Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may grow in, on, or around plant tissues, stimulate plant growth by a plethora of mechanisms. These bacteria are collectively known as PGPR (plant growth promoting rhizobacteria). The search for PGPR and investigation of their modes of action are increasing at a rapid pace as efforts are made to exploit them commercially as biofertilizers. After an initial clarification of the term biofertilizers and the nature of associations between PGPR and plants (i.e., endophytic versus rhizospheric), this review focuses on the known, the putative, and the speculative modes-of-action of PGPR. These modes of action include fixing N2, increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant–microbe symbioses. The combination of these modes of actions in PGPR is also addressed, as well as the challenges facing the more widespread utilization of PGPR as biofertilizers.

Plant growth promoting rhizobacteria as biofertilizers

Free-living plant growth-promoting rhizobacteria (PGPR) can be used in a variety of ways when plant growth enhancements are required. The most intensively researched use of PGPR has been in agriculture and horticulture. Several PGPR formulations are currently available as commercial products for agricultural production. Recently developing areas of PGPR usage include forest regeneration and phytoremediation of contaminated soils. As the mechanisms of plant growth promotion by these bacteria are unravelled, the possibility of more efficient plant-bacteria pairings for novel and practical uses will follow. The progress to date in using PGPR in a variety of applications with different plants is summarized and discussed here.

Applications of free living plant growth-promoting rhizobacteria

This review presents a critical and comprehensive documentation and analysis of the developments in agricultural, environmental, molecular, and physiological studies related to Azospirillum cells, and to Azospirillum interactions with plants, based solely on information published between 1997 and 2003. It was designed as an update of previous reviews (Bashan and Levanony 1990; Bashan and Holguin 1997a), with a similar scope of interest. Apart from an update and critical analysis of the current knowledge, this review focuses on the central issues of Azospirillum research today, such as, (i) physiological and molecular studies as a general model for rhizosphere bacteria; (ii) co-inoculation with other microorganisms; (iii) hormonal studies and re-consideration of the nitrogen contribution by the bacteria under specific environmental conditions; (iv) proposed Azospirillum as a non-specific plant-growth-promoting bacterium; (v) re-introduction of the "Additive Hypothesis," which suggests involvement of multip...

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Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003)

Learning the lessons.

Rhizobacteria that exert beneficial effects on plant growth and development are referred to as plant growth promoting rhizobacteria (PGPR) In recent years, the use of PGPR to promote plant growth has increased in various parts of the world PGPR can affect plant growth by production and release of secondary metabolites (plant growth regulators/phytohormones/biologically active substances), lessening or preventing deleterious effects of phytopathogenic organisms in the rhizosphere and/or facilitating the availability and uptake of certain nutrients from the root environment Selection of effective PGPR is the most critical aspect to have maximum benefits from this technology Researchers are using different approaches for screening rhizobacteria to select effective PGPR including promotion of root/shoot growth under gnotobiotic conditions, in vitro production of plant growth regulators/biologically active substances and assessing of ACC-deaminase activity of the rhizobacteria However, the combined use of two or more approaches for screening of rhizobacteria could be more useful to select effective PGPR These screening approaches and practical applications of PGPR in agriculture are the major focus of this review

Plant Growth Promoting Rhizobacteria: Applications and Perspectives In Agriculture

There is a growing awareness that science education should center not just on knowledge acquisition but developing the foundation for lifelong learning. However, for intentional learning of science to occur in school, out of school, and after school, there needs to be a motivation to learn science. Prior research had shown that students' motivation to learn science tends to decrease during adolescence (Anderman and Young (1994) Journal of Research in Science Teaching 31: 811-831; Lee and Anderson (1993) American Educational Research Journal 30: 585-610; Simpson and Oliver (1990) Science Education 74: 1-18). This study compared 5th through 8th grade students' self-reported goal orientations, engagement in science class, continuing motivation for science learning, and perceptions of their schools' and parents' goals emphases, in Israeli traditional and democratic schools. The results show that the aforementioned decline in adolescents' motivation for science learning in school and out of school is not an inevitable developmental trend, since it is apparent only in traditional schools but not in democratic ones. The results suggest that the non-declining motivation of adolescents in democratic schools is not a result of home influence but rather is related to the school culture. 2010 Wiley Periodicals, Inc., J Res Sci Teach 48: 199-216, 2011

Adolescents' Declining Motivation to Learn Science: Inevitable or Not?

