Auxin: regulation, action, and interaction.

Glucosinolates are sulfur-rich, anionic natural products that upon hydrolysis by endogenous thioglucosidases called myrosinases produce several different products (e.g., isothiocyanates, thiocyanates, and nitriles). The hydrolysis products have many different biological activities, e.g., as defense compounds and attractants. For humans these compounds function as cancer-preventing agents, biopesticides, and flavor compounds. Since the completion of the Arabidopsis genome, glucosinolate research has made significant progress, resulting in near-complete elucidation of the core biosynthetic pathway, identification of the first regulators of the pathway, metabolic engineering of specific glucosinolate profiles to study function, as well as identification of evolutionary links to related pathways. Although much has been learned in recent years, much more awaits discovery before we fully understand how and why plants synthesize glucosinolates. This may enable us to more fully exploit the potential of these compounds in agriculture and medicine.

Biology and biochemistry of glucosinolates

Plant secondary metabolites are unique sources for pharmaceuticals, food additives, flavors, and industrially important biochemicals. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Secondary metabolites play a major role in the adaptation of plants to the environment and in overcoming stress conditions. Environmental factors viz. temperature, humidity, light intensity, the supply of water, minerals, and CO2 influence the growth of a plant and secondary metabolite production. Drought, high salinity, and freezing temperatures are environmental conditions that cause adverse effects on the growth of plants and the productivity of crops. Plant cell culture technologies have been effective tools for both studying and producing plant secondary metabolites under in vitro conditions and for plant improvement. This brief review summarizes the influence of different abiotic factors include salt, drought, light, heavy metals, frost etc. o...

https://www.tandfonline.com/doi/pdf/10.4161/psb.6.11.17613

Influence of abiotic stress signals on secondary metabolites in plants

Variation of glucosinolate accumulation among different organs and developmental stages of Arabidopsis thaliana.

Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated. Here, we provide biochemical evidence that the tricarboxylic acid cycle intermediate L-malic acid (MA) secreted from roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Root secretions of L-MA are induced by the foliar pathogen Pseudomonas syringae pv tomato (Pst DC3000) and elevated levels of L-MA promote binding and biofilm formation of FB17 on Arabidopsis roots. The demonstration that roots selectively secrete L-MA and effectively signal beneficial rhizobacteria establishes a regulatory role of root metabolites in recruitment of beneficial microbes, as well as underscores the breadth and sophistication of plant-microbial interactions.

Root-secreted malic acid recruits beneficial soil bacteria

Glucosinolate content in susceptible and resistant chinese cabbage varieties during development of clubroot disease

Cytokinin-like substances in both healthy and infected (Plasmodiophora brassicae) Wor. strain S) roots of Brassica campestris L. ssp. pekinensis cv. Granat have been tentatively identified and quantified by HPLC. The isomers of the cytokinins could be seperated on a reversed phase column using a gradient elution with increasing amounts of methanol. Secondary plasmodia were isolated mechanically from Plasmodiophora brassicae infected roots. The time course of adenine uptake and its conversion to cytokinins were investigated. Evidence is presented for the incorporation of [U-14C]-adenine into trans-zeatin by secondary plasmodia.

Isomers of zeatin and zeatin riboside in clubroot tissue: evidence for trans‐zeatin biosynthesis by Plasmodiophora brassicae

Auxin conjugates play a role in the regulation of free indole-3-acetic acid (IAA) content in plants. Not much is known about the enzymes involved in either conjugate synthesis or hydrolysis. In this study we have isolated and characterized an auxin conjugate hydrolase from Chinese cabbage seedlings and investigated it during the development of both the Chinese cabbage plants and the clubroot disease. The hydrolase isolated from light- and dark-grown seedlings accepted the amide conjugates indole-3-acetic acid-alanine (IAAla), IAA-phenylalanine (IAPhe), but not IRA-aspartate (lAAsp) as substrates. We also found a substantial amount of hydrolysis of an ester conjugate (IAA-glucose, IAGlu) in our enzyme preparation. The tentative reaction product IAA was identified by HPLC and subsequent GC-MS analysis. The pH optima for the different substrates were not identical, suggesting several hydrolase isoforms. After gel filtration chromatography we found at least two peaks containing different hydrolase isoforms. The isoform, which converted IAGlU to IAA, exhibited a molecular mass of ca 63 kDa, and an isoform of ca 21 kDa converted IAAla and IAPhe. The increased free IAA content in clubroot-diseased roots of Brassicaceae can be due to either de novo synthesis or release of IAA from conjugates. To answer this question free, ester- and amide-bound IAA was measured in 24- and 30-day-old leaves and roots of healthy and Plasmodiophora brassicae-infected Chinese cabbage, and the hydrolase activity with different substrates measured in the same tissues. The amide conjugates were dramatically enhanced in infected roots, whereas free IAA was only slightly enhanced compared to the control tissue. Hydrolase activity was also enhanced in clubbed roots, but the substrate specificity differed from that found in the seedlings. Especially, IAAsp hydrolysis was induced after inoculation with P. brassicae. We conclude that different auxin conjugates can be hydrolyzed at different developmental stages or under stress.

