1 Ecology , Genetic Diversity and Screening Strategies of Plant Growth Promoting Rhizobacteria ( PGPR )

TheGerman agronomist Hiltner first defined the rhizosphere, in 1904, as the ‘effect’ of the roots of legumes on the surrounding soil, in terms of higher microbial activity because of the organic matter released by the roots. Until the end of the twentieth century, this ‘effect’ was not considered to be an ecosystem. It is interesting to make some brief observations about the size, in terms of energy and extension, of this ecosystem to determine its impact on how the biosphere functions. First, in extension, the rhizosphere is the largest ecosystem on earth. Second, the energy flux in this system is enormous. Some authors estimate that plants release between 20 and 50% of their photosynthates through their roots [1,2]. Thus, rhizosphere’s impact on how the biosphere functions is fundamental. A large number of macroscopic organisms and microorganisms such as bacteria, fungi, protozoa and algae coexist in the rhizosphere. Bacteria are the most abundant among them. Plants select those bacteria contributing most to their fitness by releasing organic compounds through exudates [3], creating a very selective environment where diversity is low [4,5]. A complex web of interactions takes place among them, and this may affect plant growth, directly or indirectly. Since bacteria are the most abundant microorganisms in the rhizosphere, it is highly probable that they influence the plant’s physiology to a greater extent, especially considering their competitiveness in root colonization [6]. Bacterial diversity can be defined in terms of taxonomic, genetic and functional diversity [7]. In the rhizosphere, the metabolic versatility of a bacterial population (functional diversity) is based on its genetic variability and on possible interactions with other prokaryotic and eukaryotic organisms such as plants. j1