Role of Arginine decarboxylase (ADC) in Arabidopsis thaliana defence against the pathogenic bacterium Pseudomonas viridiflava

Polyamine biosynthesis starts with putrescine production through the decarboxylation of arginine or ornithine. In Arabidopsis thaliana, putrescine is synthesised exclusively by arginine decarboxylase (ADC), which exists as two isoforms (ADC1 and 2) that are differentially regulated by abiotic stimuli, but their role in defence against pathogens has not been studied in depth. This work analysed the participation of ADC in Arabidopsis defence against Pseudomonas viridiflava. ADC activity and expression, polyamine levels and bacterial resistance were analysed in null mutants of each ADC isoform. In non-infected wild-type (WT) plants, ADC2 expression was much higher than ADC1. Analysis of adc mutants demonstrated that ADC2 contributes to a much higher extent than ADC1 to basal ADC activity and putrescine biosynthesis. In addition, adc2 mutants showed increased basal expression of salicylic acid- and jasmonic acid-dependent PR genes. Bacterial infection induced putrescine accumulation and ADC1 expression in WT plants, but pathogen-induced putrescine accumulation was blocked in adc1 mutants. Results suggest a specific participation of ADC1 in defence, although basal resistance was not decreased by dysfunction of either of the two ADC genes. In addition, and as opposed to WT plants, bacterial infection increased ADC2 expression and ADC activity in adc1 mutants, which could counterbalance the lack of ADC1. Results demonstrate a major contribution of ADC2 to total ADC activity and the specific induction of ADC1 in response to infection. A certain degree of functional redundancy between the two isoforms in relation to their contribution to basal resistance is also evident.

Cheese whey: an alternative growth and protective medium for Rhizobium loti cells

Cheese whey (CW)-based growth medium efficiently protects Rhizobium loti cells during freezing and desiccation and can maintain their growth in a manner similar to that of traditional mannitol-based medium (YEM). The cheese-whey-based medium (CW) improved viability when used to re-suspend cell pellets kept at -20 degrees C and -80 degrees C and resulted in the survival of over 90% of the cells. Moreover, bacterial pellets obtained from cells grown in CW withstand desiccation better than cells grown in YEM. Survival was over 60% after 30 days at 4 degrees C. No differences were observed in nodulation efficiency between YEM-grown and CW-grown cells. Fast protein liquid chromatography (FPLC) protocols are presented for total protein profile analyses of sweet and acid cheese whey.

The effect of polyamine biosynthesis inhibition on growth and differentiation of the phytopathogenic fungus Sclerotinia sclerotiorum

We studied the effects of several polyamine biosynthesis inhibitors on growth, differentiation, free polyamine levels and in vivo and in vitro activity of polyamine biosynthesis enzymes in Sclerotinia sclerotiorum. Alpha-Difluoromethylornithine (DFMO) and alpha-difluoromethylarginine (DFMA) were potent inhibitors of mycelial growth. The effect of DFMO was due to inhibition of ornithine decarboxylase (ODC). No evidence for the existence of an arginine decarboxylase (ADC) pathway was found. The effect of DFMA was partly due to inhibition of ODC, presumably after its conversion into DFMO by mycelial arginase, as suggested by the high activity of this enzyme detected both in intact mycelium and mycelial extracts. In addition, toxic effects of DFMA on cellular processes other than polyamine metabolism might have occurred. Cyclohexylamine (CHA) slightly inhibited mycelial growth and caused an important decrease of free spermidine associated with a drastic increase of free putrescine concentration. Methylglyoxal bis-[guanyl hydrazone] (MGBG) had no effect on mycelial growth. Excepting MGBG, all the inhibitors strongly decreased sclerotial formation. Results demonstrate that sclerotial development is much more sensitive to polyamine biosynthesis inhibition than mycelial growth. Our results suggest that mycelial growth can be supported either by spermidine or putrescine, while spermidine (or the putrescine/spermidine ratio) is important for sclerotial formation to occur. Ascospore germination was completely insensitive to the inhibitors.