RNA-based technologies for insect control in plant production.

RNA interference (RNAi) is a biological process in which small RNA (sRNA) molecules sequence-specifically silence gene expression at the transcriptional or post-transcriptional level, either by directing inhibitory chromatin modifications or by decreasing the stability or translation potential of the targeted mRNA. The trigger for gene silencing is double-stranded RNA (dsRNA) generated from an endogenous genomic locus or a foreign source, such as a transgene or virus. The process of gene silencing can be exploited in agriculture to control plant diseases and pests. Of the pests that impact crop yield (including nematodes, arthropods, rodents, snails, slugs and birds), insects constitute the largest and most diverse group. Here, we review the "pros" and "cons" of using RNAi technology mediated by dsRNA-expressing transgenic plants (host-induced gene silencing, HIGS) or direct application of chemically synthesized dsRNA to control plant-damaging insects. Rapid progress in elucidating RNAi mechanisms has led to the first commercial products on the market. Given the high potential of RNAi strategies, their use in agriculture, horticulture, and forestry will likely be extensive in the future. However, further studies are needed to improve the efficacy of RNAi-based plant protection strategies and to assess their associated safety risks.

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future.

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

How does the multifaceted plant hormone salicylic acid combat disease in plants and are similar mechanisms utilized in humans?

Salicylic acid (SA) is an important plant hormone that regulates many aspects of plant growth and development, as well as resistance to (a)biotic stress. Efforts to identify SA effector proteins have revealed that SA binds to and alters the activity of multiple plant proteins-this represents a shift from the paradigm that hormones mediate their functions via one or a few receptors. SA and its derivatives also have multiple targets in animals; some of these proteins, like their plant counterparts, are associated with pathological processes. Together, these findings suggest that SA exerts its defense-associated effects in both kingdoms via a large number of targets.

SOS - too many signals for systemic acquired resistance?

Following pathogen infection, activation of systemic acquired resistance (SAR) in uninfected tissues requires transmission of a signal(s) from the infected tissue via the vasculature. Several candidates for this long-distance signal have been identified, including methyl salicylate (MeSA), an SFD1/GLY1-derived glycerol-3-phosphate (G3P)-dependent signal, the lipid-transfer protein DIR1, the dicarboxylic acid azelaic acid (AzA), the abietane diterpenoid dehydroabietinal (DA), jasmonic acid (JA), and the amino acid-derivative pipecolic acid (Pip). Some of these signals work cooperatively to activate SAR and/or regulate MeSA metabolism. However, Pip appears to activate SAR via an independent pathway that may impinge on these other signaling pathway(s) during de novo salicylic acid (SA) biosynthesis in the systemic tissue. Thus, a complex web of cross-interacting signals appears to activate SAR.

Salicylic Acid biosynthesis and metabolism.

Salicylic acid (SA) has been shown to regulate various aspects of growth and development; it also serves as a critical signal for activating disease resistance in Arabidopsis thaliana and other plant species. This review surveys the mechanisms involved in the biosynthesis and metabolism of this critical plant hormone. While a complete biosynthetic route has yet to be established, stressed Arabidopsis appear to synthesize SA primarily via an isochorismate-utilizing pathway in the chloroplast. A distinct pathway utilizing phenylalanine as the substrate also may contribute to SA accumulation, although to a much lesser extent. Once synthesized, free SA levels can be regulated by a variety of chemical modifications. Many of these modifications inactivate SA; however, some confer novel properties that may aid in long distance SA transport or the activation of stress responses complementary to those induced by free SA. In addition, a number of factors that directly or indirectly regulate the expression of SA biosynthetic genes or that influence the rate of SA catabolism have been identified. An integrated model, encompassing current knowledge of SA metabolism in Arabidopsis, as well as the influence other plant hormones exert on SA metabolism, is presented.

Salicylic Acid, a multifaceted hormone to combat disease.

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed.