Microwounding is a pivotal factor for the induction of actin-dependent penetration resistance against fungal attack

Development of fusarium wilt resistant mutants of Musa spp. cv.Rasthali (AAB, Silk subgroup) and comparative proteomic analysis along with its wild type

Induced mutagenesis using embryogenic cell suspension (ECS) explants with toxin based screening is an effective tool to create non-chimeral Fusarium wilt resistant mutants in banana. Global proteomics unravel the molecular mechanism behind resistance. Race 1 of Fusarium wilt is a serious threat to Musa spp. cv.Rasthali (AAB, Silk subgroup) which is a choice variety traditionally grown in most of the south East Asian countries. Resistant gene introgression into susceptible varieties through conventional breeding has several limitations and the predominant ones being sterility and long generation time. Under such circumstances, induced mutagenesis combined with toxin based in vitro screening remains as the viable alternative for the development of fusarium wilt resistant Rasthali. Therefore, induced mutagenesis was attempted by using ethylmethane sulfonate (EMS) in embryogenic cell suspension (ECS) of Rasthali followed by in vitro screening for fusarium wilt resistance using new generation toxins and pot screening through challenge inoculation with Foc race 1. This ultimately resulted in the identification of 15 resistant lines. Global proteomic analysis in one of the resistant mutant lines namely NRCBRM15 and its wild type revealed 37 proteins, of which 20 showed differential expression. Out of 20 proteins, nineteen were significantly abundant in NRCBRM15 and only one was abundant in wild Rasthali. A total of nine genes based on protein expression were further validated using quantitative real time polymerase chain reaction (qRT-PCR). Annotation results revealed that some of the genes namely Enolase, ATP synthase-alpha subunit, Actin 2, Actin 3,-glucanase, UTP-glucose-1-phosphate uridylyltransferase, Respiratory burst oxidase homolog, V type proton ATPase catalytic subunit A and DUF292 domain containing protein are involved in diverse functions such as carbohydrate metabolism, energy production, electron carrier, response to wounding, binding proteins, cytoskeleton organization, extracellular region, structural molecule and defense.

From filaments to function: The role of the plant actin cytoskeleton in pathogen perception, signaling and immunity

The eukaryotic actin cytoskeleton is required for numerous cellular processes, including cell shape, development and movement, gene expression and signal transduction, and response to biotic and abiotic stress. In recent years, research in both plants and animal systems have described a function for actin as the ideal surveillance platform, linking the function and activity of primary physiological processes to the immune system. In this review, we will highlight recent advances that have defined the regulation and breadth of function of the actin cytoskeleton as a network required for defense signaling following pathogen infection. Coupled with an overview of recent work demonstrating specific targeting of the plant actin cytoskeleton by a diversity of pathogens, including bacteria, fungi and viruses, we will highlight the importance of actin as a key signaling hub in plants, one that mediates surveillance of cellular homeostasis and the activation of specific signaling responses following pathogen perception. Based on the studies highlighted herein, we propose a working model that posits changes in actin filament organization is in and of itself a highly specific signal, which induces, regulates and physically directs stimulus-specific signaling processes, most importantly, those associated with response to pathogens.

Proton flows across the plasma membrane in microperforated characean internodes: tonoplast injury and involvement of cytoplasmic streaming

Microperforation of characean cell wall with a glass micropipette in the absence of the tonoplast impalement was found to cause rapid alkalinization of the apoplast by 2-3 pH units, which may rigidify the cell wall structure, thus protecting the cell from further injury. A similar but a deeper insertion of a microneedle, associated with piercing the tonoplast and with an action potential generation, led to a considerable delay in the apoplast alkalinization without affecting the amplitude of the eventual increase in pH. The retardation by the mechanically elicited action potential of the incision-mediated pH transients in the apoplast contrasted sharply to the enhancement of these pH transients by the action potential triggered electrically before the microperforation. Hence, the delay of the apoplast alkalinization was not related to basic ionic mechanisms of plant action potentials. Measurements of the vacuolar pH after mechanical elicitation of an action potential indicate that the tonoplast piercing was accompanied by leakage of protons from the vacuole into the cytoplasm, which may strongly acidify the cytoplasm around the wounded area, thus collapsing the driving force for H(+) influx from the medium into the cytoplasm. The lag period preceding the onset of external alkalinization was found linearly related to the duration of temporal cessation of cytoplasmic streaming. The results suggest that the delayed alkalinization of the apoplast in the region of tonoplast wounding reflects the localized recovery of the proton motive force across the plasmalemma during replacement of the acidic cytoplasm with fresh portions of unimpaired cytoplasm upon restoration of cytoplasmic streaming.

Actin marker lines in grapevine reveal a gatekeeper function of guard cells

Resistance to abiotic and biotic stress is a central topic for sustainable agriculture, especially in grapevine, one of the field crops with the highest economic output per acreage. As early cellular factors for plant defense, actin microfilaments (AF) are of high relevance. We therefore generated a transgenic actin marker line for grapevine by expressing a fusion protein between green fluorescent protein and the second actin-binding domain of Arabidopsis (Arabidopsis thaliana) fimbrin, AtFIM1. Based on this first cytoskeletal-marker line in grapevine, the response of AFs to phytopathogenic microorganisms could be followed in vivo. Upon inoculation with fluorescently labeled strains of phytopathogenic bacteria, actin responses were confined to the guard cells. In contrast, upon contact with zoospores of Plasmopara viticola, not only the guard cells, but also epidermal pavement cells, where no zoospores had attached responded with the formation of a perinuclear actin basket. Our data support the hypothesis that guard cells act as pacemakers of defense, dominating the responses of the remaining epidermal cells.

Rice OsPAD4 functions differently from Arabidopsis AtPAD4 in host-pathogen interactions

The extensively studied Arabidopsis phytoalexin deficient 4 (AtPAD4) gene plays an important role in Arabidopsis disease resistance; however, the function of its sequence ortholog in rice is unknown. Here, we show that rice OsPAD4 appears not to be the functional ortholog of AtPAD4 in host-pathogen interactions, and that the OsPAD4 encodes a plasma membrane protein but that AtPAD4 encodes a cytoplasmic and nuclear protein. Suppression of OsPAD4 by RNA interference (RNAi) increased rice susceptibility to the biotrophic pathogen Xanthomonas oryzae pv. oryzae (Xoo), which causes bacteria blight disease in local tissue. OsPAD4-RNAi plants also show compromised wound-induced systemic resistance to Xoo. The increased susceptibility to Xoo was associated with reduced accumulation of jasmonic acid (JA) and phytoalexin momilactone A (MOA). Exogenous application of JA complemented the phenotype of OsPAD4-RNAi plants in response to Xoo. The following results suggest that OsPAD4 functions differently than AtPAD4 in response to pathogen infection. First, OsPAD4 plays an important role in wound-induced systemic resistance, whereas AtPAD4 mediates systemic acquired resistance. Second, OsPAD4-involved defense signaling against Xoo is JA-dependent, but AtPAD4-involved defense signaling against biotrophic pathogens is salicylic acid-dependent. Finally, OsPAD4 is required for the accumulation of terpenoid-type phytoalexin MOA in rice-bacterium interactions, but AtPAD4-mediated resistance is associated with the accumulation of indole-type phytoalexin camalexin.