassette that integrates information perceived at the plant-insect interface into broad-spectrum defense responses. Specificity can be achieved via JA-independent processes and spatio-temporal changes of JA-modulating hormones, including ethylene (ET), salicylic acid (SA), abscisic acid (ABA), auxin, cytokinins (CK), brassinosteroids (BR) and gibberellins (GB). The identification of receptors and ligands and an integrative view of hormone-mediated response systems are crucial to understand specificity in plant immunity to herbivores.     

Reconsidering the rhizotoxicity of hydroxyl, sulphate, and fluoride complexes of aluminium

Recent years have witnessed some convergence of opinion regardingthe identity of the rhizotoxic species of aluminium.     

Role of phytohormones in organogenic ability of elm multiplicated shoots

The study presents the comparative analyses of endogenous contents of auxin (IAA), cytokinins (CKs), polyamines (PAs), and phenolic acids (PhAs) in apical and basal parts of elm multiplicated shoots with regard to the organogenic potential. The shoot-forming capacity was higher in the apical part than in the basal part. However, the timing of root formation was in the apical type of explant significantly delayed (compared with the organogenic potential of basal part). Significantly higher contents of free bases, ribosides and ribotides of isopentenyl adenine, zeatin and dihydrozeatin that were found in the apical segments, might be considered as the most important factor affecting in vitro shoot formation. The content of endogenous free IAA was approximately three times higher in the basal shoot parts than in the apical parts. The amounts of putrescine and spermidine were higher in the apical part which generally contains less differentiated tissues than the basal part of shoot. The predominant PhA in both types of explants was caffeic acid, and concentrations of other PhAs decreased in the following order: p-coumaric, ferulic, sinapic, vanillic, chlorogenic, p-hydroxybenzoic and gallic acids. The contents of all determined PhAs in their free forms and higher contents of glycoside-bound p-coumaric, ferulic and sinapic acids, precursors for lignin biosynthesis, were found in the basal parts.     

Biosynthesis of phytohormones in algae

Like in the case of higher plants, algal growth and development are controlled by the hormonal regulatory system. Essentially all known phytohormones were identified in various algal taxa, and the range of their physiological activities was confirmed. At the same time, our knowledge of enzymes involved in the phytohormone synthesis in algae is rather limited. Data concerning genes encoding these enzymes are still more fragmentary. Current data about proteomes of some algae allow the revealing of amino acid sequences with homology to those of the higher plant enzymes and their conserved domains.     

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.     

Al3+-Ca2+ interactions in aluminum rhizotoxicity

Several mineral rhizotoxicities, including those induced by Al³⁺, H⁺, and Na⁺, can be relieved by elevated Ca²⁺ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca²⁺ from transport channels or surface ligands that must be occupied by Ca²⁺ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al³⁺, that (i) Ca²⁺, Mg²⁺, and Sr²⁺ are equally ameliorative, (ii) that root elongation does not increase as Ca²⁺ replaces Mg²⁺ or Sr²⁺ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al³⁺ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca²⁺ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC³⁺), in contrast to Al³⁺, was specifically relieved by Ca²⁺ at the membrane surface. The rhizotoxicity induced by H⁺ exhibited a weak specific response to Ca²⁺ at the membrane surface. We conclude that the Ca²⁺-displacement hypothesis fails in the case of Al³⁺ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al³⁺. However, TEC³⁺, but not Al³⁺, may be toxic because it inhibits Ca²⁺ uptake. The nature of the specific H⁺-Ca²⁺ interaction is uncertain.Abbreviations {Al³⁺ }₀ chemical activity of Al³⁺ at the root-cell membrane surface - {Al³⁺ }_E chemical activity of Al³⁺ in the external rooting medium - E₀ electrical potential at the root-cell membrane surface - HXM²⁺ hexamethonium ion - TEC³⁺ tris(ethylenediamine)cobaltic ion     

Novel phytohormones involved in long-range signaling

Communication between distant organs is an essential feature of multicellular organisms. Plants are no exception to this rule and long-range signals are involved in the regulation of many aspects of organ growth and development. In this review, we use two specific examples to illustrate this point. The first is a novel upwardly mobile hormone involved in the regulation of shoot branching. The second is a root-derived hormonal signal that regulates leaf development.     

The role of phytohormones in cotton fiber development

Since 1974, when Beasley and Ting discovered that fertilized ovules of cotton can be cultured in media supplemented with GA along with auxin, the effect of all types of phytohormones on fiber development has been widely studied. Many phytohormones, including GA, IAA, brassinosteroid (Br), ABA, ethylene (Et), and cytokinins (Ck), all have been demonstrated to play important roles during cotton fiber development. In recent years, the rapid development of genomic analysis and the accumulation of high-quality cotton ESTs allowed us to probe phytohormonal gene expression during fiber development. Many phytohormonal genes, including GA-, IAA-, ABA-, Br-, Et-, and Ck-related genes, participating in phytohormone biosynthesis pathways and signal transduction pathway accumulated in the process of cotton fiber development.     

