Seeing the Light: The Roles of Red- and Blue-Light Sensing in Plant Microbes

Plants collect, concentrate and conduct light throughout their tissues, thus enhancing light availability to their resident microbes. This review explores the role of photosensing in the biology of plant-associated bacteria and fungi, including the molecular mechanisms of red light sensing by phytochromes and blue light sensing by LOV-domain proteins in these microbes. Bacteriophytochromes function as major drivers of the global transcriptome and in the lightmediated suppression of virulence, motility and conjugation in some phytopathogens, and in the light-mediated induction of the photosynthetic apparatus in a stem-nodulating symbiont. Bacterial LOV proteins also influence light-mediated changes in both symbiotic and pathogenic phenotypes. Although red light sensing by fungal phytopathogens is poorly understood, fungal LOV proteins contribute to blue light regulation of traits including asexual development and virulence. Collectively, these studies highlight that plant microbes have evolved to exploit light cues, and that light sensing is often coupled with sensing other environmental signals

A war over water when bacteria invade leaves

A bacterial leaf pathogen actively targets plant cell processes to create an aqueous environment favorable for growth, revealing that control of water is a fundamental element of bacterial virulence

Metabolic coupling on roots

The coupling of root exudation of nutrients with microbial nutrient uptake preferences helps drive the assembly of rhizosphere microbiomes, enabling the use of metabolite interaction traits for engineering favorable microbial communities on roots

Bacterial wilt symptoms are impacted by host age and involve net downward movement of Erwinia tracheiphila in muskmelon

Cucurbit bacterial wilt, caused by Erwinia tracheiphila, is a damaging disease of cucurbit crops in the Midwest and Northeast U.S. Current management of bacterial wilt relies primarily on insecticide applications to control striped and spotted cucumber beetles (Acalymma vittatum and Diabrotica undecimpunctata howardi, respectively), which vector E. tracheiphila. Development of alternative management strategies is constrained by a lack of understanding of bacterial wilt etiology. The impact of host age on rate on symptom development and extent of bacterial movement in the xylem of muskmelon (Cucumis melo cv. Athena) was evaluated following wound inoculation of 2- to 8-week-old plants in growth chamber experiments. Wilting occurred more rapidly in plants after inoculating E. tracheiphila into 2- or 4-week-old plants than 6- or 8-week-old plants. Recovery of viable cells from stem segments revealed that vascular spread of E. tracheiphila was more extensive below than above the inoculation point. These findings provide experimental evidence that host age impacts the rate of symptom development in cucurbit bacterial wilt and that movement of the xylem-inhabiting pathogen E. tracheiphila within muskmelon plants occurs primarily in the downward direction

Physiological and Transcriptional Responses to Osmotic Stress of Two Pseudomonas syringae Strains That Differ in Epiphytic Fitness and Osmotolerance

The foliar pathogen Pseudomonas syringae is a useful model for understanding the role of stress adaptation in leaf colonization. We investigated the mechanistic basis of differences in the osmotolerance of two P. syringae strains, B728a and DC3000. Consistent with its higher survival rates following inoculation onto leaves, B728a exhibited superior osmotolerance over DC3000 and higher rates of uptake of plant-derived osmoprotective compounds. A global transcriptome analysis of B728a and DC3000 following an osmotic upshift demonstrated markedly distinct responses between the strains; B728a showed primarily upregulation of genes, including components of the type VI secretion system (T6SS) and alginate biosynthetic pathways, whereas DC3000 showed no change or repression of orthologous genes, including downregulation of the T3SS. DC3000 uniquely exhibited improved growth upon deletion of the biosynthetic genes for the compatible solute N-acetylglutaminylglutamine amide (NAGGN) in a minimal medium, due possibly to NAGGN synthesis depleting the cellular glutamine pool. Both strains showed osmoreduction of glnA1 expression, suggesting that decreased glutamine synthetase activity contributes to glutamate accumulation as a compatible solute, and both strains showed osmoinduction of 5 of 12 predicted hydrophilins. Collectively, our results demonstrate that the superior epiphytic competence of B728a is consistent with its strong osmotolerance, a proactive response to an osmotic upshift, osmoinduction of alginate synthesis and the T6SS, and resiliency of the T3SS to water limitation, suggesting sustained T3SS expression under the water-limited conditions encountered during leaf colonization

Survival of the Goss’s Wilt Bacterium and Management Implications

Goss’s wilt is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn). The survival of Cmn in soil and crop residues was examined by Schuster (1975). Pure cultures of the bacterium in soil did not survive for long (less than two weeks), however the bacterium was able to survive for up to 10 months in infested surface crop residue. When the crop residue (leaves, stalks, cobs and ears) was buried at 4 inches or 8 inches, the bacterium was only detected in stalks residue after 10 months. Thus, conservation tillage practices that partially bury infested crop residue should reduce survival of the Goss’s wilt bacterium. Any tillage done must take into account soil conservation. Rotating to a non-host crop, such as soybean, will allow time for infested residues to breakdown and inoculum levels to decrease

