RBOHF2 of Barley Is Required for Normal Development of Penetration Resistance to the Parasitic Fungus Blumeria graminis f. sp. hordei

Abstract

Plant respiratory burst oxidase homologs are prominent sources of reactive oxygen species (ROS) in signal transduction and in interaction with microbes. However, the function of respiratory burst oxidase homologue (RBOH) genes in interaction with microbes might differ for certain plant and pathogen species. We produced transgenic barley knock down (KD) for the HvRBOHF2 isoform of NADPH oxidases. Young HvRBOHF2 KD shoots did not show obvious morphological alterations from the wild type but adult HvRBOHF2 KD plants developed fewer tillers, were less fertile, and showed spontaneous cell death in leaf mesophyll. Additionally, HvRBOHF2 KD plants were unable to contain wound-induced cell death. Before developmental failure became obvious, young HvRBOHF2 KD seedlings were much more susceptible to penetration by the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei. Strikingly, the B. graminis f. sp. hordei-induced cell-wall-associated oxidative burst was not substantially attenuated in HvRBOHF2 KD plants but enhanced susceptibility apparently influenced the subcellular site of hydrogen peroxide accumulation. Taken together, misexpression of HvRBOHF2 caused failure of barley to normally develop penetration resistance to B. graminis f. sp. hordei and to control leaf cell death.

Reactive oxygen species generation and signaling in plants

“…109,110 Rbohs are involved in defense against pathogen attacks, e.g. by activating hypersensitive responses and regulating innate immunity in plants [111][112][113][114] and extracellular lignin production (115, and ref. therein), but also in nodule formation in Medicago truncatula, 116 abiotic stress responses such as heat, cold, drought, salinity and high light.…”

“…149 Another examples is the rbohC rbohD double knockout line which is strongly impaired in apoplastic ROS production in response to pathogen attack, or the rbohC mutant impaired in root hair development due to the lack of apoplastic ROS production in the appropriate cells. 114 Considering the complex ROS scavenging network 169 and gene families involved in ROS production, large number of new mutants, mutant combinations or overexpressor lines need to be analyzed in the future. 169 There are also other problems in defining specificity in ROS signaling: For instance, transcriptional analyses from the flu mutant have shed some light on the specific effects of 1 O 2 , 26,181 but, unlike in the wild-type, the 1 O 2 production is not associated with excess excitation of the photosystem II.…”

Resistance to powdery mildew in one Spanish barley landrace hardly resembles other previously identified wild barley resistances

“…Considering the superficial hyphal growth, cell death in neighbouring mesophyll cells might be triggered by signalling factors built up within the epidermis, such as those derived from the oxidative burst, more than by intracellular ESH development (Thordal-Christensen et al 1997;Proels et al 2010). It was expected that lower HR would allow greater numbers of germlings to develop ESH and would promote mycelial development.…”

Barley

“…Transgenic barley plants were developed with stably knocked down expression of HvRBOHF2 which were unable to contain wound-induced cell death and revealed developmental alterations from the threeleaf stage onward. The results suggest that RBOHF2 is required for normal development of penetration resistance to the fungus (Proels et al 2010 ). Moreover, barley plants carrying an HvBI-1 RNA interference (RNAi) construct having lower levels of HvBI-1 respond with less susceptibility to powdery mildew than wild-type plants accompanied by enhanced resistance to penetration by B. graminis f. sp.…”