The chaperones, heat shock proteins (HSPs), stabilize proteins to minimize proteotoxic stress, especially during heat stress (HS) and polyamine (PA) oxidases (PAOs) participate in the modulation of the cellular homeostasis of PAs and reactive oxygen species (ROS). An interesting interaction of HSP90s and PAOs was revealed in Arabidopsis thaliana by using the pLFY:HSP90RNAi line against the four AtHSP90 genes encoding cytosolic proteins, the T-DNA Athsp90-1 and Athsp90-4 insertional mutants, the Atpao3 mutant and pharmacological inhibitors of HSP90s and PAOs. Silencing of all cytosolic HSP90 genes resulted in several-fold higher levels of soluble spermidine (S-Spd), acetylated Spd (N8-acetyl-Spd) and acetylated spermine (N1-acetyl-Spm) in the transgenic Arabidopsis thaliana leaves. Heat shock induced increase of soluble-PAs (S-PAs) and soluble hydrolyzed-PAs (SH-PAs), especially of SH-Spm, and more importantly of acetylated Spd and Spm. The silencing of HSP90 genes or pharmacological inhibition of the HSP90 proteins by the specific inhibitor radicicol, under HS stimulatory conditions, resulted in a further increase of PA titers, N8-acetyl-Spd and N1-acetyl-Spm, and also stimulated the expression of PAO genes. The increased PA titers and PAO enzymatic activity resulted in a profound increase of PAO-derived hydrogen peroxide (H2O2) levels, which was terminated by the addition of the PAO-specific inhibitor guazatine. Interestingly, the loss-of-function Atpao3 mutant exhibited increased mRNA levels of selected AtHSP90 genes. Taken together, the results herein reveal a novel function of HSP90 and suggest that HSP90s and PAOs cross-talk to orchestrate PA acetylation, oxidation, and PA/H2O2 homeostasis.

In spring 2009 and 2010 significant bacterial disease symptoms were observed in potato (Solanum tuberosum cv.Spunta) crops in the Messara plain of Crete in Greece. Affected plants had blackleg and rotting symptoms on stem base, brown discolouration of vascular tissues, wilting symptoms on the foliage and soft rot in daughter tubers (Fig. 1). Incidence varied from 5%-50%. Isolations were made from soft-rotted tubers and plants with typical blackleg symptoms on crystal violet pectate (CVP) and nutrient agar glucose (NAG) media. Single bacterial colonies having characteristic pectinolytic activity (CVP) and/or "fried egg" colonies (NAG) (Fig. 2) were subcultured, purified and used for further characterisation. Twenty isolates were initially characterised as Dickeya sp. (syn: Erwinia chrysanthemi, Pectobacterium chrysanthemi). They were Gram-negative rods, facultative anaerobes, oxidase negative, sensitive to erythromycin (15 μg/disk), positive for phosphatase and indole production; grew at 37°C, negative for acid production from α-methylglucoside and trehalose; and caused soft rot in potato tuber slices (Sławiak et al., 2009). Six isolates were selected for further analysis and characterised as Dickeya sp. biovar 3. They grew on arabinose, melibiose, raffinose, mannitol and inulin but did not utilise tartrate or hydrolyse arginine under anaerobic conditions, and grew poorly at 39°C (Laurila et al., 2010). These strains were biochemically identical to the reference strain Dickeya sp. IPO2222 (kindly provided by Dr. van der Wolf) while biochemically distinct from the reference strain Dickeya dianthicola BPIC2098.Molecular characterisation was performed on four isolates (Ds3386, Ds3378, Ds3400, Ds3405) using BOX-and ERIC-PCR fingerprinting (Fig. 3), as well as dnaX gene sequencing (Sławiak et al., 2009). The obtained dnaX sequences have been deposited in GenBank (Accession Nos. JN663794 to JN663803). Results from BOX-ERIC profiling and dnaX sequencing revealed identical profiles for the isolated stains and the reference strain Dickeya sp. IPO2222; while they were clearly distinct from the related species D. dianthicola BPIC2098, P. carotovorum TEIC3036, P. atrosepticum TEIC3211 and other Erwinia spp. (Fig. 3).Pathogenicity assays were performed on potato (cv. Spunta) either by injecting 20 μl bacterial suspension (10 7 cfu/ml) into the stem or by stabbing a tuber at the stolon end with a toothpick charged with bacterial growth. Typical disease symptoms were observed within 10 days (Fig. 2). Re-isolated strains had an identical profile with the inoculated strains and the reference strain Dickeya sp. IPO2222. This is the first report of potato blackleg caused by Dickeya sp. biovar 3 in Greece. In the last five years, Dickeya sp. has caused economic losses of up to €30M annually to the production of potato propagation material in the Netherlands (Toth et al., 2011). Further spread of the pathogen in potato production in Greece is expected to have a high economic impact. ReferencesLaurila J, Hannukkala A, Nykyri J, Pasanen...

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Tomato bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis (Cmm) (Li et al. 2018), is a common destructive disease worldwide. The bacterium colonizes plant’s vascular system causing characteristic symptoms such as wilting, cankers, brown and black discoloration of vessels, white spots on the fruit and leaves. In the present work, isolates were collected from infected tomato plants of 11 Greek Prefectures during the years 2003-2018, and the genetic variability was investigated. After initial identification, 93 selected Cmm strains were subjected to phylogenetic analyses based on the sequencing of multiple loci (Multi-Locus Sequencing Analysis, MLSA) utilizing four housekeeping genes (atpD, ppk, kdpA and sdhA). MLSA highlighted 4.1% variability in nucleotide sequences and classified the 93 Cmm strains into 38 haplotypes. Nevertheless, the phylogenetic grouping of the strains was not in accordance with the geographical distribution, a fact that probably advocates the introduction of the disease from infected propagating material. However, strains isolated from the same area in a different year were placed in the same group which means that the pathogen can remain active between growing seasons. The study provides important epidemiological information on the disease outbreaks and the spread of Cmm in Greece.

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