Secretion and regulation Hrp mutants of Erwinia amylovora trigger different responses in apple

The interaction between two Hrp mutants of Erwinia amylovora, the parental virulent strain, and a host plant (Malus sp.) was studied with apple seedlings and apple flowers, using inoculation procedures which reduce the possibility of injury. We showed that one hrp regulatory mutant protected apple tissue against the virulent strain to a greater degree than a secretion mutant. The dynamics of bacterial growth in this interaction indicated that the protection was associated with the inhibition of the multiplication of the virulent strain. The activity of two enzymes involved in plant defence responses, phenylalanine-ammonia lyase and guaiacol peroxidase, increased in leaves treated with the regulatory mutant, but not in the leaves treated with the secretion mutant. The role of regulatory genes of the hrp cluster in this interaction is discussed.     

Involvement of cytosolic ascorbate peroxidase and Cu/Zn-superoxide dismutase for improved tolerance against drought stress

In order to understand the role of cytosolic antioxidant enzymes in drought stress protection, transgenic tobacco (Nicotiana tabacum cv. Xanthi) plants overexpressing cytosolic Cu/Zn-superoxide dismutase (cytsod) (EC 1.15.1.1) or ascorbate peroxidase (cytapx) (EC 1.11.1.1) alone, or in combination, were produced and tested for tolerance against mild water stress. The results showed that the simultaneous overexpression of Cu/Znsod and apx or at least apx in the cytosol of transgenic tobacco plants alleviates, to some extent, the damage produced by water stress conditions. This was correlated with higher water use efficiency and better photosynthetic rates. In general, oxidative stress parameters, such as lipid peroxidation, electrolyte leakage, and H(2)O(2) levels, were higher in non-transformed plants than in transgenic lines, suggesting that, at the least, overexpression of cytapx protects tobacco membranes from water stress. In these conditions, the activity of other antioxidant enzymes was induced in transgenic lines at the subcellular level. Moreover, an increase in the activity of some antioxidant enzymes was also observed in the chloroplast of transgenic plants overexpressing cytsod and/or cytapx. These results suggest the positive influence of cytosolic antioxidant metabolism on the chloroplast and underline the complexity of the regulation network of plant antioxidant defences during drought stress.     

Modulation of defense responses of Malus spp. during compatible and incompatible interactions with Erwinia amylovora

Erwinia amylovora is the causal agent of fire blight, a disease affecting members of subfamily Maloideae. In order to analyze mechanisms leading to compatible or incompatible interactions, early plant molecular events were investigated in two genotypes of Malus with contrasting susceptibility to fire blight, after confrontation with either E. amylovora or the incompatible tobacco pathogen Pseudomonas syringae pv. tabaci. Many defense mechanisms, including generation of an oxidative burst and accumulation of pathogenesis-related proteins, were elicited in both resistant and susceptible genotypes by the two pathogens at similar rates and according to an equivalent time course. This elicitation was linked with the functional hypersensitive reaction and pathogenicity (hrp) cluster of E. amylovora, because an hrp secretion mutant did not induce such responses. However, a delayed induction of several genes of various branch pathways of the phenylpropanoid metabolism was recorded in tissues of the susceptible genotype challenged with the wild-type strain of E. amylovora, whereas these genes were quickly induced in every other plant-bacteria interaction, including interactions with the hrp secretion mutant. This suggests the existence of hrp-independent elicitors of defense in the fire blight pathogen as well as hrp-dependant mechanisms of suppression of these nonspecific inductions.     

Genetic diversity and population structure of Pepino mosaic virus in tomato crops of Spain and Morocco

Pepino mosaic virus (PepMV, genus Potexvirus) is an emergent and highly infectious pathogen responsible for economically important diseases in tomato crops. An extensive survey of tomato plants showing PepMV‐like symptoms was carried out in 2017 to study the PepMV genetic diversity and populations structure in different tomato‐producing areas of Spain and Morocco. Molecular dot‐blot hybridization analysis showed that virus populations from Spain and Morocco were mainly composed of isolates belonging to the Chilean 2 (CH2) strain, although isolates of the European (EU) strain were detected in significant proportions in Spanish populations, mainly in mixed infections. A few isolates of the American (US1) strain were also detected in Tenerife (Canary Islands, Spain) crops. Eighty‐five isolates were randomly selected and sequenced in the genomic region that encodes the triple gene block and capsid protein genes. Our phylogenetic and population genetics analyses confirmed the presence of the CH2, EU and US1 PepMV strains. Despite the high genetic similarity observed within populations, variants were maintained at low frequency under purifying selection, and differentiation among more geographically distant locations was identified, with potential gene flow contributing to the shaping of the PepMV populations structure.     

Using quantitative real-time PCR to detect chimeras in transgenic tobacco and apricot and to monitor their dissociation

Background  

The routine generation of transgenic plants involves analysis of transgene integration into the host genome by means of Southern blotting. However, this technique cannot distinguish between uniformly transformed tissues and the presence of a mixture of transgenic and non-transgenic cells in the same tissue. On the other hand, the use of reporter genes often fails to accurately detect chimerical tissues because their expression can be affected by several factors, including gene silencing and plant development. So, new approaches based on the quantification of the amount of the transgene are needed urgently.     

Priming of Nicotiana benthamiana antioxidant defences using brown seaweed extracts

Extracts from the brown seaweeds Cystoseira myriophylloides, Laminaria digitata and Fucus spiralis were evaluated as plant defence inducers against the wild fire disease of Nicotiana benthamiana. Seeds’ imbibition in aqueous seaweed extracts (SE) results in plants with reduced disease severity. In addition, bacterial populations were significantly reduced in these plants when compared to those germinated in sterile distilled water. They primed N. benthamiana for H₂O₂ accumulation and for enhanced activity of several antioxidant enzymes including catalase, ascorbate peroxidase, guaiacol peroxidase and polyphenol oxidase. These results revealed that soaking of seeds in SE before sowing allows N. benthamiana to reduce pathogen attack and can be easily applied in practice.