A biochemical analysis of the interaction of victorin and oats

Victoria blight of oats is caused by the fungus Cochhobolus victoriae. This fungus is pathogenic due to its ability to produce the host-selective toxin victorin. Previously, a 100-kD protein that binds victorin in vivo only in susceptible genotypes was identified as the P protein of the glycine decarboxylase complex (GDC). Victorin is a potent in vivo inhibitor of GDC. Leaf slices pretreated with victorin displayed an effective Victorin inhibited the concentration for 50% inhibition (EC₅₀) of 81 μM for GDA. glycine-bicarbonate exchange reaction in vitro with an EC₅₀ of 23 μM. We also identified a 15-kD mitochondrial protein in susceptible and resistant genotypes that hound victorin. Amino acid sequence analysis indicated this protein is the H protein component of the GDC. Thus, victorin specifically binds to two components of the GDC. Victorin had no detectable effect on GDC in isolated mitochondria, apparently due to the inability of isolated mitochondria to import victorin. The interaction of victorin with the GDC may be central to victorin's mode of action. Supporting this observation is the finding that CO₂ gives partial protection against victorin. Elevated CO₂ is known to ameliorate the effect of GDC inhibition. Victorin treated plants incubated in the light develop more severe symptoms than dark-incubated plants. Victorin appears to induce a plant-wide signal transduction cascade, resulting in diverse effects. Victorin induces specific proteolytic cleavage of the Rubisco large subunit (LSU). Leaf slices incubated with victorin for 4 hours in the dark accumulate a form of LSU which is cleaved after the N-terminal lysine 14. LSU cleavage in leaf slices is prevented by the protease inhibitors E-64 and calpeptin. LaCl₃ prevents this cleavage of LSU and LaCl₃ also confers complete protection against victorin at the whole plant level. Victorin also causes lipid peroxidation as measured by MDA accumulation. DNA laddering is seen in leaves after 3 hr treatment with toxin. The ethylene inhibitors AOA and STS give significant protection against victorin at the whole plant level, and also prevent LSU cleavage

Differential effects of environment on potato phenylpropanoid and carotenoid expression

<p>Abstract</p> <p>Background</p> <p>Plant secondary metabolites, including phenylpropanoids and carotenoids, are stress inducible, have important roles in potato physiology and influence the nutritional value of potatoes. The type and magnitude of environmental effects on tuber phytonutrients is unclear, especially under modern agricultural management that minimizes stress. Understanding factors that influence tuber secondary metabolism could facilitate production of more nutritious crops. Metabolite pools of over forty tuber phenylpropanoids and carotenoids, along with the expression of twenty structural genes, were measured in high-phenylpropanoid purple potatoes grown in environmentally diverse locations in North America (Alaska, Texas and Florida).</p> <p>Results</p> <p>Phenylpropanoids, including chlorogenic acid (CGA), were higher in samples from the northern latitudes, as was the expression of phenylpropanoid genes including phenylalanine ammonia lyase (<it>PAL</it>), which had over a ten-fold difference in relative abundance. Phenylpropanoid gene expression appeared coordinately regulated and was well correlated with metabolite pools, except for hydroxycinnamoyl-CoA:quinatehydroxcinnamoyl transferase (<it>HQT</it>; r = -0.24). <it>In silico </it>promoter analysis identified two cis-acting elements in the HQT promoter not found in the other phenylpropanoid genes. Anthocyanins were more abundant in Alaskan samples and correlated with flavonoid genes including <it>DFR </it>(r = 0.91), <it>UFGT </it>(r = 0.94) and <it>F3H </it>(r = 0.77). The most abundant anthocyanin was petunidin-3-coum-rutinoside-5-glu, which ranged from 4.7 mg g^-1in Alaska to 2.3 mg g^-1in Texas. Positive correlations between tuber sucrose and anthocyanins (r = 0.85), suggested a stimulatory effect of sucrose. Smaller variation was observed in total carotenoids, but marked differences occurred in individual carotenoids, which had over a ten-fold range. Violaxanthin, lutein or zeaxanthin were the predominant carotenoids in tubers from Alaska, Texas and Florida respectively. Unlike in the phenylpropanoid pathway, poor correlations occurred between carotenoid transcripts and metabolites.</p> <p>Conclusion</p> <p>Analysis of tuber secondary metabolism showed interesting relationships among different metabolites in response to collective environmental influences, even under conditions that minimize stress. The variation in metabolites shows the considerable phenotypical plasticity possible with tuber secondary metabolism and raises questions about to what extent these pathways can be stimulated by environmental cues in a manner that optimizes tuber phytonutrient content while protecting yields. The differences in secondary metabolites may be sufficient to affect nutritional quality.</p

