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Università degli studi di Milano (2012)

‘MORE CROP PER DROP’ : THE CONTRIBUTE OF PLANT GROWTH PROMOTING BACTERIA

MARASCO, RAMONA

Titre : ‘MORE CROP PER DROP’ : THE CONTRIBUTE OF PLANT GROWTH PROMOTING BACTERIA

Auteur : MARASCO, RAMONA

Université de soutenance : Università degli studi di Milano

Grade : Doctoral Thesis 2012

Résumé partiel
Environmental stresses are main factors limiting crop production worldwide. Water stress is a primary cause of crop losses, reducing average yields for most of the major crops by more than 50%, particularly in arid and semi-arid regions. Moreover, the increasing frequency of dry periods in many regions of the world, as a consequence of the global warming, frequently results in saline soils with low agricultural potential. Bacteria are among the first colonizers of soil habitats at different stages of development, largely contributing in determining soil structure and composition. In this context, understanding the diversity of soil- and plant-associated bacteria and their roles in soil structure maintenance, protection of plant health and development under harsh conditions is crucial, assuming that these associations could be manipulated to increase the productivity and sustainability of agro-ecosystems. A new emerging approach to improve crop production without extending agriculture surfaces and agro-chemicals use, is the exploitation of the natural microbiota contribute in soil structure determination and of the association of plants with Plant Growth Promoting Bacteria (PGPB). During the last couple of decades, the use of PGPB for sustainable agriculture was increased in different parts of the world resulting in significant improvement in growth and yield of agronomical important crops, also in arid land. These beneficial bacteria can colonize the rhizosphere soil or the root system, and even thrive in the plant tissues as endophytic populations. Bacterial mechanisms of plant growth promotion include biological nitrogen fixation, synthesis of phytormones, stress reduction or protection from pathogens. In addition, plant-associated communities also increase availability of nutrients such as phosphorous, iron and other elements, playing key roles in ecosystem processes such as nutrient cycling and conservation of soil structure. To remove the erratic performance of microbial inoculation, the selection of native bacteria acclimatized to harsh environmental condition has been advocated. Recent reports documented the efficiency of PGP bacteria isolated from plants growing in arid and semi-arid regions. This PhD thesis aimed to explore the microbial contribution in soil neogenesis in deglaciating environments where a soil fertility gradient can be envisaged. Besides, this thesis has the proposal to exploit the PGP properties of endophytes and rhizobacteria adapted to harsh conditions to develop an eco-friendly approach for the improvement of crop productivity in arid environments. For this purpose, a large collection of 5000 bacteria associated to plants grown in arid areas was established. Bacteria were isolated from the root tissues and soil fraction at increasing distance from the plant : the rhizosphere, the root surrounding soil and the bulk soil. The selected plants were Olea europea (olive tree), Vitis vinifera (grapevine) and Capsicum annuum (pepper) growing in arid and semi-arid regions of Tunisia, Egypt and Italy. These agricultural crops have a relevant economic interest in the Mediterranean basin and are known to be highly adapted to extreme stressful conditions of arid land, thus allowing to improve the chances to select drought resistant bacteria. Analysis of the abundance and diversity of the cultivable bacterial fraction from root tissues and rhizosphere showed a difference in the spatial distribution of bacterial genera associated to the plants. In olive tree we observed higher bacterial densities in the rhizosphere compared to endophytic populations, due to the release of exudates in the rhizosphere from living roots. 2 Besides, the low phylogenetic diversity observed in the endophyte fraction indicated that plant tissues may select specific bacterial colonizers. Particularly, the endophytes were dominated by the sporeformer Bacillales order represented by the genera Bacillus, Paenibacillus and Brevibacillus. The dominant order cultured from soil fractions was Actinobacteria, with dominant genera such as Arthrobacter and Streptomyces, frequently isolated from dry soils and desert crusts. Differently, in pepper plants sampled in Egypt and Italy the endophytic isolates were affiliated exclusively to the Bacilli class. Differently, isolates belonging to the gamma subgroup of the class Proteobacteria were predominant in rhizosphere, root surrounding soil and bulk soil of pepper plants from Egypt, with many of these isolates assigned to Enterobacteriaceae family, in particular to Klebsiella, Citrobacter, Raoultella genera and to the genus Pseudomonas. The family Enterobacteraceae comprises many species with enteric habitat, which origin could be attributable to the use of irrigation water of low hygienic quality.

Mots Clés  : PLANT GROWTH PROMOTING BACTERIA ; DROUGHT ; ARID LAND ; MICROBIAL DIVERSITY

Présentation

Page publiée le 4 juin 2012, mise à jour le 15 mars 2019