Bacterial endophytes in agricultural crops

Abstract

Endophytic bacteria are ubiquitous in most plant species, residing latently or actively colonizing plant tissues locally as well as systemically. Several definitions have been proposed for endophytic bacteria; in this review endophytes will be defined as those bacteria that can be isolated from surface-disinfested plant tissue or extracted from within the plant, and that do not visibly harm the plant. While this definition does not include nonextractable endophytic bacteria, it is a practical definition based on experimental limitations and is inclusive of bacterial symbionts, as well as internal plant-colonizing nonpathogenic bacteria with no known beneficial or detrimental effects on colonized plants. Historically, endophytic bacteria have been thought to be weakly virulent plant pathogens but have recently been discovered to have several beneficial effects on host plants, such as plant growth promotion and increased resistance against plant pathogens and parasites. In general, endophytic bacteria originate from the epiphytic bacterial communities of the rhizosphere and phylloplane, as well as from endophyte-infested seeds or planting materials. Besides gaining entrance to plants through natural openings or wounds, endophytic bacteria appear to actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase. Since these enzymes are also produced by pathogens, more knowledge on their regulation and expression is needed to distinguish endophytic bacteria from plant pathogens. In general, endophytic bacteria occur at lower population densities than pathogens, and at least some of them do not induce a hypersensitive response in the plant, indicating that they are not recognized by the plant as pathogens. Evolutionarily, endophytes appear to be intermediate between saprophytic bacteria and plant pathogens, but it can only be speculated as to whether they are saprophytes evolving toward pathogens, or are more highly evolved than plant pathogens and conserve protective shelter and nutrient supplies by not killing their host. Overall, the endophytic microfloral community is of dynamic structure and is influenced by biotic and abiotic factors, with the plant itself constituting one of the major influencing factors. Since endophytic bacteria rely on the nutritional supply offered by the plant, any parameter affecting the nutritional status of the plant could consequently affect the endophytic community. This review summarizes part of the work being done on endophytic bacteria, including their methodology, colonization, and establishment in the host plant, as well as their role in plant–microbe interactions. In addition, speculative conclusions are raised on some points to stimulate thought and research on endophytic bacteria.Key words: endophytic bacteria, methods, localization, diversity, biological control.

New advances in plant growth-promoting rhizobacteria for bioremediation

“…PGPR can be divided into two groups according to their relationship with the plants: symbiotic bacteria and free-living rhizobacteria (Khan, 2005). As reviewed by Compant et al (2005), Glick (1995Glick ( , 2001, Hall (2002), Hallman et al (1997), Lucy et al (2004), Sturz et al (2000), and Welbaum et al (2004), a lot of work have been done on the mechanisms and principles of the PGPR-plant relationship, which was accepted widely as rhizosphere effect. Generally, PGPR function in three different ways (Glick, 1995(Glick, , 2001): synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the environment (Çakmakçi et al, 2006;Lucas García et al, 2004a,b;Siddiqui and Mahmood, 2001), and lessening or preventing the plants from diseases (Guo et al, 2004;Jetiyanon and Kloepper, 2002;Raj et al, 2003;Saravanakumar et al, in press).…”

Aumento da eficiência nutricional de tomateiros inoculados com bactérias endofíticas promotoras de crescimento

“…Quando o estímulo do crescimento das plantas é direto, o microrganismo produz fitormônios ou substâncias análogas destes reguladores de crescimento capazes de estimular o crescimento e desenvolvimento da planta (Bashan & Holguin, 1997). Quando o efeito é indireto, o crescimento é estimulado pela redução da população de microrganismos deletérios ou patogênicos às plantas, ou seja, pelo controle biológico de fitopatógenos (Hallmann et al, 1997). Outro fator que contribui para o crescimento das plantas é a capacidade de algumas bactérias endofíticas de aumentar a absorção de nutrientes minerais e água e melhorar a disponibilização destes nutrientes (Hallmann et al, 1997;Lazarovits & Nowak, 1997).…”

“…Quando o efeito é indireto, o crescimento é estimulado pela redução da população de microrganismos deletérios ou patogênicos às plantas, ou seja, pelo controle biológico de fitopatógenos (Hallmann et al, 1997). Outro fator que contribui para o crescimento das plantas é a capacidade de algumas bactérias endofíticas de aumentar a absorção de nutrientes minerais e água e melhorar a disponibilização destes nutrientes (Hallmann et al, 1997;Lazarovits & Nowak, 1997). Como exemplo, cita-se o gênero Azospirillum, que, além de ser diazotrófico, isto é, capaz de fixar o N 2 atmosférico, também secreta fitormônios, principalmente auxinas, e promove o aumento da absorção de nutrientes e água, com conseqüente crescimento da planta (Schloter & Hartmann, 1998).…”

Influence of growth conditions on production of poly(3-hydroxybutyrate) by Bacillus cereus HAL 03 endophytic to Helianthus annuus L.

“…The culturable endophytic bacterial strains used in this study were isolated from the surface sterilized leaf, stem and root segments of freshly collected H. annuus L. plants following the techniques of Hallmann et al [20] and Reinhold-Hurek and Hurek [21]. Pure cultures of these isolates were maintained at 4ºC on slopes of tryptic soy agar by subculturing at regular interval.…”