Acyl CoA Binding Proteins are Required for Cuticle Formation and Plant Responses to Microbes

Fatty acids (FA) and lipids are well known regulators of plant defense. Our previous studies have shown that components of prokaryotic (plastidal) FA biosynthesis pathway regulate various aspects of plant defense. Here, we investigated the defense related roles of the soluble acyl CoA binding proteins (ACBP), which are thought to facilitate the intracellular transport of FA/lipids. We show that ACBP3 and 4 are required for maintaining normal lipids levels and that ACBP3 contributes to the lipid flux between the prokaryotic and eukaryotic pathways. We also show that loss of ACBP 3, 4, or 6 impair normal development of the cuticle and affect both basal and resistance protein-mediated defense against bacterial and fungal pathogens. Loss of ACBP3, 4, or 6 also inhibits the induction of systemic acquired resistance (SAR) due to the plants inability to generate SAR inducing signal(s). Together, these data show that ACBP3, ACBP4 and ACBP6 are required for cuticle development as well as defense against microbial pathogens

COP1, a Negative Regulator of Photomorphogenesis, Positively Regulates Plant Disease Resistance via Double-Stranded RNA Binding Proteins

The E3 ubiquitin ligase COP1 (Constitutive Photomorphogenesis 1) is a well known component of the light-mediated plant development that acts as a repressor of photomorphogenesis. Here we show that COP1 positively regulates defense against turnip crinkle virus (TCV) and avrRPM1 bacteria by contributing to stability of resistance (R) protein HRT and RPM1, respectively. HRT and RPM1 levels and thereby pathogen resistance is significantly reduced in the cop1 mutant background. Notably, the levels of at least two double-stranded RNA binding (DRB) proteins DRB1 and DRB4 are reduced in the cop1 mutant background suggesting that COP1 affects HRT stability via its effect on the DRB proteins. Indeed, a mutation in either drb1 or drb4 resulted in degradation of HRT. In contrast to COP1, a multi-subunit E3 ligase encoded by anaphase-promoting complex (APC) 10 negatively regulates DRB4 and TCV resistance but had no effect on DRB1 levels. We propose that COP1-mediated positive regulation of HRT is dependent on a balance between COP1 and negative regulators that target DRB1 and DRB4

Enhanced Disease Susceptibility 1 and Salicylic Acid Act Redundantly to Regulate Resistance Gene-Mediated Signaling

Resistance (R) protein–associated pathways are well known to participate in defense against a variety of microbial pathogens. Salicylic acid (SA) and its associated proteinaceous signaling components, including enhanced disease susceptibility 1 (EDS1), non–race-specific disease resistance 1 (NDR1), phytoalexin deficient 4 (PAD4), senescence associated gene 101 (SAG101), and EDS5, have been identified as components of resistance derived from many R proteins. Here, we show that EDS1 and SA fulfill redundant functions in defense signaling mediated by R proteins, which were thought to function independent of EDS1 and/or SA. Simultaneous mutations in EDS1 and the SA–synthesizing enzyme SID2 compromised hypersensitive response and/or resistance mediated by R proteins that contain coiled coil domains at their N-terminal ends. Furthermore, the expression of R genes and the associated defense signaling induced in response to a reduction in the level of oleic acid were also suppressed by compromising SA biosynthesis in the eds1 mutant background. The functional redundancy with SA was specific to EDS1. Results presented here redefine our understanding of the roles of EDS1 and SA in plant defense

