Heat Stress Pre-Exposure May Differentially Modulate Plant Defense to Powdery Mildew in a Resistant and Susceptible Barley Genotype

Abstract

Heat stress negatively affects barley production and under elevated temperatures defense responses to powdery mildew (<i>Blumeria graminis</i> f. sp. <i>hordei,</i> Bgh) are altered. Previous research has analyzed the effects of short-term (30 s to 2 h) heat stress, however, few data are available on the influence of long-term exposure to heat on powdery mildew infections. We simultaneously assessed the effects of short and long term heat pre-exposure on resistance/susceptibility of barley to Bgh, evaluating powdery mildew infection by analyzing symptoms and Bgh biomass with RT-qPCR in barley plants pre-exposed to high temperatures (28 and 35 °C from 30 s to 5 days). Plant defense gene expression after heat stress pre-exposure and inoculation was also monitored. Our results show that prolonged heat stress (24, 48 and 120 h) further enhanced Bgh susceptibility in a susceptible barley line (MvHV118-17), while a resistant line (MvHV07-17) retained its pathogen resistance. Furthermore, prolonged heat stress significantly repressed the expression of several defense-related genes (<i>BAX inhibitor-1</i>, <i>Pathogenesis related-1b</i> and <i>Respiratory burst oxidase homologue F2</i>) in both resistant and susceptible barley lines. Remarkably, heat-suppressed defense gene expression returned to normal levels only in MvHV07-17, a possible reason why this barley line retains Bgh resistance even at high temperatures.