Temporal transcriptional changes in SAR and sugar transport-related genes during wheat and leaf rust pathogen interactions

Abstract

Leaf rust (Puccinia triticina Erikss.) is one of the most damaging pathogens of wheat (Triticum aestivum L.). With the rapid evolution of new races, worldwide distribution, and high genetic diversity, P. triticina has the ability to cause severe epidemics in wheat growing areas. In plants, salicylic acid (SA) and sugar-mediated defense pathways are expected to provide durable and broad-spectrum resistance. To understand the role of SA and sugar-mediated resistance mechanisms in wheat during early leaf rust infection, expression profiles of the key regulators of SA (TaEDS1, TaPAD4, TaNDR1, TaRAR1, TaSGT1, TaHSP90, TaEDS5, TaPAL, and TaNPR1) and sugar (TaHTP, TaSTP13A) pathways were analyzed in time-course experiments between two wheat near-isogenic lines (NILs) differing in the leaf rust resistance gene, Lr24. The quantification of candidate gene expression using reverse transcription quantitative real-time PCR at different time points post inoculation showed stage-specific transcriptional reprogramming between compatible and incompatible interactions. Interestingly, two distinct expression patterns were observed between two types of interactions. The genes acting upstream of SA in the SA pathway (TaEDS1, TaPAD4, TaNDR1, TaRAR1, TaSGT1, TaHSP90, TaEDS5) showed strong expressions at a later stage [48 h post inoculation (hpi)] of leaf rust infection in the compatible interaction compared to unchanged or slightly changed expressions in the incompatible interaction. Further, these genes showed similar expression patterns in either of the interactions, suggesting their cooperative or coordinated functions. On the other hand, the genes involved in SA biosynthesis (TaPAL), SA downstream signaling (TaNPR1), and sugar transportation (TaHTP, TaSTP13A) showed a strong expression at mid phase of infection between 6 and 24 hpi in the incompatible interaction compared to the compatible interaction. These expression patterns suggest that TaPAL and TaNPR1 play a positive regulatory role in the SA-mediated resistance pathway whereas TaHTP (Lr67) plays an important role in the sugar-mediated resistance pathway activated by the leaf rust resistance gene, Lr24.