While DCA and INA behave similar in nahG plants, these two compounds differ profoundly in their level of dependency on the transcriptional co-factor NPR1 . Although the defense inducing activity of INA is fully blocked in npr1-3 plants, DCA is only partially reduced in this mutant. Thus, with CMP442, DCA and INA a small set of synthetic elicitors is now available that has different activating effects on the SA-dependent plant defense network . These molecular probes along with additional synthetic elicitors from our screen and genetic mutations are likely to prove highly useful for the fine dissection of this complex regulatory network.Chemical pesticides currently in use typically rely on direct antibiotic or biocidal activity, which often leads to undesirable toxic environmental side effects . In response to these concerns the U.S. Environmental Protection Agency has established a program to facilitate registration of new reduced-risk pesticides that have a reduced impact on human health and other non-target organisms . Synthetic elicitors identified by our project protect plants by inducing their natural immune responses. As their primary mode-of action does not involve the inhibition of key metabolic or developmental steps in target organisms, they are likely to be less harmful for humans and the environment than conventional pesticides. Due to the continuous pollution of the environment caused by the massive use of traditional pesticides and the increasing awareness of environmental protection issues of consumers and farmers in the US, Europe and other parts of the globe,vertical grow system innovative “green” pesticides suitable for conventional farming practices are urgently needed.

A possible disadvantage of the use of synthetic elicitors for crop protection is that permanent defense activation often results in fitness costs, due to the phytotoxicity of some defensive plant products and resource allocation away from growth or reproduction. For example, as a result of its long-term activity, the synthetic elicitor 2,6-isonicotinic acid , which was developed in the 1990s by Ciba Geigy, was insufficiently tolerated by some crop plants to warrant practical use as a plant protection compound . However, we found DCA and CMP442 to be promising in this respect when contrasted to other known defense elicitors, such as INA or BTH , their defense-inducing activity is only transient and weakens within several days after application . In addition, low doses of CMP442 proved to be beneficial for plant growth. Arabidopsis and tomato grown on solid medium containing low concentrations of CMP442 developed significantly longer roots than untreated seedlings. In addition, single root drench application of CMP442 enhanced growth of aerial parts of soil-grown Arabidopsis and tomato. Thus, CMP442 appears to be uniquely suited to provide plant seedlings with both protection from diseases and enhancement of vigor. We also found several other synthetic elicitors, including DCA, that have similar effects on root growth at low concentrations. However, of those synthetic elicitors we tested so far CMP442 is the most efficient one in this respect. CMP442 strongly enhanced growth of Arabidopsis roots at a concentration of 1 µM. At this concentration CMP442 still can induce defense responses.As shown in Figure 2.3, 10 µM of CMP442 was sufficient to significantly suppress the development of HpaNoco2 spores in Arabidopsis. Despite these promising observations, further studies are needed to explore the full potential of CMP442 for simultaneous disease protection and growth enhancement for crops.

Several regulatory proteins were found to contribute to both defense and developmental processes . These include the Arabidopsis proteins SGT1b, AS1 and AtTIP49a . For example, SGT1b, a regulatory component of SCF complex ubiquitin ligases, was found to be involved in controlling stability of several R proteins as well as the activation of ETI, but also SCFTIR1-mediated auxin responses, such as root development and apical dominance . We recently reported that Enhanced Downy Mildew 2 , which is required for R-mediated resistance of Arabidopsis against the Hiks1 isolate of Hpa, positively affects floral transition . EDM2 has additional roles in plant development, such as promoting proper leaf pavement cell development and controlling the succession of leaf types formed during early vegetative stages of Arabidopsis. An increasing number of studies are reporting on similar molecular links between plant immune and developmental processes. The molecular nature and biological purpose of crosstalk between both types of processes is poorly understood at this point. CMP442, with its clear effects on both plant immunity and growth, is likely to serve as a valuable tool for the dissection of molecular crosstalk between defense and development. We are currently testing its effects on both defense induction and growth enhancement in a variety of known Arabidopsis signaling mutants. Results from these and related studies should shed light on the fascinating, but yet enigmatic, link between seemingly unrelated types of physiological processes in plants. CMP442 may also allow for the discovery of fundamental causes of the general phenomenon of hormesis. Although widely described for numerous types of organisms and physical, chemical or biological stimuli, the genetic and molecular basis of hormesis is largely unknown.

