Production, amplification and systemic propagation of redox messengers in plants? The phloem can do it all!

Rapid long-distance signalling is an emerging topic in plant research, and is particularly associated with responses to biotic and abiotic stress. Systemic acquired resistance (SAR) to pathogen attack is dependent on nitric oxide (NO) and reactive oxygen species (ROS) such as hydrogen peroxide (H2O2). By comparison, systemic wound responses (SWRs) and systemic acquired acclimation (SAA) to abiotic stress encounters are triggered by rapid waves of H2O2, calcium and electrical signalling. Efforts have been made to decipher the relationship between redox messengers, calcium and other known systemic defence signals. Less is known about possible routes of signal transduction throughout the entire plant. Previously, the phloem has been suggested to be a transport conduit for mobile signals inducing SAR, SWR and SAA. This review highlights the role of the phloem in systemic redox signalling by NO and ROS. A not yet identified calcium-dependent NO source and S-nitrosoglutathione reductase are candidate regulators of NO homeostasis in the phloem, whereas ROS concentrations are controlled by NADPH oxidases and the H2O2-scavenging enzyme ascorbate peroxidase. Possible amplification mechanisms in phloem-mediated systemic redox signalling are discussed

Epidemiological analysis of the damage potential of Pgt-Ug99 in Central East, North East Africa; Iran and Punjab (India)

In the Rift valley Epidemiological Zone that falls in the predicted Puccinia graminis tritici-Ug99 spread route from Uganda to India, is the presence of bi-modular annual rainy season, absence of a hot dry summer and the practice of two wheat growing seasons (the green bridge). This ensures the survival of Pgt in situ and the endogenous primary inoculum initiates the recurrence of the disease. While in the Indian Punjab Epidemiological Zone the exogenous primary inoculum of Pgt from the Himalaya to the plains of Indian Punjab gets subdued and at best builds-up traces of disease severity before the harvest of wheat. The Iranian Epidemiological Zone is similar to Indian Punjab, except that there is no monsoon and the exogenous inoculum of Pgt initiates stem rust development year and again. It is felt that Pgt-Ug99 lacks pre-eminence to threaten wheat production in the Indian Punjab

Mechanistic and genetic overlap of barley host and non-host resistance to Blumeria graminis

Non-host resistance of barley to Blumeria graminis f.sp. tritici (Bgt), an inappropriate forma specialis of the grass powdery mildew fungus, is associated with formation of cell wall appositions (papillae) at sites of attempted fungal penetration and a hypersensitive cell death reaction (HR) of single attacked cells. Penetration resistance and HR are also typical features of race-non-specific and race-specific resistance of barley to the appropriate Blumeria graminis f.sp. hordei (Bgh), raising the question of whether genotypic differences in the cellular response of barley to Bgt are delectable. First, we analysed fungal penetration frequencies and HR in different barley accessions known to show altered non-host resistance. In genotypes with limited resistance to inappropriate cereal rust fungi, we concomitantly detected low penetration resistance to Bgt and significant differences of HR rates during attack from Bgt. Second, we tested barley mutants known to show altered host responses to Bgh. The rar1-mutation that suppresses many types of race-cultivar-specific resistances did not influence the non-host response of the Bgt-isolate used in this study. However, mutants of Ror1 and Ror2, two genes required for full race non-specific penetration resistance of mlo-barley to barley powdery mildew fungus, exhibited altered defence response to Bgt, including higher frequencies of fungal penetration. On these mutants, growth of the inappropriate fungus was arrested subsequent to penetration by HR. Together, the data show that barley defence response to the wheat powdery mildew fungus is determined by similar factors as race-specific and race-non-specific resistance to appropriate Bgh

Systemic induction of NO-, redox-, and cGMP signaling in the pumpkin extrafascicular phloem upon local leaf wounding.

Cucurbits developed the unique extrafascicular phloem (EFP) as a defensive structure against herbivorous animals. Mechanical leaf injury was previously shown to induce a systemic wound response in the EFP of pumpkin (Cucurbita maxima). Here, we demonstrate that the phloem antioxidant system and protein modifications by NO are strongly regulated during this process. Activities of the central antioxidant enzymes dehydroascorbate reductase, glutathione reductase and ascorbate reductase were rapidly down-regulated at 30 min with a second minimum at 24 h after wounding. As a consequence levels of total ascorbate and glutathione also decreased with similar bi-phasic kinetics. These results hint toward a wound-induced shift in the redox status of the EFP. Nitric oxide (NO) is another important player in stress-induced redox signaling in plants. Therefore, we analyzed NO-dependent protein modifications in the EFP. Six to forty eight hours after leaf damage total S-nitrosothiol content and protein S-nitrosylation were clearly reduced, which was contrasted by a pronounced increase in protein tyrosine nitration. Collectively, these findings suggest that NO-dependent S-nitrosylation turned into peroxynitrite-mediated protein nitration upon a stress-induced redox shift probably involving the accumulation of reactive oxygen species within the EFP. Using the biotin switch assay and anti-nitrotyrosine antibodies we identified 9 candidate S-nitrosylated and 6 candidate tyrosine-nitrated phloem proteins. The wound-responsive Phloem Protein 16-1 (PP16-1) and Cyclophilin 18 (CYP18) as well as the 26.5 kD isoform of Phloem Protein 2 (PP2) were amenable to both NO modifications and could represent important redox-sensors within the cucurbit EFP. We also found that leaf injury triggered the systemic accumulation of cyclic guanosine monophosphate (cGMP) in the EFP and discuss the possible function of this second messenger in systemic NO and redox signaling within the EFP

The <em>N</em>-acyl homoserine-lactone depleted <em>Rhizobium radiobacter</em> mutant RrF4NM13 shows reduced growth-promoting and resistance-inducing activities in mono- and dicotyledonous plants.

The AlphaproteobacteriumRhizobium radiobacter(syn.Agrobacterium tumefaciens, "A. fa brum") can live in close association with the sebacinoid fungusSerendipita(syn.Piriformospora)indicathat forms a mutualistic Sebacinalean symbiosis with a wide range of host plants. The endobacterial strainR. radiobacterF4 (RrF4), which was originally isolated from the fungus, has plant growth promotion and resistance-inducing activities resembling the beneficial activities known from the endobacteria-containingS. indica. The mechanism by which free endobacterial cells influence growth and disease resistance of colonized host plants is not fully understood. Here, we show thatRrF4 produces a spectrum of quorum sensing-mediatingN-acyl-homoserine lactones (AHLs) with acyl chains of C8, C10, and C12 as well as hydroxyl- or oxo-substitutions at the C3 position. In addition, and in line with previous findings showing that AHLs increase plant biomass and induce systemic resistance, the AHL-depleted lactonase-overexpressing transconjugantRrF4NM13 was partially compromised in promoting growth and inducing resistance against bacterial pathogens in bothArabidopsis thalianaand wheat (Triticum aestivum). Scanning and transmission electron microscopy proved thatRrF4NM13, in contrast toRrF4, does not form cellulose-like fiber scaffolds for efficient root surface attachment. Moreover,RrF4NM13 does not penetrate into the intercellular space of the cortical tissue, which in contrast is strongly colonized byRrF4. We discuss the possibility that AHLs contribute to the outcome of the Sebacinalean symbiosis