Trafficking at the host cell surface during plant immune responses

The plasma membrane (PM) of eukaryotic cells is not only an outermost covering to contain and protect inner molecules required for cell viability but also a place where communications dynamically occur with adjacent cells and environments including pathogens. However the selective permeability limits the free translocation of information across the PM between cells. Therefore, eukaryotic cells have invented an elaborate machinery to safely export and import proteins and small molecules within a membrane-wrapped container called a vesicle. Upon infection, a host plant cell also actively interacts with a phytopathogen to achieve its goal, defense to frustrate the pathogen attempt. To understand communications between pathogens and plants, hence this review is mainly focused on molecular transport events that occur at the host PM during plant immune responses.     

Vesicle trafficking in plant immune responses

In plants, perception of pathogen-associated molecular patterns at the surface is the first line of defence in cellular immunity. This review summarizes recent evidence of the involvement of vesicle trafficking in the plant's immune response against pathogens. I first discuss aspects of ligand-stimulated receptor endocytosis. The best-characterized pattern-recognition receptor (PRR), FLS2, is a transmembrane leucine-rich repeat receptor kinase that recognizes bacterial flagellin. FLS2 was recently shown to undergo internalization upon activation with its cognate ligand. An animal PRR, TLR4 that mediates perception of bacterial-derived lipopolysaccharides, similarly exhibits ligand-stimulated endocytosis. The second focus is N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-mediated immunity involving syntaxins and their cognate partners. One of the genes involved in basal immunity in Arabidopsis, PEN1, encodes a syntaxin that focally accumulates at fungal penetration sites, raising the possibility that induced exocytosis is important for active defence. Pathogen-triggered endocytic and exocytic processes have to be balanced to ensure host cell homeostasis. Thus, understanding how phytopathogens have evolved strategies to exploit host cell vesicle trafficking to manipulate immune responses is currently an area of intense study.     

Implications of endophyte-plant crosstalk in light of quorum responses for plant biotechnology

Quorum sensing, the cell-to-cell communication system mediated by autoinducers, is responsible for regulation of virulence factors, infections, invasion, colonization, biofilm formation, and antibiotic resistance within bacterial populations. Concomitantly, quorum quenching is a process that involves attenuation of virulence factors by inhibiting or degrading quorum signaling autoinducers. Survival of endophytic microorganisms, commonly known as endophytes, in planta is a continuous mêlée with invading pathogens and pests. In order to survive in their microhabitats inside plants, endophytes have co-evolved to not only utilize an arsenal of biologically active defense compounds but also impede communication between invading pathogens. Such antivirulence strategies prevent pathogens from communicating with or recognizing each other and thus, colonizing plants. The quenching phenomena often involves microbial crosstalk within single or mixed population(s) vis-à-vis gene expression, and production/modulation of quenching enzymes coupled to various antagonistic and synergistic interactions. This concept is particularly interesting because it can be biotechnologically translated in the future to quorum inhibiting antivirulence therapies without triggering resistance in bacteria, which is currently a major problem worldwide that cannot be tackled only with antimicrobial therapies. In this mini-review, we highlight the quorum quenching capacity of endophytes with respect to attenuation of virulence factors and aiding in plant defense response. Further, benefits and potential challenges of using such systems in biotechnology are discussed.     

Making sense of microarrays

A report on the 'Critical Assessment of Microarray Data Analysis' (CAMDA 2000) meeting, Durham, North Carolina, USA, December 18-19,2000.     

Investigating the functions of the RIN4 protein complex during plant innate immune responses

Pathogen recognition by the plant innate immune system invokes a sophisticated signal transduction network that culminates in disease resistance. The Arabidopsis protein RIN4 is a well-known regulator of plant immunity. However, the molecular mechanisms by which RIN4 controls multiple immune responses have remained elusive. in our recently published study, we purified components of the RIN4 protein complex from A. thaliana and identified several novel RIN4-associated proteins.¹ we found that one class of RIN4-associated proteins, the plasma membrane H⁺-ATPases AHA1 and AHA2, play a crucial role in resisting pathogen invasion. Plants use RIN4 to regulate H⁺-ATPase activity during immune responses, thereby controlling stomatal apertures during pathogen attack. Stomata were previously identified as active regulators of plant immune responses during pathogen invasion, but how the plant innate immune system coordinates this response was unknown.^2,3 Our investigations have revealed a novel function of rin4 during pathogenesis. Here, we discuss the rin4-AHA1/2 interaction and highlight additional RIN4-associated proteins (RAPs) as well as speculate on their potential roles in plant innate immunity.Key words: RIN4, PAMP-triggered immunity, effector-triggered immunity, protein complex, innate immunity     

