Summary• The role of abscisic acid (ABA) during the establishment of the arbuscular mycorrhiza (AM) was studied using ABA sitiens tomato ( Lycopersicon esculentum ) mutants with reduced ABA concentrations.• Sitiens plants and wild-type (WT) plants were colonized by Glomus intraradices . Trypan blue and alkaline phosphatase histochemical staining procedures were used to determine both root colonization and fungal efficiency. Exogenous ABA and silver thiosulfate (STS) were applied to establish the role of ABA and putative antagonistic cross-talk between ABA and ethylene during AM formation, respectively.• Sitiens plants were less susceptible to the AM fungus than WT plants. Microscopic observations and arbuscule quantification showed differences in arbuscule morphology between WT and sitiens plants. Both ABA and STS increased susceptibility to the AM fungus in WT and sitiens plants. Fungal alkaline phosphate activity in sitiens mutants was completely restored by ABA application.• The results demonstrate that ABA contributes to the susceptibility of tomato to infection by AM fungi, and that it seems to play an important role in the development of the complete arbuscule and its functionality. Ethylene perception is crucial to AM regulation, and the impairment of mycorrhiza development in ABA-deficient plants is at least partly attributable to ethylene.

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Acetylene reduction activity (ARA) and leghemoglobin (Lb) content in nodules were sigificantly reduced when pea (Pisum sativum L. cv. Lincoln) plants were subjected to 50 mM sodium chloride stress for 3 weeks. C2H2 reduction activity by bacteriods isolated from pea nodules was drastically inhibited by saline stress, and malate appeared to be a more appropriate substrate than glucose or succinate in maintaining this activity. Salt added directly to the incubation mixture of bacteriods or to the culture medium of plants inhibited O2 uptake by bacteroids. Nodule cytosolic phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and bacteriod malate dehydrogenase (MDH; EC 1.1.1.37) activities were strongly enhanced by salt stress. Under these conditions, malate concentration was depressed in bacteroids and cytosol, whereas total soluble sugar (TSS)content slightly increased in both fractions. The effect of salt stress on TSS and malate content suggests that the utilization of carbohydrate within nodules could be inhibited during salt stress. The inhibitory effect of NaCl on N2 fixation activity of bacteroids and to the decrease in bacteroid respiration. The stimulation of fermentative metabolism induced by salinity suggests some reduction in O2 availability within the nodule. Salt stress was also responsible for a decrease of the cytosolic protein content, specifically of leghemoglobin, in the nodules.