3Pathogenic microorganisms affecting plant health are a major and chronic threat to food production and ecosystem stability worldwide. As agricultural production intensified over the past few decades, producers became more and more dependent on agrochemicals as a relatively reliable method of crop protection helping with economic stability of their operations. However, increasing use of chemical inputs causes several negative effects, i.e., development of pathogen resistance to the applied agents and their nontarget environmental impacts (44, 62). Furthermore, the growing cost of pesticides, particularly in less-affluent regions of the world, and consumer demand for pesticide-free food has led to a search for substitutes for these products. There are also a number of fastidious diseases for which chemical solutions are few, ineffective, or nonexistent (62). Biological control is thus being considered as an alternative or a supplemental way of reducing the use of chemicals in agriculture (44,62,136,188).There has been a large body of literature describing potential uses of plant associated bacteria as agents stimulating plant growth and managing soil and plant health (reviewed in references 63, 70, 143, 165, and 188). Plant growth-promoting bacteria (PGPB) (8) are associated with many, if not all, plant species and are commonly present in many environments. The most widely studied group of PGPB are plant growth-promoting rhizobacteria (PGPR) (82) colonizing the root surfaces and the closely adhering soil interface, the rhizosphere (82, 84). As reviewed by Kloepper et al. (84) or, more recently, by Gray and Smith (65), some of these PGPR can also enter root interior and establish endophytic populations. Many of them are able to transcend the endodermis barrier, crossing from the root cortex to the vascular system, and subsequently thrive as endophytes in stem, leaves, tubers, and other organs (10, 28,65,70). The extent of endophytic colonization of host plant organs and tissues reflects the ability of bacteria to selectively adapt to these specific ecological niches (65, 70). Consequently, intimate associations between bacteria and host plants can be formed (28, 70, 84) without harming the plant (70,83,84,92,191). Although, it is generally assumed that many bacterial endophyte communities are the product of a colonizing process initiated in the root zone (102, 165, 177, 188), they may also originate from other source than the rhizosphere, such as the phyllosphere, the anthosphere, or the spermosphere (70).Despite their different ecological niches, free-living rhizobacteria and endophytic bacteria use some of the same mechanisms to promote plant growth and control phytopathogens (15, 46,63,70,92,165). The widely recognized mechanisms of biocontrol mediated by PGPB are competition for an ecological niche or a substrate, production of inhibitory allelochemicals, and induction of systemic resistance (ISR) in host plants to a broad spectrum of pathogens (15,63,66,67,97,146) and/or abiotic stresses (reviewed in referen...
In both managed and natural ecosystems, beneficial plant-associated bacteria play a key role in supporting and/or increasing plant health and growth. Plant growth-promoting bacteria (PGPB) can be applied in agricultural production or for the phytoremediation of pollutants. However, because of their capacity to confer plant beneficial effects, efficient colonization of the plant environment is of utmost importance. The majority of plant-associated bacteria derives from the soil environment. They may migrate to the rhizosphere and subsequently the rhizoplane of their hosts before they are able to show beneficial effects. Some rhizoplane colonizing bacteria can also penetrate plant roots, and some strains may move to aerial plant parts, with a decreasing bacterial density in comparison to rhizosphere or root colonizing populations. A better understanding on colonization processes has been obtained mostly by microscopic visualisation as well as by analysing the characteristics of mutants carrying disfunctional genes potentially involved in colonization. In this review we describe the individual steps of plant colonization and survey the known mechanisms responsible for rhizosphere and endophytic competence. The understanding of colonization processes is important to better predict how bacteria interact with plants and whether they are likely to establish themselves in the plant environment after field application as biofertilisers or biocontrol agents.
SUMMARYActinobacteriaare Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. ManyActinobacteriahave a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture.Actinobacteriaplay diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species ofCorynebacterium,Mycobacterium,Nocardia,Propionibacterium, andTropheryma), soil inhabitants (e.g.,MicromonosporaandStreptomycesspecies), plant commensals (e.g.,Frankiaspp.), and gastrointestinal commensals (Bifidobacteriumspp.).Actinobacteriaalso play an important role as symbionts and as pathogens in plant-associated microbial communities. This review presents an update on the biology of this important bacterial phylum.
Patterns of colonization of Vitis vinifera L. cv. Chardonnay plantlets by a plant growth-promoting bacterium, Burkholderia sp. strain PsJN, were studied under gnotobiotic conditions. Wild-type strain PsJN and genetically engineered derivatives of this strain tagged with gfp (PsJN::gfp2x) or gusA (PsJN::gusA11) genes were used to enumerate and visualize tissue colonization. The rhizospheres of 4-to 5-week-old plantlets with five developed leaves were inoculated with bacterial suspensions. Epiphytic and endophytic colonization patterns were then monitored by dilution plating assays and microscopic observation of organ sections. Bacteria were chronologically detected first on root surfaces, then in root internal tissues, and finally in the fifth internode and the tissues of the fifth leaf. Analysis of the PsJN colonization patterns showed that this strain colonizes grapevine root surfaces, as well as cell walls and the whole surface of some rhizodermal cells. Cells were also abundant at lateral root emergence sites and root tips. Furthermore, cell wall-degrading endoglucanase and endopolygalacturonase secreted by PsJN explained how the bacterium gains entry into root internal tissues. Host defense reactions were observed in the exodermis and in several cortical cell layers. Bacteria were not observed on stem and leaf surfaces but were found in xylem vessels of the fifth internode and the fifth leaf of plantlets. Moreover, bacteria were more abundant in the fifth leaf than in the fifth internode and were found in substomatal chambers. Thus, it seems that Burkholderia sp. strain PsJN induces a local host defense reaction and systemically spreads to aerial parts through the transpiration stream.
This review presents an overview of eutypa dieback, esca and botryosphaeria dieback, the predominant grapevine trunk diseases worldwide. It covers their symptomatologies in the trunk, leaves and berries; the characteristics of the different fungal species associated with them; and host-pathogen interactions. Here, the host-pathogen relationship is defined at the cytological, physiological and molecular levels. Currently available experimental tools for studying these diseases, both in vitro and in the field, are discussed. Finally, a progress report on their control, which, since the ban of sodium arsenite, comprises chemical, biological and ⁄ or sanitation methods, is presented.
Apart from water availability, low temperature is the most important environmental factor limiting the productivity and geographical distribution of plants across the world. To cope with cold stress, plant species have evolved several physiological and molecular adaptations to maximize cold tolerance by adjusting their metabolism. The regulation of some gene products represents an additional mechanism of cold tolerance. A consequence of these mechanisms is that plants are able to survive exposure to low temperature via a process known as cold acclimation. In this review, we briefly summarize recent progress in research and hypotheses on how sensitive plants perceive cold. We also explore how this perception is translated into changes within plants following exposure to low temperatures. Particular emphasis is placed on physiological parameters as well as transcriptional, post-transcriptional and post-translational regulation of cold-induced gene products that occur after exposure to low temperatures, leading to cold acclimation.
Both in natural and in managed ecosystems, bacteria are common inhabitants of the phytosphere and the internal tissues of plants. Probably the most diverse and environmentally adaptable plant-associated bacteria belong to the genus Burkholderia. This genus is well-known for its human, animal and plant pathogenic members, including the Burkholderia cepacia complex. However, it also contains species and strains that are beneficial to plants and can be potentially exploited in biotechnological processes. Here we present an overview of plant-associated Burkholderia spp. with special emphasis on beneficial plant-Burkholderia interactions. A discussion of the potential for utilization of stable plant-Burkholderia spp. associations in the development of low-input cropping systems is also provided.
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