Roles of F-box proteins in plant hormone responses

The F-box protein is an important component of the E3 ubiquitin ligase Skpl-Cullin-F-box protein complex. It binds specific substrates for ubiquitin-mediated proteolysis. The F-box proteins contain a signature F-box motif at their amino-terminus and some protein-protein interaction motifs at their carboxyterminus, such as Trp-Asp repeats or leucine rich repeats. Many F-box proteins have been identified to be involved in plant hormone response as receptors or important medial components. These breakthrough findings shed light on our current understanding of the structure and function of the various F-box proteins, their related plant hormone signaling pathways, and their roles in regulating plant development.     

Making sense of chromatin states

Researchers find new pieces in the puzzle of genome regulation.     

Sex hormone modulation of human uterine epithelial cell immune responses

Sexually transmitted infections are a major worldwide publichealth problem affecting millions of people. A number of bacteria,fungi, viruses, and protozoa can infect reproductive tissues,resulting in varying degrees of pathology ranging from littlediscomfort to death. The female reproductive tract has evolvedinnate and adaptive immune mechanisms that protect from microbialinfection, thereby reducing infection and disease. Central tothis protection are the epithelial cells that line the femalereproductive tract. In the uterus, columnar epithelial cellsprovide a physical barrier to microbial infection, possess toll-likereceptors that detect pathogens and secrete a number of constitutiveand induced factors that directly or indirectly hinder infection.For example, uterine epithelial cells secrete peptides thatdestroy pathogenic microbes. In addition, epithelial cells producechemokines and cytokines that attract and activate innate immunecells and serve as a link to the adaptive immune system. Further,uterine epithelial cells serve as a conduit for secretory antibodiesto enter the lumen and can present antigen to T cells. Theseprotective mechanisms contribute to an environment in the uterusthat is generally considered sterile, unlike the environmentin the lower female reproductive tract. The uterine environmentis in constant flux due to the concentration changes in sexhormones that occur in preparation for reproduction. The sexhormones estrogen and progesterone alter the local immune systemto prepare for conception, influence how well the immune systemwill tolerate antigenic sperm and a semi-allogeneic fetus andyet provide a network of protective immune mechanisms againstmicrobial pathogens. Understanding how sex hormones influenceuterine epithelial cell function will provide a basis for immuneprotection in the uterus.     

Making sense of cilia and flagella

Data reported at an international meeting on the sensory and motile functions of cilia, including the primary cilium found on most cells in the human body, have thrust this organelle to the forefront of studies on the cell biology of human disease.     

Making sense of lung-cancer gene-expression profiles

A report on the Critical Assessment of Microarray Data Analysis (CAMDA'03) meeting and competition, Durham, USA, 12-14 November 2003.     

The perplexing role of autophagy in plant innate immune responses

Autophagy is a major intracellular process for the degradation of cytosolic macromolecules and organelles in the lysosomes or vacuoles for the purposes of regulating cellular homeostasis and protein and organelle quality control. In complex metazoan organisms, autophagy is highly engaged during the immune responses through interfaces either directly with intracellular pathogens or indirectly with immune signalling molecules. Studies over the last decade or so have also revealed a number of important ways in which autophagy shapes plant innate immune responses. First, autophagy promotes defence‐associated hypersensitive cell death induced by avirulent or related pathogens, but restricts unnecessary or disease‐associated spread of cell death. This elaborate regulation of plant host cell death by autophagy is critical during plant immune responses to the types of plant pathogens that induce cell death, which include avirulent biotrophic pathogens and necrotrophic pathogens. Second, autophagy modulates defence responses regulated by salicylic acid and jasmonic acid, thereby influencing plant basal resistance to both biotrophic and necrotrophic pathogens. Third, there is an emerging role of autophagy in virus‐induced RNA silencing, either as an antiviral collaborator for targeted degradation of viral RNA silencing suppressors or an accomplice of viral RNA silencing suppressors for targeted degradation of key components of plant cellular RNA silencing machinery. In this review, we summarize this important progress and discuss the potential significance of the perplexing role of autophagy in plant innate immunity.