Indole-3-acetic acid in microbial and microorganism-plant signaling

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.     

拟南芥 MeIAA 抗性突变体的筛选和初步图位克隆分析

生长素是最重要的植物激素之一, 参与了植物生长发育的各个方面。植物体内游离的IAA是生长素的主要活性形式, 在IAA甲基转移酶1(IAMT1)的作用下, IAA可以转变为IAA甲酯 (MeIAA)。MeIAA本身没有活性, 在植物体内的MeIAA酯解酶作用下可以重新转变为IAA。 MeIAA是非极性分子, 能够在植物体内自由扩散。利用MeIAA的这种特殊性质筛选突变体, 可以分离到MeIAA代谢途径或者IAA途径中新的成分。我们对拟南芥种子进行EMS诱变, 通过观察黑暗下下胚轴的生长情况, 筛选MeIAA的抗性突变体。我们成功分离到了8株可能的抗性突变体, 并对其中的一个Methyl -IAA resistant 1 (mir1) 突变体进行了深入分析。MeIAA抗性突变体的筛选将为进一步了解MeIAA的代谢、IAA稳态调控和响应机理提供新的材料。     

拟南芥MeIAA抗性突变体的筛选和初步图位克隆分析

生长素是最重要的植物激素之一,参与了植物生长发育的各个方面。植物体内游离的IAA是生长素的主要活性形式,在IAA甲基转移酶1（IAMT1）的作用下,IAA可以转变为IAA甲酯（MelAA）。MelAA本身没有活性,在植物体内的MelAA酯解酶作用下可以重新转变为IAA。MelAA是非极性分子,能够在植物体内自由扩散。利用MelAA的这种特殊性质筛选突变体,可以分离到MelAA代谢途径或者IAA途径中新的成分。我们对拟南芥种子进行EMS诱变,通过观察黑暗下下胚轴的生长情况,筛选MelAA的抗性突变体。我们成功分离到了8株可能的抗性突变体,并对其中的一个Methyl-JAAresistant1（mir1）突变体进行了深入分析。MelAA抗性突变体的筛选将为进一步了解MelAA的代谢、IAA稳态调控和响应机理提供新的材料。     

Competitive antagonism between IAA and indole alkaloid hypaphorine must contribute to regulate ontogenesis

Since 1995 the role of fungal hypaphorine in plants has been widely investigated and its IAA-antagonist activity recognized. Evidence of competitive antagonism includes organ development, gene expression or molecule–molecule interaction levels. Based on present knowledge, three sites of hypaphorine/IAA competition and subsequent signalling pathways have been hypothesized: the extracellular signalling pathway, the intracellular signalling pathway, and the transmembrane signalling pathway. Hypaphorine with other active indole alkaloids should be regarded as a new class of IAA antagonist finely regulating specific steps of plant growth or development.     

The Physiological Function of Melatonin in Plants

Melatonin (N-acetyl-5-methoxytryptamine), a well-known animal hormone, was discovered in plants in 1995 but very little research into it has been carried out since. It is present in different parts of all the plant species studied, including leaves, stems, roots, fruits and seeds. This brief review will attempt to provide an overview of melatonin (its discovery, presence and functions in different organisms, biosynthetic route, etc.) and to compile a practically complete bibliography on this compound in plants. The common biosynthetic pathways shared by the auxin, indole-3-acetic, and melatonin suggest a possible coordinated regulation in plants. More specifically, our knowledge to date of the role of melatonin in the vegetative and reproductive physiology of plants is presented in detail. The most interesting aspects for future physiological studies are presented.Key Words: antioxidant, auxin, flowering, growth, IAA, melatonin, plant hormone, reproductive development, rooting, vegetative developmentMelatonin (N-acetyl-5-methoxytryptamine), an “old friend” and well known as an animal hormone but “new” to plant biology is arousing great interest due to its broad distribution in the biological kingdom and the recent data on its possible physiological role in plants. Many studies on melatonin, as a phytochemical compound with potentially interesting health-related properties, have recently appeared, but no more than 15–20 papers with a plant physiological focus have been published since 1995. Besides mentioning the most interesting data on melatonin related with plants, this review will hopefully trigger more studies into this molecule to deepen our understanding of the different physiological roles that it might play in plants. We shall briefly look at the well-known function of melatonin in vertebrates, its discovery in plants and other organisms, and its presence in plants as a possible medicinal phytochemical. The joint biosynthetic pathways of melatonin and the auxin indole-3-acetic acid (IAA) will be described. Thus, we reveal the new and emerging field of melatonin studies in plants, the limited physiological data available and its possible role in plants.     

