Heat treatments at varying ambient temperatures and durations differentially affect plant defense to Blumeria hordei in a resistant and a susceptible Hordeum vulgare line

Abstract

Our previous research showed that a powdery mildew resistant barley line (MvHV07-17) maintains its resistance to Blumeria hordei (Bh) even if plants are exposed to long-term high temperature of 35 ⁰C for 120 h before Bh inoculation, while such high temperature pre-treatment further increases susceptibility to infection in the susceptible barley line MvHV118-17. In the present study, we extend this approach using short-term high-temperature water treatment (49 ⁰C 30 s) in order to determine how it affects powdery mildew resistance in these barley lines. We found that this short-term heat shock (HS) impaired plant defense responses, as reflected by development of Bh colonies and visible necrotic spots on leaves of MvHV07-17, which does not develop visible symptoms upon Bh inoculation under optimal growth conditions. In contrast, both HS and long-term heat stress enhanced susceptibility to Bh in MvHV118-17 plants. These results were supported by the measurement of Bh biomass using a qPCR method. Furthermore, microscopic examinations showed that HS elevated the rate of successful Bh penetration events, the spread of cell death in the surrounding mesophyll area and allowed colony formation and sporulation in resistant barley while early and effective plant defense responses, such as papilla formation and single-cell epidermal hypersensitive response were significantly reduced. Furthermore, we found that the accumulation of hydrogen peroxide in both resistant and susceptible barley was correlated with susceptibility induced by HS and long-term heat-stress. This study may contribute to a better understanding of plant defense responses to Bh in barley exposed to heat.