Narciclasine与植物激素的相互作用研究

主要研究了天然生物活性物质Narciclasine(NCS)与植物激素的相互作用。发现NCS对于IAA刺激小麦胚芽鞘伸长过程、GA3对大麦胚乳中α-淀粉两诱导作用及6-BA对离体萝卜子叶光下增大以及转绿过程的促进作用,均显示出一定的抑制作用,而且其抑制强度均随浓度的增加而增加。对于不同的生理过程,NCS表现出不同的抑制强度,其中对离体萝卜子叶光下转绿过程的抑制作用较强。对于离体子叶的光下增大和转绿过程,不同植物激素的表现不同,6-BA和GA3对离体子叶增大及光下转绿具有促进作用;ABA和NCS则显示出一定的抑制作用。     

植物激素调控丛枝菌根发育的作用机制研究进展

丛枝菌根（arbuscular mycorrhiza,AM）是土壤中AM真菌和绝大多数维管植物根系长期进化过程中相互识别、相互作用形成的互利共生体。AM的发育与功能效应依赖AM真菌-寄主植物之间精准的“分子对话”,同时受到环境条件特别是土壤养分水平、干旱和盐渍化的制约。植物激素作为低浓度的小分子有机物,是参与调控AM共生过程的重要信号分子。其中,主要有9种植物激素参与AM发育过程且分工各有不同：独脚金内酯（strigolactones,SLs）参与AM真菌-寄主植物之间最初的共生识别,脱落酸（abscisic acid,ABA）和油菜素内酯（brassinosteroid,BR）促进前期的菌丝入侵,但水杨酸（salicylic acid,SA）和乙烯（ethylene,ET）抑制前期的菌丝入侵,生长素（auxin,Aux）、ABA和BR促进随后的丛枝形成而ET和赤霉素（gibberellin,GA）的作用则相反,茉莉酸（jasmonic acid,JA）对菌丝入侵与丛枝形成均可能存在正调控或负调控作用。目前细胞分裂素（cytokinin,CTK）在AM发育中的作用尚不明确。更为复杂的是,通常植物激素信号之间的交叉互作决定AM的发育进程。本文针对AM发育过程总结了不同植物激素的调控作用特点和不同植物激素信号之间的互作（协同或拮抗）,以及胁迫条件下不同植物激素信号的可能调控机制。深入研究和系统阐明植物激素调控AM真菌-寄主植物共生的生理/分子机制,将有助于促进生物共生学理论研究及菌根技术的应用。     

The role of phytohormones in tumor formation in radish

Tumor formation was studied in inbred radish lines that produce tumors on plant roots during flowering. In all radish lines under consideration, the sequences homologous to oncogenes tmr/tml of Agrobacterium tumefaciens were revealed by Southern hybridization. No sequences homologous to the tms locus of A. tumefaciens and the oncogenes of A. rhizogenes were determined. It was found that auxin sensitivity and the tumor-producing capacity were coinherited. We suggest that tumor phenotype arise as a result of a combination between agrobacterial "cytokinin" oncogenes and certain alleles of "auxin" radish genes.     

The involvement of phytohormones in the plant sex regulation

The data concerning the plant sex regulation by phytohormones are presented. Functioning of signaling pathways regulating floral development and sex expression, including those with phytohormone involvement, are considered. The role of phytohormones in the functioning of systems and mechanisms of sex regulation is analyzed. The examples of sex reversion by plant treatment with phytohormones are presented. It is demonstrated that many genes determining sex encode proteins involved in the phytohormone metabolism. The significance of phytohormone investigation for the understanding of molecular mechanisms of plant sex regulation is discussed.     

植物激素在植物细胞壁扩展中的作用

细胞壁不仅是植物细胞结构的重要组成部分,而且控制着细胞的大小、形状和生长。细胞经有丝分裂后,原生质体吸水膨胀,细胞壁重塑,新生壁物质合成,纤维素定向沉积等引发细胞壁生长。在这些过程中,乙烯（ethylene,ET）、生长素（auxin）、赤霉素（gibberellin,GA）、油菜素甾醇（brassinosteroids,BR）等植物激素调控细胞壁生长相关酶类如纤维素合酶复合体（cellulose synthase A,CESA）、扩展素（expansin,EXP）、木葡聚糖内糖基转移酶／水解酶（xyloglucan endotran glucosylase／hydrolase,XET／XTH）的表达活性,进而调控细胞壁扩展,促使细胞壁的生长。     

Modern aspects of studying phytohormones. Cytokinins

The review presents current data on mechanisms of cytokinin action in plants. By analogy with the first part (Ivanova et al., 1999), in which general principles of phytohormone action and cardinal trends of phytohormone investigations were examined, here the relevant information on mechanisms of action of auxins and gibberellins has been given, and taking cytokines as example an attempt has been done to summarize the literature data on the number of questions offered for analysing hormones of high animals (Gudwin, Merser, 1986). The review demonstrates that mechanisms of cytokine action at the cellular level are not known in many cases. One of the most significant factors in the action of phytohormones of this class on plants is their concentration, determined by their synthesis, transportation and further chemical conversions. This paper points to a poor knowledge of the relative role of these processes in regulation of cytokinin contents and their distribution among plant organs. Two possible ways of studying cytokinin action at the present day stage of investigations have been designated: 1) revealing the cytokinin expressed genes and establishing mechanisms of their action; 2) estimation of endogenous cytokinin alteration and the influence of this alteration on definite processes in the cell with the help of ipt-gene from t-DNA of Agrobacterium tumefaciens.