Auxin-conjugate hydrolysis in Chinese cabbage: characterization of an amidohydrolase and its role during infection with clubroot disease.

The in vitro conversion of [14C]-tryptophan to [14C]-indole-3-acetaldoxime (IAOX) by microsomal membranes of Chinese cabbage (Brassica campestris ssp. pekinensis cv. Granat) has been studied. The reaction product was identified by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC). Furthermore. IAOX was identified as an endogenous compound of Chinese cabbage by mass spectroscopy. The tryptophan-oxidizing enzyme (TrpOxE) was characterized. MnCl2 was required as cofactor, H2O2, and 2,4-dichlorophenol (DCP) stimulated the reaction. The enzyme showed a pH optimum at pH 8–9 and a Km for l-tryptophan of 20 μM. The membranes containing TrpOxE activity were identified as plasma membranes by means of aqueous polymer two-phase partitioning. The TrpOxE from Chinese cabbage was purified 3-fold from plasma membranes by solubilization followed by (NH4)2SO4-fractionation, affinity-chromatography with concanavalin A, and native gel electrophoresis. Enzyme activity was reduced by a tunicamycin pretreatment. Several other plant species, e.g. maize (Zea mays L. Inrakorn), sunflower (Helianthus annuus L. cv. Hohes Sonnengold), tobacco (Nicotiana tabacum L. cv. White Burley), and pea (Pisum sativum L. cv. Krombeck) showed a similar conversion of [14C]-tryptophan to [14C]-IAOX by phase-partitioned plasma membranes.

A plasma membrane-bound enzyme oxidizes L-tryptophan to indole-3-acetaldoxime.

The host range of the soilborne obligate biotroph, Plasmodiophora brassicae was investigated. Evidence is presented that infection by P. brassicae might occur in non-Brassica species, leading to the potential formation of resting spores. Structures resembling P. brassicae were found in the root cortex of Tropaeolum majus, Carica papaya, Reseda alba and Beta vulgaris as demonstrated by scanning electron microscopy. Inoculation of Brassica rapa roots with spores extracted from either T. majus or B. vulgaris roots which had been previously inoculated with P. brassicae led to development of clubroot in the roots of B. rapa. It was also shown that the development of the symptom might be correlated with glucosinolate content, although other host factors are implicated in the B. vulgaris interaction with P. brassicae. In the glucosinolate-containing non-Brassicas, T. majus and C. papaya, the concentrations of benzylglucosinolate increased markedly in roots inoculated with P. brassicae, compared with the controls. There were also increases in concentrations of benzylglucosinolate in leaves of T. majus after P. brassicae infection. However, in R. alba roots, the total glucosinolate content decreased after inoculation with P. brassicae compared with the controls. High root concentrations of 2-OH-2-phenylethylglucosinolate (glucobarbarin) compared with low root indole glucosinolates in this species might limit P. brassicae infection and development. The importance of our investigations in relation to cultivation of non-Brassica species on fields infested with P. brassicae is discussed.

Willy Hilgenberg

Papers

Glucosinolate content in susceptible and resistant chinese cabbage varieties during development of clubroot disease

Isomers of zeatin and zeatin riboside in clubroot tissue: evidence for trans‐zeatin biosynthesis by Plasmodiophora brassicae

Auxin-conjugate hydrolysis in Chinese cabbage: characterization of an amidohydrolase and its role during infection with clubroot disease.

A plasma membrane-bound enzyme oxidizes L-tryptophan to indole-3-acetaldoxime.

The host range of Plasmodiophora brassicae and its relationship to endogenous glucosinolate content