Changes in the level of enzyme activities involved in lignin biosynthesis during the temperature sensitive resistant response of wheat (Sr6) to stem rust (P6)

Near-isogenic lines of the wheat cultivar Marquis, carrying either the Sr6-gene for resistance or the corresponding sr6-allele for susceptibility were grown either at 18°C or at 27°C. Seven-day-old plants were inoculated with uredospores of race 32 of the stem rust fungus Puccinia graminis f. sp. tritici (P6). At different times after inoculation, activities of the enzymes phenylalanine ammonia-lyase (PAL), 4-coumarate:CoA ligase (4CL), and peroxidases (PO) were determined. In the incompatible Sr6/P6 interaction at 18°C, increases in these enzyme activities were detected at the time of the hypersensitive resitant response. In contrast, in the compatible interactions, Sr6/P6 at 27°C and sr6/P6 at both temperatures, activities of PAL and 4CL rapidly increased at the time of beginning sporulation, whereas PO activities remained low. When plants were treated with an elicitor derived from stem rust germlings both isolines exhibited typical symptoms of the resistant reaction, i.e., chloroses, necroses, yellow autofluorescence under UV-light, and increase in PAL activity, at both temperatures. At 18°C, the Sr6/P6 interaction, thus, shows changes in lignin biosynthetic enzyme activities which are typical for highly incompatible interactions; while at 27°C, these changes reflect the fully compatible nature of this same interaction.