The MAPK Kinase Kinase GmMEKK1 Regulates Cell Death and Defense Responses

MAPK signaling pathways play critical roles in plant immunity. Here, we silenced multiple genes encoding MAPKs using virus-induced gene silencing mediated by Bean pod mottle virus to identify MAPK genes involved in soybean (Glycine max) immunity. Surprisingly, a strong hypersensitive response (HR) cell death was observed when soybean MAPK KINASE KINASE1 (GmMEKK1), a homolog of Arabidopsis (Arabidopsis thaliana) MEKK1, was silenced. The HR was accompanied by the overaccumulation of defense signaling molecules, salicylic acid (SA) and hydrogen peroxide. Genes involved in primary metabolism, translation/transcription, photosynthesis, and growth/development were down-regulated in GmMEKK1-silenced plants, while the expression of defense-related genes was activated. Accordingly, GmMEKK1-silenced plants were more resistant to downy mildew (Peronospora manshurica) and Soybean mosaic virus compared with control plants. Silencing GmMEKK1 reduced the activation of GmMPK6 but enhanced the activation of GmMPK3 in response to flg22 peptide. Unlike Arabidopsis MPK4, GmMPK4 was not activated by either flg22 or SA. Interestingly, transient overexpression of GmMEKK1 in Nicotiana benthamiana also induced HR. Our results indicate that GmMEKK1 plays both positive and negative roles in immunity and appears to differentially activate downstream MPKs by promoting GmMPK6 activation but suppressing GmMPK3 activation in response to flg22. The involvement of GmMPK4 kinase activity in cell death and in flg22- or SA-triggered defense responses in soybean requires further investigation

SAG101 Forms a Ternary Complex with EDS1 and PAD4 and Is Required for Resistance Signaling against Turnip Crinkle Virus

EDS1, PAD4, and SAG101 are common regulators of plant immunity against many pathogens. EDS1 interacts with both PAD4 and SAG101 but direct interaction between PAD4 and SAG101 has not been detected, leading to the suggestion that the EDS1-PAD4 and EDS1-SAG101 complexes are distinct. We show that EDS1, PAD4, and SAG101 are present in a single complex in planta. While this complex is preferentially nuclear localized, it can be redirected to the cytoplasm in the presence of an extranuclear form of EDS1. PAD4 and SAG101 can in turn, regulate the subcellular localization of EDS1. We also show that the Arabidopsis genome encodes two functionally redundant isoforms of EDS1, either of which can form ternary complexes with PAD4 and SAG101. Simultaneous mutations in both EDS1 isoforms are essential to abrogate resistance (R) protein-mediated defense against turnip crinkle virus (TCV) as well as avrRps4 expressing Pseudomonas syringae. Interestingly, unlike its function as a PAD4 substitute in bacterial resistance, SAG101 is required for R-mediated resistance to TCV, thus implicating a role for the ternary complex in this defense response. However, only EDS1 is required for HRT-mediated HR to TCV, while only PAD4 is required for SA-dependent induction of HRT. Together, these results suggest that EDS1, PAD4 and SAG101 also perform independent functions in HRT-mediated resistance

Plant Antioxidants Affect Human and Gut Health, and Their Biosynthesis Is Influenced by Environment and Reactive Oxygen Species

Many environmental stresses cause an increase in reactive oxygen species in plants and alter their nutritional value. Plants respond to many stresses by producing increased amounts of compounds with antioxidant properties including vitamins, phenylpropanoids and carotenoids. Such compounds have wide-ranging health-promoting effects in humans that are partly due to their antioxidant function because oxidative stress underlies many human diseases. Some of these compounds have complex interactions with the gut, promoting gut health and changing the gut microbiome, whereas the gut influences the bioavailability of the ingested compounds and may metabolize them into products with different effects on health than the original compound. Substantial efforts have been made to increase the nutritional value of crops through breeding or transgenic approaches, but comparatively little effort has been directed towards increasing nutritional value through crop management and environment, which may present another approach to enhance the nutritional quality