This is a mix methods follow-up study in which we reconfirm the findings from an earlier study [Vedder-Weiss & Fortus [2011] Journal of Research in Science Teaching, 48(2), 199–216]. The findings indicate that adolescents' declining motivation to learn science, which was found in many previous studies [Galton [2009] Moving to secondary school: Initial encounters and their effects. Perspectives on Education, 2(Primary-secondary Transfer in Science), 5–21. Retrieved from www.wellcome.ac.uk/perspectives; Osborne, Simon, & Collins, [2003] International Journal of Science Education 25(9), 1049–1079], is not an inevitable phenomenon since it appears not to occur in Israeli democratic schools. In addition to reinforcing previous results in a different sample, new results show that the differences between the two school types are also apparent in terms of students' self-efficacy in science learning, students' perceptions of their teachers' goals emphases, and students' perception of their peers' goals orientation. Quantitative results are accompanied by rich verbal examples of ways in which students view and articulate their own and their teachers' goal emphases. Content analysis of students' interviews showed that students in traditional schools are directed more towards goals that are external and related to the outcome of learning in comparison to democratic school students who are motivated more by goals that are internal and related to the process of learning. Structure analysis of these interviews suggests that democratic school students experience a greater sense of autonomy in their science learning than traditional school students do. Implications for research on students' motivation are discussed, such as considering not only the teacher and the classroom but also the school culture. © 2012 Wiley Periodicals, Inc. J Res Sci Teach 49: 1057–1095, 2012

Adolescents' declining motivation to learn science: A follow-up study

Continuing motivation for science learning may be manifested through engagement in extracurricular science-related activities, which are not the result of school or other external requirements. Very few articles have appeared in the last decade on this important aspect of science learning. This article presents a survey based on seven Likert-type items for measuring adolescents' continuing motivation for science. It describes how the survey was developed, tested, and used to explore the relations between school type, grade, and gender and adolescents' continuing motivation for science learning. Data on the continuing motivation of 2,958 Israeli 5th–8th grade students, from traditional and democratic schools, were collected and analyzed using polytomous Rasch techniques and hierarchical linear modeling. The results indicate that in both types of schools girls had lower continuing motivation for science than boys, and that while the continuing motivation of both boys and girls in traditional schools decreased between 5th and 8th grade, the continuing motivation of students in democratic schools remained constant during this period. © 2014 Wiley Periodicals, Inc. J Res Sci Teach 51: 497–522, 2014

Measuring students' continuing motivation for science learning

Achievement goal theory distinguishes between mastery goals (the goals of developing competence) and performance goals (the goals of demonstrating competence) [Ames [1992] Journal of Educational Psychology 84: 261–271]. In this study, we employed this theory aiming to better understand why adolescents' motivation to learn science declines with age in many schools yet not in others. We collected survey data from 5th to 8th grade Israeli students (N = 1,614). Utilizing Structural Equation Modeling (SEM) methods, we investigated the relations between students' perceptions of goals emphases in their environment (by parents, peers, teachers, and schools), their own goals orientations and their engagement in science learning in and out of school (classroom and extra-curricular engagement). In addition, we compared between these relations in traditional and democratic schools and in elementary and middle school grade levels. Findings show that: (A) perceptions of the goals that significant adults (parents and teachers) emphasize were better predictors of students' motivation, in and out of school, than perceptions of the goals that peers and schools emphasize; (B) perceptions of teachers' performance goals emphases negatively predicted classroom engagement; (C) the relative effect of perceived parents' mastery emphasis on extra-curricular engagement was higher in elementary grades than in middle school grades; (D) the relative effect of perceived school's mastery emphasis was higher in middle school grades than in elementary grades; and (E) students' mastery goals orientation in science class was a strong predictor of their extra-curricular engagement. Implications for both research and practice are discussed. © 2013 Wiley Periodicals, Inc. J Res Sci Teach 50: 952–988, 2013

School, Teacher, Peers, and Parents' Goals Emphases and Adolescents' Motivation to Learn Science in and out of School.

Most research and practice in teacher in-service learning focuses on formal professional development activities. This article calls for paying greater attention to the informal conversations that a...

Dana Vedder-Weiss

Papers

Adolescents' Declining Motivation to Learn Science: Inevitable or Not?

Adolescents' declining motivation to learn science: A follow-up study

Measuring students' continuing motivation for science learning

School, Teacher, Peers, and Parents' Goals Emphases and Adolescents' Motivation to Learn Science in and out of School.

Relocating Research on Teacher Learning: Toward Pedagogically Productive Talk: