Unmasking host and microbial strategies in the Agrobacterium-plant defense tango

Abstract

Coevolutionary forces drive adaptation of both plant-associated microbes and their hosts. Eloquently captured in the Red Queen Hypothesis, the complexity of each plant–pathogen relationship reflects escalating adversarial strategies, but also external biotic and abiotic pressures on both partners. Innate immune responses are triggered by highly conserved pathogen-associated molecular patterns, or PAMPs, that are harbingers of microbial presence. Upon cell surface receptor-mediated recognition of these pathogen-derived molecules, host plants mount a variety of physiological responses to limit pathogen survival and/or invasion. Successful pathogens often rely on secretion systems to translocate host-modulating effectors that subvert plant defenses, thereby increasing virulence. Host plants, in turn, have evolved to recognize these effectors, activating what has typically been characterized as a pathogen-specific form of immunity. Recent data support the notion that PAMP-triggered and effector-triggered defenses are complementary facets of a convergent, albeit differentially regulated, set of immune responses. This review highlights the key players in the plant’s recognition and signal transduction pathways, with a focus on the aspects that may limit Agrobacterium tumefaciens infection and the ways it might overcome those defenses. Recent advances in the field include a growing appreciation for the contributions of cytoskeletal dynamics and membrane trafficking to the regulation of these exquisitely tuned defenses. Pathogen counter-defenses frequently manipulate the interwoven hormonal pathways that mediate host responses. Emerging systems-level analyses include host physiological factors such as circadian cycling. The existing literature indicates that varying or even conflicting results from different labs may well be attributable to environmental factors including time of day of infection, temperature, and/or developmental stage of the host plant.

Belowground Microbiota and the Health of Tree Crops

“…On the other hand, the soil/rhizosphere microbiota also harbors a range of soil-borne plant pathogenic agents. Besides the prokaryotes Rhizobium radiobacter and R. rhizogenes ( Rhizobiaceae , Rhizobiales, Proteobacteria, formerly known as Agrobacterium tumefaciens and A. rhizogenes , respectively) capable of inducing tumor formation in many economically relevant tree crops (Hwang et al, 2015 ), the most important negative effectors of tree health in the soil microbiota are fungus-like organisms (i.e., oomycetes) and higher fungi. A brief overview of the most relevant is presented below.…”

Artificial Agrobacterium tumefaciens strains exhibit diverse mechanisms to repress Xanthomonas oryzae pv. oryzae‐induced hypersensitive response and non‐host resistance in Nicotiana benthamiana

“…Alternatively, LBA4404 and EHA105 may target other T3SS effector gene(s) or other types of gene(s) to repress Xoo ‐induced HR. Moreover, the interactions between plants and Agrobacterium are very complex (Gohlke and Deeken, ; Hwang et al ., ). Agrobacterium is able to suppress diverse plant defences at various stages during infection (Gohlke and Deeken, ; Hwang et al ., ; Veena et al ., ).…”

“…Moreover, the interactions between plants and Agrobacterium are very complex (Gohlke and Deeken, ; Hwang et al ., ). Agrobacterium is able to suppress diverse plant defences at various stages during infection (Gohlke and Deeken, ; Hwang et al ., ; Veena et al ., ). Therefore, it is also possible that these Agrobacterium strains compromise plant pathways responsible for the generation of the Xoo ‐induced ROS burst, thereby blocking the Xoo ‐induced HR in N. benthamiana .…”

“…The capacity for DNA transfer has made this bacterium the workhorse of gene transformation and transient gene expression studies in plants (Choi et al ., ; Untergasser et al ., ). The interactions between plants and A. tumefaciens are increasingly being understood (Gohlke and Deeken, ; Hwang et al ., ). Nevertheless, the role of A. tumefaciens in antibiosis to other pathogens inside the host plant is rarely documented.…”

Editorial: “Agrobacterium biology and its application to transgenic plant production”

“…The authors summarize important genomic, epigenomic, transcriptomic, and metabolomic studies that reveal epigenetic changes associated with T-DNA integration and gall development. Subsequently, Hwang et al ( 2015 ) review important pathogenic elicitors, host cell receptor molecules, and their downstream signal transduction pathways in host plants during the PAMP-triggered immune response. They highlight recent discoveries linking plant immunity to endomembrane trafficking and actin dynamic changes.…”