Optimization of Ustilago nuda Inoculum Concentration for Screening Un8-Mediated Loose Smut Resistance in Barley Reveals a Resistance Reaction that Disrupts Seed Germination and Suggests a Role for Abscisic Acid in Disease Development.

Barley loose smut has been effectively controlled for decades through resistance conferred by the Un8 gene. However, evaluation of loose smut reaction using floret inoculation at the standard inoculum concentration is associated with the production of small, discolored seeds in Un8 carriers and susceptible genotypes. Interestingly, Un8 carriers also displayed significantly poorer germination than susceptible genotypes and produce short-lived seedlings following inoculation. To understand these observations, a Un8 carrier (TR11698) and susceptible non-Un8 carrier (CDC Austenson) were assessed for seed traits, Ustilago nuda biomass in the seed, infection rate, and phytohormone profile across a range of lower inoculum concentrations. At lower inoculum concentrations, seed appearance and weight improved in both genotypes, and infection rate increased in CDC Austenson. Pathogen load in the seed was similar in both genotypes and was positively correlated with the CDC Austenson infection rate. No infection was ever observed in TR11698. Significantly, germination rate improved in CDC Austenson, whereas the very low germination rate and short-lived seedlings remained associated with TR11698. It appears that poor seed appearance in both genotypes and low germination rate in the susceptible genotype can be improved by lowering the inoculum concentration. However, the very low germination rates and seedling death associated with the Un8 carrier TR11698 are indicative of Un8-mediated resistance to loose smut. Finally, profiling of 38 phytohormones revealed that larger seeds observed at some inoculum concentrations compared with mock inoculation had higher abscisic acid concentrations. This could represent a pathogen survival strategy by ensuring better growth of the host.