Stress Ameliorative Effects of Indole Acetic Acid on <i>Hordeum vulgare</i> L. Seedlings Subjected to Zinc Toxicity

The heavy metals present in the environment accumulate in the plants and affect their productivity and yield. By entering the food chain, metals cause several serious health problems in human beings as well as in other organisms. Indole acetic acid (IAA) is known to act as a signaling molecule between symbiotic association of metal accumulating plants and plant growth promoting rhizobacteria (PGPR). Present study demonstrated a protective role of IAA against surplus Zinc (Zn)-induced toxicity to Hordeum vulgare seedlings. Elevated Zn concentrations suppressed the plant growth, caused a reduction in leaf relative water contents (RWC) and elevated free proline and non-protein thiols (NPT) accumulation. Zinc treatment also led to enhanced lipid peroxidation (MDA contents) as well as the activity of ascorbate peroxidase (APX), showing the involvement of antioxidative defense mechanism to reduce Zn induced toxicity. IAA oxidase activity was also observed to increase due to Zn treatment. IAA pretreatment of H. vulgare caryopsis could partly revert the Zn-induced toxicity in seedlings.     

A Bacterial Indole-3-acetyl-L-aspartic Acid Hydrolase Inhibits Mung Bean (Vigna radiata L.) Seed Germination Through Arginine-rich Intracellular Delivery

Indole-3-acetyl-L-aspartic acid (IAA-Asp) is a natural product in many plant species and plays many important roles in auxin metabolism and plant physiology. IAA-Asp hydrolysis activity is, therefore, believed to affect plant physiology through changes in IAA metabolism in plants. We applied a newly discovered technique, arginine-rich intracellular delivery (AID), to deliver a bacterial IAA-Asp hydrolase into cells of mung bean (Vigna radiata) seeds and measured its effects on mung bean seed germination. IAA-Asp hydrolase inhibited seed germination about 12 h after the enzyme was delivered into cells of mung bean seeds both covalently and noncovalently. Mung bean seed germination was delayed by 36 h when the enzyme protein was noncovalently attached to the AID peptide and longer than 60 h when the enzyme protein was covalently attached to the AID peptide. Root elongation of mung bean plants was inhibited as much as 90% or 80%, respectively, when the IAA-Asp hydrolase was delivered with the AID peptide by covalent or noncovalent association. Further thin-layer chromatography analysis of plant extracts indicated that the levels of IAA increased about 12 h after treatment and reached their peak at 24 h. This result suggests that IAA-Asp hydrolase may increase IAA levels and inhibit seed germination of mung bean plants and that the AID peptide is a new, rapid, and efficient experimental tool to study the in vivo activity of enzymes of interest in plant cells.     

Indole-3-acetic acid is a physiological inhibitor of TORC1 in yeast

Indole-3-acetic acid (IAA) is the most common, naturally occurring phytohormone that regulates cell division, differentiation, and senescence in plants. The capacity to synthesize IAA is also widespread among plant-associated bacterial and fungal species, which may use IAA as an effector molecule to define their relationships with plants or to coordinate their physiological behavior through cell-cell communication. Fungi, including many species that do not entertain a plant-associated life style, are also able to synthesize IAA, but the physiological role of IAA in these fungi has largely remained enigmatic. Interestingly, in this context, growth of the budding yeast Saccharomyces cerevisiae is sensitive to extracellular IAA. Here, we use a combination of various genetic approaches including chemical-genetic profiling, SAturated Transposon Analysis in Yeast (SATAY), and genetic epistasis analyses to identify the mode-of-action by which IAA inhibits growth in yeast. Surprisingly, these analyses pinpointed the target of rapamycin complex 1 (TORC1), a central regulator of eukaryotic cell growth, as the major growth-limiting target of IAA. Our biochemical analyses further demonstrate that IAA inhibits TORC1 both in vivo and in vitro. Intriguingly, we also show that yeast cells are able to synthesize IAA and specifically accumulate IAA upon entry into stationary phase. Our data therefore suggest that IAA contributes to proper entry of yeast cells into a quiescent state by acting as a metabolic inhibitor of TORC1.     

Stimulation of Agrobacterium tumefaciens virulence with indole-3-acetic acid

The phytopathogen Agrobacterium tumefaciens incites the production of crown-gall on a wide range of dicotyledonous plants. Gall formation is dependent upon indole-3-acetic acid (IAA) and cytokinin production by the transformed plant cells. Upon incubation of Agrobacterium tumefaciens C58 with the plant hormone indole-3-acetic acid (IAA), bacterial virulence on cucumber plants was stimulated up to tenfold. Stimulation was maximized after exposure of bacteria to 50 or 100 μg ml^-1 IAA for 3 h. This was shown to be at the early log phase of bacterial growth.
The authors suggest that the excretion of IAA by the transformed plant cells stimulates bacterial virulence mechanism(s) encoded by the Ti plasmid, the chromosome, or both.     