Hormesis is characterized by a biphasic dose-response to a treatment which stimulates at low doses and has an inhibitory or toxic effect at higher concentrations . Biologically, hormesis is believed to be an adaptive response at either the cellular or organismal level to stress. The exposure to low doses of herbicides to produce enhanced growth has been widely reported on . Recent research has revealed some signaling pathways and mechanisms that are responsible for specific hormetic responses. These involve certain ion channels, protein kinases, deacetylases, transcription factors, chaper ones, antioxidant enzymes, and glutathione peroxidase . Another noteworthy observation is that some inducers of hormetic responses can protect the respective cells or organisms against a variety of additional stressors later on . Although the phenomenon of hormesis has been known for several decades, our knowledge of its biological basis is fragmentary at best and much remains to be explained. In particular, it is unclear if the great variety of hormesis-like phenomena have a common functional basis, or if they are mechanistically unrelated. A comprehensive comparison of molecular responses triggered by a variety of hormesis-inducing stimuli in a single type of organism, such as the versatile molecular genetics model Arabidopsis, may allow defining common denominators for this complex phenomenon. Plant diseases can be caused by pathogens with different types of lifestyles. While biotrophic pathogens require living host tissues to complete their life cycles, necrotrophs feed off dead plant cells. The phytohormones salicylic acid , jasmonic acid , and ethylene are known to coordinate plant defense responses to combat the respective type of infecting pathogen. Currently, most documented interactions between JA- and SA-dependent signaling processes are antagonistic, but their interactions are complex and details of crosstalk between them are not fully understood. Upon recognition of necrotrophs, an increase in JA and ET synthesis occurs along with enhanced transcript levels of defense genes, such as Plant Defensin 1.2 ,mobile grow system which is often used as a marker for induction of the JA pathway. Plant defensins are small peptides that can be found throughout the plant kingdom and are encoded by small gene families. Here I report on the development of a screening procedure to identify synthetic elicitors that activate the JA-/ET-dependent branch of the defense network. Towards this end, a set of genes was identified that display SA-independent upregulation in response to infection with the biotrophic oomycete Hyaloperonospora arabidopsidis . Four of the five genes are PDF members including PDF1.2b. Additionally, efforts to create Arabidopsis thaliana lines containing RNA silencing transgenes cosilencing closely related PDF family members are described. Plants are constantly assaulted by a variety of biotic stressors, such as microbial pathogens. Most pathogens are unable to infect plants, making disease the exception, not the rule . Plant pathogens are typically divided into two main categories: biotrophs, which obtain nutrients through living tissue and necrotrophs, which must kill plant tissue to acquire nutrients .

Plants evolved the ability to recognize pathogens and tailor their defense responses to the type of infecting pathogen . They possess an inducible immune system enabling them to specifically recognize molecular features of pathogens and activate transcriptional cascades defending the plant from disease . When these mechanisms are absent or inactivated by pathogen effector molecules, plants are rendered susceptible . Pathogen effectors are proteins or small molecules secreted into host cells that attenuate defense signaling processes weakening plant immune responses. Strong immunity against pathogens can be mediated by plant disease resistance genes, which encode receptors that specifically recognize effectors from distinct pathogen races . Thus, such race-specific immunity is based on interactions of complementary R– and effector-genes . A hallmark of R-mediated disease resistance is the hypersensitive response , a programmed form of plant cell death localized to pathogen infection sites. HR is an effective defense reaction against biotrophic pathogens, which are dependent on live plant tissue . During compatible interactions, basal defense is activated, which is a weakened form of plant immunity that does not involve HR and is typically insufficient to prevent disease . R-mediated disease resistance is frequently facilitated through the SA-dependent branch of the defense network, which is often attributed to defense responses against biotrophic pathogens whereas the JA- and ET-dependent mechanisms seem preferentially to mediate immunity against necrotrophs Thus, the plant immune system is able to specifically tailor distinct responses against different types of pathogens. Fine tuning of these responses is mediated by complex crosstalk between individual signaling branches . The timing, amplitudes and spatial distributions of certain defense signals determine the individual defense reactions activated in response to a given type of pathogen. One pathogen widely utilized to study plant defense is Hyaloperonospora arabidopsidis , an oomycete and obligate biotroph known to exclusively infect the model plant species Arabidopsis thaliana. The study of this pathosystem has facilitated the identification of more gene-for-gene interactions than any other plant and pathogen combination . Thus far, SA-dependent defenses have been directly attributed to limiting Hpa growth in this pathosystem with JA/ET having no distinguishable role . The majority of plant disease research indicates that interactions between JA- and SA-signaling are antagonistic, although it has also been demonstrated that at low concentrations they can act synergistically . JA is a lipid-derived signal that has many vital roles in plants . Responses to JA are controlled by a regulatory apparatus consisting of four key components: the JA signal, the ubiquitin ligase SCFCOI1, the jasmonate ZIM-domain repressor proteins, and transcription factors that positively regulate expression of JA-responsive genes . JA-signaling is involved in complex processes such as pollen maturation, response to wounding, fruit ripening, root growth, and even tendril coiling . The role of JA in the defense response to insect wounding was first suggested in 1992 . JA- and ET-dependent regulatory processes can act cooperatively . For example, both JA- and ETsignaling contribute to resistance against necrotrophic pathogens and are knownto inhibit the formation of HR. This dual function is advantageous for the plant, since necrotrophs feed off dead tissue and may benefit from HR . Activation of JA-mediated defenses are preceded by the accumulation of jasmonates synthesized by the octadecanoid pathway . However, knowledge of the JA-signaling pathway is still incomplete. Differences exist between the JA-signaling systems studied in different plant species. For example, although the system in pathway induces systemic JA responses in tomato, no evidence has yet indicated such a pathway exists in Arabidopsis . When some plants are subjected to predation, the JA pathway is activated, requiring a 200-amino acid precursor, prosystemin. Prosystemin then produces the 18-amino-acid peptide, system in, through proteolytic processing . Systemin induces the production of H2O2, followed by the biosynthesis of JA and leading to the activation of defense-related genes . The first steps of JA biosynthesis occur in the chloroplast where membrane-derived linoleic acid is converted to 12-oxo-phytodienoic acid using multiple biosynthetic enzymes . OPDA is then transported to the peroxisome, where it is reduced to OPC-8:0 by OPDA reductase3 undergoing three rounds of β-oxidation, resulting in the production of -7-iso-JA . Studies found that a main bioactive form of JA is JA-isoleucine , which is produced by conjugation of JA to Ile by Jasmonate Resistant 1 .