Phenylpropanoid compounds encompass a wide range of structural classes and biological functions. Limiting discussion to stress-induced phenylpropanoids eliminates few of the structural classes, because many compounds thst are constitutive in one plant species or tissue can be induced by various stresses in another species or in another tissue of the same plant (Beggs et al., 1987; Christie et al., 1994).

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Stress-Induced Phenylpropanoid Metabolism.

Reactive oxygen species (ROS) are produced as a normal product of plant cellular metabolism. Various environmental stresses lead to excessive production of ROS causing progressive oxidative damage and ultimately cell death. Despite their destructive activity, they are well-described second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. Whether ROS would serve as signaling molecules or could cause oxidative damage to the tissues depends on the delicate equilibrium between ROS production, and their scavenging. Efficient scavenging of ROS produced during various environmental stresses requires the action of several nonenzymatic as well as enzymatic antioxidants present in the tissues. In this paper, we describe the generation, sites of production and role of ROS as messenger molecules as well as inducers of oxidative damage. Further, the antioxidative defense mechanisms operating in the cells for scavenging of ROS overproduced under various stressful conditions of the environment have been discussed in detail.

https://downloads.hindawi.com/archive/2012/217037.pdf

Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

Contrasting Mechanisms of Defense Against Biotrophic and Necrotrophic Pathogens

The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

Rapid generation of superoxide and accumulation of H2O2 is a characteristic early feature of the hypersensitive response following perception of pathogen avirulence signals. Emerging data indicate that the oxidative burst reflects activation of a membrane-bound NADPH oxidase closely resembling that operating in activated neutrophils. The oxidants are not only direct protective agents, but H2O2 also functions as a substrate for oxidative cross-linking in the cell wall, as a threshold trigger for hypersensitive cell death, and as a diffusible signal for induction of cellular protectant genes in surrounding cells. Activation of the oxidative burst is a central component of a highly amplified and integrated signal system, also involving salicylic acid and perturbations of cytosolic Ca2+, which underlies the expression of disease-resistance mechanisms.

The oxidative burst in plant disease resistance

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction--the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizofiltration--the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization--the use of plants to eliminate the bioavailability of toxic metals in soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

Why Use Phytoremediation? (B. Ensley). ENVIRONMENTAL POLLUTION AND GREEN PLANTS. Phytoremediation's Economic Potential (D. Glass). Phytoremediation and Public Acceptance (R. Tucker & J. Shaw). Regulatory Considerations for Phytoremediation (S. Rock & P. Sayre). TECHNOLOGIES FOR METAL PHYTOREMEDIATION. Phytoextraction of Metals (M. Baylock & J. Huang). Phytostabilization of Metals (S. Cunningham & W. Berti). Phytofiltration of Metals (Y. Kapulnik & S. Dushenkov). The Use of Plants for the Treatment of Radionuclide (M. Negri & R. Hinchman). Photostabilization of Metals Using Hybrid Poplar Trees (J. Schnoor). Phytoreduction of Environmental Mercury Pollution (C. Rugh, et al.). The Physiology and Biochemistry of Selenium Volatilization By Plants (M. de Souza, et al.). BIOLOGY OF METAL PHYTOREMEDIATION. Metal Accumulating Plants (R. Reeves & A. Baker). Mechanisms of Metal Hyperaccumulation in Plants (D. Salt & U. Kramer). Mechanisms of Metal Resistance: Phytochelatins and Metalothioneins (C. Cobbett & P. Goldsborough). Molecular Mechanisms of Ion Transport in Plant Cells (M. Guerinot).

Phytoremediation of toxic metals : using plants to clean up the environment

A small number of wild plants which grow on metal contaminated soil accumulate large amounts of heavy metals in their roots and shoots This property may be exploited for soil reclamation if an easily cultivated, high biomass crop plant able to accumulate heavy metals is identified Therefore, the ability of various crop plants to accumulate Pb in shoots and roots was compared While all crop Brassicas tested accumulated Pb, some cultivars of Brassica juncea (L) Czern showed a strong ability to accumulate Pb in roots and to transport Pb to the shoots (1083 mg Pb/g DW in the roots and 345 mg Pb/g DW in the shoots) B juncea was also able to concentrate Cr{sup -6}, Cd, Ni, Zn, and Cu in the shoots 58, 52, 31, 17, and 7 fold, respectively, from a substrate containing sulfates and phosphates as fertilizers The high metal accumulation by some cultivars of B juncea suggests that these plants may be used to clean up toxic metal-contaminated sites in a process termed phytoextraction

Phytoextraction: The Use of Plants To Remove Heavy Metals from Soils

Some cultivars of tobacco are resistant to tobacco mosaic virus (TMV) and synthesize pathogenesis-related (PR) proteins upon infection. In a search for the signal or signals that induce resistance or PR genes, it was found that the endogenous salicylic acid levels in resistant, but not susceptible, cultivars increased at least 20-fold in infected leaves and 5-fold in uninfected leaves after TMV inoculation. Induction of PRl genes paralleled the rise in salicylic acid levels. Since earlier work has demonstrated that treatment with exogenous salicylic acid induces PR genes and resistance, these findings suggest that salicylic acid functions as the natural transduction signal.

https://science.sciencemag.org/content/sci/250/4983/1002.full.pdf

Salicylic acid : a likely endogenous signal in the resistance response of tobacco to viral infection

INTRODUCTION . . . . . . . .... . . . . . . ... . . . . . .. . . . . . . . ........ .. . . . . . . . . ..... . . . . . . . . . . . . . . .. . . . . . . . . . . . . 439 History of Salicylates .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 General Properties of Salicylic Acid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 Salicylic Acid Levels in Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 EFFECTS OF EXOGENOUS SALICYLIC A CID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 Salicylic Acid and Flowering . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 Allelopathic Properties of Salicylic Acid : Effect on Membranes and Ion Uptake. .. . 444 Other Effects of Exogenously Applied Salicylic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 SALICYLIC ACID A ND H EAT PRODU CTION IN PLANTS . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 445 Thermogenic Plants and Search for Calorigen. . . . . . . .. . . . . . . . . . ...... . . . . 445 Salicylic Acid : A Natural Inducer of Thermogenesis in Arum Lilies . . . . . . . . . .... . . . . . . 446 SALICYLIC ACID A ND D IS EAS E RES ISTANCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Disease Resistance in Plants: Effects of Salicylic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Salicylic Acid : A Likely Signal for Disease Resistance in Plants. .... . . . . . . . . . . . . . . . .. 450 SALICYLIC ACID BIOSYNTHES IS IN PLANTS . .. . . ... .. . . . . . . . . . . . . ...... . . . . . . . . . . . . . . .. 451 Biosynthetic Pathway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Biosynthetic Enzymes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 SALICYLIC ACID M ETABOLISM . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 MI CRO BIAL PRODU CT IO N OF SALICYLIC ACID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454 CO NCLUDING REMARKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ilya Raskin

Papers

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

Phytoremediation of toxic metals : using plants to clean up the environment

Phytoextraction: The Use of Plants To Remove Heavy Metals from Soils

Salicylic acid : a likely endogenous signal in the resistance response of tobacco to viral infection

Role of Salicylic Acid in Plants