Summary• Chitosan, a component of the cell wall of many fungi, has been widely used to mimic pathogen attack and has been shown to induce several defence responses.• Here we show that low concentrations (50 µg ml − 1 ) of chitosan are able to induce an increase in cytosolic Ca 2+ concentration ([Ca 2+ ] cyt ), accumulation of H 2 O 2 in the culture medium, induction of the defence gene chalcone synthase ( chs ), and cell death in soybean cells ( Glycine max ).• Chitosan-induced cell death occurred through cytoplasmic shrinkage, chromatin condensation and activation of caspase 3-like protease, suggesting the activation of a programmed cell death (PCD) pathway. Buffering extracellular Ca 2+ with the Ca 2+ chelator EGTA prevents [Ca 2+ ] cyt elevation, H 2 O 2 production and all downstream PCD features, but not cell death.• Higher doses (200 µg ml − 1 ) of chitosan evoked neither Ca 2+ transient and H 2 O 2 production nor caspase 3-like activation, but caused cell death, possibly as a result of plasma membrane disturbance.

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Summary• Legume-rhizobium symbiosis requires a complex dialogue based on the exchange of diffusible signals between the partners. Compatible rhizobia express key nodulation (nod) genes in response to plant signals -flavonoids -before infection. Host plants sense counterpart rhizobial signalling molecules -Nod factorsthrough transient changes in intracellular free-calcium. Here we investigate the potential involvement of Ca 2+ in the symbiotic signalling pathway activated by flavonoids in Rhizobium leguminosarum bv. viciae.• By using aequorin-expressing rhizobial strains, we monitored intracellular Ca 2+ dynamics and the Ca 2+ dependence of nod gene transcriptional activation.• Flavonoid inducers triggered, in R. leguminosarum, transient increases in the concentration of intracellular Ca 2+ that were essential for the induction of nod genes. Signalling molecules not specifically related to rhizobia, such as strigolactones, were not perceived by rhizobia through Ca 2+ variations. A Rhizobium strain cured of the symbiotic plasmid responded to inducers with an unchanged Ca 2+ signature, showing that the transcriptional regulator NodD is not directly involved in this stage of flavonoid perception and plays its role downstream of the Ca 2+ signalling event.• These findings demonstrate a key role played by Ca 2+ in sensing and transducing plant-specific flavonoid signals in rhizobia and open up a new perspective in the flavonoid-NodD paradigm of nod gene regulation.

The first identification and characterization of a catalytic activity associated with NadR protein is reported. A computer-aided search for sequence similarity revealed the presence in NadR of a 29-residue region highly conserved among known nicotinamide mononucleotide adenylyltransferases. The Escherichia coli nadR gene was cloned into a T7-based vector and overexpressed. In addition to functionally specific DNA binding properties, the homogeneous recombinant protein catalyzes NAD synthesis from nicotinamide mononucleotide and ATP.

In order to ascertain the presence and the in muro localization of cellulose, the enzyme-gold affinity test was applied to algal cell walls. The high specificity of affinity cytochemistry allowed us, by using the enzyme cellulase, to confirm the available biochemical data and to give a map of the cellulose localization in different algal groups. Taking into account the complex skeletal polysaccharide structure and composition of the algal cell walls, this method proved to be a reliable tool in this field