Genetic and Physiological Characterization of Gramillin Sensitivity in Barley

Abstract

Microbes produce ionophores, metabolites which facilitate ion transport across membranes, to disrupt host membranes and cellular function. Plant hosts can perceive ionophore-based disruption, and some genotypes have developed ionophore insensitivity. The basis of ionophore insensitivity is unclear but could involve avoidance of membrane interaction with the ionophore, ionophore degradation or membrane recovery. Here, we describe the insensitivity of the Lowe barley against the gramillin ionophore, a virulence factor produced by Fusarium graminearum that induces host stress responses which then promote fungal secondary metabolite gene expression during infection. During barley leaf infection, higher F. graminearum trichothecene and culmorin biosynthetic gene expression was observed in gramillin-sensitive barley compared to Lowe. Lowe degrades gramillin to a similar extent as gramillin-sensitive genotypes and can induce callose production and transcriptional reprogramming in response to gramillin. However, gramillin-induced stress responses are smaller in Lowe compared to gramillin-sensitive barley genotypes. At 4 hours after gramillin treatment, only 1,175 genes are differentially expressed in Lowe compared to 3,885 in gramillin-sensitive Leo barley. Gramillin-induced callose production and defense gene induction are smaller in Lowe compared to sensitive genotypes. Inducible stress responses were not required for gramillin insensitivity in Lowe. Together, this indicates that gramillin can disrupt membranes in Lowe to induce stress responses and that Lowe insensitivity is unlikely due to complete avoidance or gramillin degradation. Instead, the toxicity of gramillin is attenuated in Lowe which aligns with lower induction of F. graminearum secondary metabolite biosynthetic genes during infection.