Warm wheat does not rust - how wheat resists stripe rust when temperatures rise.

Abstract

Stripe rust is a destructive wheat disease that is estimated to cause 5.47 million tonnes of wheat loss per year (Beddow et al., 2015). The fungus that causes the disease, Puccinia striiformis f. sp. tritici (Pst), occurs in all wheat-growing regions of the world but causes more problems in cooler climates. One reason for this is that the fungus requires relatively low temperatures for spore germination, growth, and sporulation. But some wheat plants also have a defense mechanism against Pst that only operates under high temperatures. Two types of temperature-sensitive defense mechanisms are known: high-temperature seedling plant resistance and high-temperature adult plant resistance. When grown under high-temperature conditions, wheat varieties with such a defense mechanism are resistant against Pst. Because high-temperature resistance provides immunity to all Pst races, understanding the genetic basis of this defense mechanism might be valuable for breeding. Over the last decades, breeders have developed resistant cultivars by deploying race-specific resistance genes. Race-specific resistant cultivars have limited durability because pathogens evolve quickly. In the case of Pst, new races have overcome most of the known race-specific resistance genes (Fu et al., 2009). Therefore, breeding for non-race-specific resistance such as high-temperature resistance might be a more durable solution in the fight against stripe rust. Xiaoping Hu, Professor of Plant Pathology at Northwest Agricultural and Forestry University in Yangling, China, is trying to understand the molecular mechanisms involved in high-temperature resistance in seedlings. His research group performed transcriptomic analysis on the resistant cultivar Xiaoyan 6 that had been inoculated with Pst and grown under high temperatures. Among the differentially expressed genes, they found a gene encoding a cysteine-rich receptor-like kinase that they designated TaCRK10. In this issue of The Plant Journal, the researchers investigated the role of this gene (Wang et al., 2021). To investigate if TaCRK10 is involved in resistance to Pst, the authors performed virus-induced gene silencing (VIGS) in the Xiaoyan 6 cultivar and inoculated the TaCRK10-silenced plants with Pst. When plants started to show symptoms of stripe rust, they were divided into two groups. One group was placed under high-temperature conditions (20°C) for 24 h, whereas the other group was kept at normal temperatures (16°C). In the TaCRK10-silenced plants grown under high temperatures, there was abundant spore formation on the leaves, whereas non-silenced plants showed only limited sporulation. Under normal temperatures, control plants and silenced plants did not show differences. These results suggest that TaCRK10 is required for high-temperature seedling plant resistance to Pst in the Xiaoyan 6 cultivar. Furthermore, when they overexpressed TaCRK10 from Xiaoyan 6 in the susceptible wheat cultivar Fielder, then inoculated the overexpression lines with Pst, sporulation was much lower than in wild-type plants. Interestingly, the overexpression lines were resistant against Pst when grown at normal temperatures. Further analysis of these lines revealed expression of genes involved in the salicylic acid defense pathway as well as accumulation of reactive oxygen species (ROS), which are common signs of an immunity response. To deduce the molecular action of TaCRK10, the authors performed a yeast-two-hybrid assay using a library constructed from Pst-infected leaves of the Xiaoyan 6 cultivar grown at high temperature. They found that TaCRK10 interacted with the histone variant TaH2A.1. Because TaCRK10 contains a serine/threonine kinase-like domain, the authors wondered if TaCRK10 might phosphorylate histone TaH2A.1. To test this, the researchers produced recombinantly expressed proteins and mixed them in the presence of ATP. Using an electrophoresis technique that separates phosphorylated and non-phosphorylated proteins followed by Western blot analysis, they found that TaH2A.1 was indeed phosphorylated in the presence of TaCRK10. To find out if H2A.1 is involved in high-temperature seedling plant resistance to Pst, the authors used VIGS to silence the histone variant. Subsequent inoculation experiments showed that sporulation was higher on TaH2A.1-silenced plants at both normal and high temperatures than on non-silenced plants. Hence, TaH2A.1 appears to be involved in wheat resistance to Pst (Figure). Xiaoping Hu wants to better understand the TaCRK10-TaH2A.1 pathway by identifying the phosphorylation sites on H2A.1 on which CRK10 acts and to determine if phosphorylation of TaH2A.1 plays a role in high-temperature resistance to stripe rust in seedlings. He explains that the findings might be used to develop wheat varieties with a durable resistance against Pst. Overexpression of CRK10 from Xiaoyan 6 in the susceptible variety Fielder resulted in plants that were resistant to the pathogen grown at normal temperatures, showing that susceptible cultivars can become resistant in this way, but also that constitutive overexpression of the allele leads to resistance that is no longer dependent on temperature.