SnTox5–Snn5: a novel Stagonospora nodorum effector–wheat gene interaction and its relationship with the SnToxA–Tsn1 and SnTox3–Snn3–B1 interactions

The Stagonospora nodorum–wheat interaction involves multiple pathogen‐produced necrotrophic effectors that interact directly or indirectly with specific host gene products to induce the disease Stagonospora nodorum blotch (SNB). Here, we used a tetraploid wheat mapping population to identify and characterize a sixth effector–host gene interaction in the wheat–S. nodorum system. Initial characterization of the effector SnTox5 indicated that it is a proteinaceous necrotrophic effector that induces necrosis on host lines harbouring the Snn5 sensitivity gene, which was mapped to the long arm of wheat chromosome 4B. On the basis of ultrafiltration, SnTox5 is probably in the size range 10–30 kDa. Analysis of SNB development in the mapping population indicated that the SnTox5–Snn5 interaction explains 37%–63% of the variation, demonstrating that this interaction plays a significant role in disease development. When the SnTox5–Snn5 and SnToxA–Tsn1 interactions occurred together, the level of SNB was increased significantly. Similar to several other interactions in this system, the SnTox5–Snn5 interaction is light dependent, suggesting that multiple interactions may exploit the same pathways to cause disease.     

The function and activity of certain membrane enzymes when localized on – and off – the membrane

A group of enzymes known to be involved in group translocation-type transport mechanisms for the uptake of a variety of nucleotide precursors are enzymatically active both in their natural membrane milieu and in aqueous solution. The activity in aqueous solution markedly differ, however, from the enzymatic activity when the enzyme is membrane localized. The adenine phosphoribosyltransferase (PRT) of E. coli (Hochstadt-Ozer and Stadtman, 1971 a) is capable of carrying out an exchange reaction between the base moieties of adenine and AMP without requiring P-ribose-PP as an intermediate; the enzyme in aqueous solution requires P-ribose-PP, indicating a different reaction mechanism in the two environments. Like the adenine PRT of E. coli, the hypo-xanthine PRT of Salmonella typhimurium (Jackman and Hochstadt, 1976) also carried out an exchange reaction on the membrane only and also is more sensitive to a number of inhibitors in aqueous solution relative to the sensitivity when embedded in the membrane. In addition, however, the hypoxanthine PRT, while restricted to hypoxanthine as a substrate in the membrane, also accepts guanine as substrate in its soluble form. The membrane capacities reflect the in situ capacities of the enzyme and the gain of guanine specificity was determined in a guanine PRT deletion strain (Jackman and Hochstadt, 1976). Finally, in mammalian cell lines purine nucleoside phosphorylase, which translocates the ribose moiety of inosine across the plasma membrane of mouse fibroblasts undergoes a 30-fold increase in substrate turnover number upon liberation from the membrane. These data raise two important caveats with respect to study of membrane enzymes and transport. Firstly, an enzyme once solubilized and found to differ kinetically from substrate transport in situ cannot be excluded from participating in translocations in the membrane on the basis of its activity in aqueous solution. Secondly, an enzyme which “appears” largely soluble upon cell rupture cannot be assumed to be a cycloplasmic enzyme because the majority of the solubilized activity may represent only a small fraction of the enzyme molecules highly activated concomitant to their solubilization. In this latter case the ability to activate enzyme still residing on the membrane (e.g., with detergents) would be necessary in order to estimate total membrane associated activity after cell rupture.