Pseudomonas syringae pv. syringae B728a Regulates Multiple Stages of Plant Colonization via the Bacteriophytochrome BphP1

Light may be an important environmental signal for plant-associated bacteria, particularly those that live on leaves. An integrated network of red/far-red- and blue-light-responsive photosensory proteins is known to inhibit swarming motility in the foliar plant pathogen Pseudomonas syringae pv. syringae B728a. Here we elucidated factors in the red/far-red-light-sensing bacteriophytochrome BphP1 signal transduction pathway and report evidence for a role of BphP1 in multiple stages of the P. syringae B728a life cycle. We report that BphP1 signaling involves the downstream regulator Bsi (bacteriophytochrome-regulated swarming inhibitor) and an acyl-homoserine lactone (AHL) signal. Loss of bphP1 or bsi resulted in the early initiation of swarm tendrils during swarming motility, a phenotype that was dependent on red/far-red light and reversed by exogenous AHL, illustrating that the BphP1-Bsi-AHL pathway inhibits the transition from a sessile state to a motile state. Loss of bphP1 or bsi resulted in larger water-soaked lesions induced on bean (Phaseolus vulgaris) pods and enhanced movement from soil and buried plant tissues to seeds, demonstrating that BphP1 and Bsi negatively regulate virulence and bacterial movement through soil to seeds. Moreover, BphP1, but not Bsi, contributed to leaf colonization; loss of bphP1 reduced survival on leaves immediately following inoculation but enhanced the size of the subsequently established populations. Neither Bsi nor Smp, a swarm motility-promoting regulator identified here, affected leaf colonization, indicating that BphP1-mediated contributions to leaf colonization are, at least in part, independent of swarming motility. These results demonstrate that P. syringae B728a red-light sensing involves a multicomponent, branched regulatory pathway that affects several stages of its life cycle

An Overview of Plant Defenses against Pathogens and Herbivores

Plants represent a rich source of nutrients for many organisms including bacteria, fungi, protists, insects, and vertebrates. Although lacking an immune system comparable to animals, plants have developed a stunning array of structural, chemical, and protein-based defenses designed to detect invading organisms and stop them before they are able to cause extensive damage. Humans depend almost exclusively on plants for food, and plants provide many important non-food products including wood, dyes, textiles, medicines, cosmetics, soaps, rubber, plastics, inks, and industrial chemicals. Understanding how plants defend themselves from pathogens and herbivores is essential in order to protect our food supply and develop highly disease-resistant plant species. This article introduces the concept of plant disease and provides an overview of some defense mechanisms common among higher plants. A close examination of plant anatomy is presented, as well as some of the ecological relationships that contribute to plant defense and disease resistance. Special care has been taken to illustrate how products used in everyday life are derived from substances produced by plants during defense responses

Glycine Betaine Catabolism Contributes to Pseudomonas syringae Tolerance to Hyperosmotic Stress by Relieving Betaine-Mediated Suppression of Compatible Solute Synthesis

Many bacteria can accumulate glycine betaine for osmoprotection and catabolize it as a growth substrate, but how they regulate these opposing roles is poorly understood. In Pseudomonas syringae B728a, expression of the betaine catabolism genes was reduced by an osmotic upshift to an intermediate stress level, consistent with betaine accumulation, but was increased by an upshift to a high stress level, as confirmed by an accompanying increase in degradation of radiolabeled betaine. Deletion of the gbcAB betaine catabolism genes reduced osmotolerance at a high osmolarity, and this reduction was due to the relief of betaine-mediated suppression of compatible solute synthesis. This conclusion was supported by the findings that, at high osmolarity, the ΔgbcAB mutant accumulated high betaine levels and low endogenous solutes and exhibited reduced expression of the solute synthesis genes. Moreover, the ΔgbcAB mutant and a mutant deficient in the synthesis of the compatible solutes NAGGN and trehalose exhibited similar reductions in osmotolerance and also in fitness on bean leaves. Activation of betaine catabolism at high osmotic stress resulted, in part, from induction of gbdR, which encodes the transcriptional activator GbdR. Betaine catabolism was subject to partial repression by succinate under hyperosmotic stress conditions, in contrast to strong repression in the absence of stress, suggesting that betaine functions both in nutrition and as an intracellular signal modulating solute synthesis under hyperosmotic stress conditions. Collectively, these results begin to provide a detailed mechanistic understanding of how P. syringae transitions from reliance on exogenously derived betaine to the use of endogenous solutes during adaptation to hyperosmotic conditions