Role of lndole-3-acetic Acid in the Interaction of the Phanerogamic Parasite Cuscuta and Host Plants

Abstract: During infection with the phanerogamic parasite Cuscuta reflexa the incompatible host plant Lycopersicon esculentum shows characteristic anatomical tissue modifications at the infection sites that are exclusively provoked following contact with Cuscuta - striking cell elongation is observed in the hypo-dermis and collenchyma of the host plant. Due to the influence of auxins on the process of cell elongation in plants, the role of indole-3-acetic acid (IAA) in the interaction of Cuscuta and Lycopersicon was studied using a highly specific enzyme-linked im-munosorbent assay for hormone measurement. It was shown that the tissue modifications in the host plant as well as cell elongation in the parasite tissue leading to the formation of an adhesive-secretory epithelium are correlated with increasing IAA levels in the respective tissues. Both anatomical modifications can also be induced artificially by injection of IAA into control tissue. Based on the obtained data, it can be hypothesized that during the parasitic attack IAA is accumulated in the haus-toria-bearing regions of Cuscuta and exuded from the epithelial cells. Due to the close contact between host and parasite, the hormone probably enters the host plant tissue causing the observed anatomical reactions.     

Book of Abstracts

A widespread occurrence of melatonin (MEL) in plant kingdom has been reported. MEL is a highly conserved molecule occurring in evolutionary distant organisms. Its role in plants seems to be similar to that in animals. Although MEL function in plants is not well known, yet a hypothesis can be put forward that it probably functions as a night signal, coordinating responses to diurnal and photoperiodic environmental cues. It has also been suggested that MEL is an independent plant growth regulator, probably its action is analogous to IAA and it may mediate the actions of other plant growth regulators. Due to its antioxidant properties MEL may also stabilize cell red-ox status and protect them against reactive oxygen species (ROS) and other harmful environmental influence.     

The Role of Wheat Germ Agglutinin in Plant–Bacteria Interactions: A Hypothesis and the Evidence in Its Support

Wheat plants are known to develop the associative symbiosis with the rhizobacterium Azospirillum brasilense.We studied the interaction of a lectin, wheat germ agglutinin (WGA), which is also found in wheat roots, with A. brasilense, strain sp245. When added to the azospirillum culture to the final concentration of 10^–8to 10^–9M, WGA enhanced IAA production, dinitrogen fixation, and ammonium excretion by bacterial cells. WGA also promoted the synthesis of proteins, both new and those already present in bacterial cells. The hypothesis that WGA is a signal molecule rerouting the bacterial metabolism in the direction favorable for the growth and development of the host plant has been put forward. It is suggested that signal properties of WGA are the basis for one of the functions of this lectin and essential for the effective associative symbiosis.     

Polar auxin transport: an early invention

In higher plants, cell-to-cell polar auxin transport (PAT) of the phytohormone auxin, indole-3-acetic acid (IAA), generates maxima and minima that direct growth and development. Although IAA is present in all plant phyla, PAT has only been detected in land plants, the earliest being the Bryophytes. Charophyta, a group of freshwater green algae, are among the first multicellular algae with a land plant-like phenotype and are ancestors to land plants. IAA has been detected in members of Charophyta, but its developmental role and the occurrence of PAT are unknown. We show that naphthylphthalamic acid (NPA)-sensitive PAT occurs in internodal cells of Chara corallina. The relatively high velocity (at least 4-5 cm/h) of auxin transport through the giant (3-5 cm) Chara cells does not occur by simple diffusion and is not sensitive to a specific cytoplasmic streaming inhibitor. The results demonstrate that PAT evolved early in multicellular plant life. The giant Chara cells provide a unique new model system to study PAT, as Chara allows the combining of real-time measurements and mathematical modelling with molecular, developmental, cellular, and electrophysiological studies.     

Conjugation of Indole-3-Acetic Acid (IAA) in Wild-Type and IAA-Overprodcing Transgenic Tobacco Plants,and Identification of the Main Conjugates by Frit-Fast Atom Bombardment Liquid Chromatography-Mass Spectrometry

Transgenic plants overproducing indole-3-acetic acid (IAA) from expression of the Agrobacterium tumefaciens T-DNA IAA biosynthesis genes were used to study the conjugation of IAA. At the 11-node stage, free IAA, as well as ester- and amide-conjugated IAA, was analyzed in wild-type tobacco SR1 and in transgenic plants denoted 35S-iaaM/iaaH (line C) and 35S-iaaM x 35S-iaaH (line X). The transgenic plants contained increased levels of both free and conjugated IAA, and the main increase in IAA conjugates occurred in amide conjugates. Two amide conjugates were identified by fritfast atom bombardment liquid chromatography-mass spectrometry as indole-3-acetylaspartic acid (IAAsp) and indole-3-acetylglutamic acid (IAGlu), and one ester conjugate was identified as indole-3-acetylglucose. IAAsp and IAGlu were also identified as endogenous substances in wild-type plants. In wild-type plants, the percent of total IAA in the free form was significantly higher in young leaves (73 [plus or minus] 7%, SD) than in old leaves (36 [plus or minus] 8%), whereas there was no difference between young (73 [plus or minus] 8%) and old internodes (70 [plus or minus] 9%). In IAA-overproducing transformants, both free and conjugated IAA levels were increased, but the percent free IAA was maintained constant (57 [plus or minus] 10%) for both leaves and internodes, independent of the total IAA level or tissue age. These results suggest that synthesis or transport of IAA conjugates is regulated in the vegetative wild-type plant, and that different organs possess a unique balance between free and conjugated IAA. The IAA-overproducing plant, however, acquires a lower proportion of free IAA in the stem and younger leaves, presumably determined by a higher conjugation in those tissues compared with wild type.     

Utilization of the plant hormone indole-3-acetic acid for growth by Pseudomonas putida strain 1290

We have isolated from plant surfaces several bacteria with the ability to catabolize indole-3-acetic acid (IAA). One of them, isolate 1290, was able to utilize IAA as a sole source of carbon, nitrogen, and energy. The strain was identified by its 16S rRNA sequence as Pseudomonas putida. Activity of the enzyme catechol 1,2-dioxygenase was induced during growth on IAA, suggesting that catechol is an intermediate of the IAA catabolic pathway. This was in agreement with the observation that the oxygen uptake by IAA-grown P. putida 1290 cells was elevated in response to the addition of catechol. The inability of a catR mutant of P. putida 1290 to grow at the expense of IAA also suggests a central role for catechol as an intermediate in IAA metabolism. Besides being able to destroy IAA, strain 1290 was also capable of producing IAA in media supplemented with tryptophan. In root elongation assays, P. putida strain 1290 completely abolished the inhibitory effect of exogenous IAA on the elongation of radish roots. In fact, coinoculation of roots with P. putida 1290 and 1 mM concentration of IAA had a positive effect on root development. In coinoculation experiments on radish roots, strain 1290 was only partially able to alleviate the inhibitory effect of bacteria that in culture overproduce IAA. Our findings imply a biological role for strain 1290 as a sink or recycler of IAA in its association with plants and plant-associated bacteria.     

DDT降解菌株Chryseobacterium sp. PYR2对小麦的促生作用及其机理

【背景】前期结果表明,DDT降解菌株Chryseobacterium sp. PYR2可高效去除土壤中的DDT等污染物,具有潜在的应用价值,但该菌对植物的影响尚不清楚。【目的】探讨菌株Chryseobacterium sp. PYR2对植物的促生作用及其机理,为后续开发DDT降解及植物促生双效功能菌剂提供理论依据。【方法】配制该菌株的不同梯度稀释菌悬液,用纸卷发芽法和盆栽法研究菌悬液对小麦种子萌发和植株生长的影响;Salkowski法测定PYR2合成吲哚-3-乙酸(Indole-3-acetic acid,IAA)量;单因素实验研究不同培养条件对菌株生长及IAA合成的影响;液相色谱-串联质谱-多反应监测(LC-MS/MS-MRM)方法分析IAA在PYR2菌体内的生物合成途径。【结果】PYR2菌悬液可明显提高小麦种子萌发率并促进小麦植株的生长,小麦的侧根数、株高、鲜重、干重等指标均明显提高。该作用是由于菌株PYR2可以合成植物生长激素IAA。最适IAA合成条件:温度30°C,pH 7.0-8.0,盐浓度0.5%,L-色氨酸50mg/L。代谢液中检测到色醇、色胺和吲哚-3-乙酰胺3种中间代谢产物,推测PYR2体内存在3条IAA合成途径,分别为吲哚-3-丙酮酸(IPy A)、TAM和IAM途径。【结论】菌株PYR2对小麦具有明显的促生效果,是由于其具有多条高效合成IAA的代谢途径,表明其在农药污染土壤的生物修复及作物种植中具有潜在的应用前景。