Identification of novel microorganisms for the growth promotion and disease protection
of olive tree, one of the major crops in the arid lands of Southern Tunisia

Drought is a world-spread problem seriously influencing crop production and quality. The Mediterranean climate is characterised by a hot dry period in summer and a cool wet period in winter. From an ecophysiological point of view, the variability and unpredictability of precipitation imposes strong constraints on plants [Galmes et al., 2005]. Natural soil forming processes in warm and dry regions frequently produce saline soils with low agricultural potential. Another limiting factor for agricultural production is the soil structure in sandy soils. In all kinds of soil, water stress is one of the most important factors that plants should overcome. In addition to the use of traditional breeding and plant genetic engineering with production of transgenic plants, the use of plant growth-promoting bacteria (exoroot associated and endophytic bacteria) is a useful approach for developing strategies to facilitate plant growth under conditions of water deficit.

Plant growth-promoting bacteria are endophytic and free-living soil bacteria that can directly or indirectly facilitate the growth of plants. Indirect stimulation of plant growth includes a variety of mechanisms by which the bacteria prevent phytopathogenic microorganisms from inhibiting plant growth and development. This biocontrol activity is accomplished due to the production of siderophores, beta-1,3-glucanase, chitinases, antibiotics and cyanide and to the ability to degrade auxin (indolacetic acid IAA), i.e. protect the plant from high-IAA-producing bacteria [Leveau and Lindow, 2005]. Direct stimulation may include providing plants with fixed nitrogen, iron that has been sequestered by bacterial siderophores, soluble phosphate and other nutrients, and the ability to produce the correct amounts of plant hormones such as IAA, gibberellic acid and cytokinins [Bloemberg and Lugtenberg, 2001]. Relevant plant growth promoting microorganisms include mychorrizal fungi and Bacilli, for example B. azotofixans for nitrogen fixation and B. thuringiensis which has an important role in the biocontrol of phytopathogenic insects and microorganisms. Other important plant growth promoting bacteria are Pseudomonads which have been widely documented as potent plant biostimulators producing among others IAA, HCN and enzymes useful to regulate plant metabolism. Also important plant growth promoting bacteria are the free-living nitrogen fixers like Azospirilla and, for leguminous plants, Rhizobia which with the symbiotic nitrogen fixation promote plant growth and improve soil fertility.

In addition to facilitate the growth of plant roots, plant growth-promoting bacteria can protect plants from the deleterious effects of environmental stresses including heavy metals, flooding, drought, and salinity. This is due to the production of the ACC deaminase enzyme which is implicated in lowering plant ethylene levels [Mayak et al., 2004]. Thus, under stress conditions, ACC deaminase-producing bacteria that restrain the production of ethylene may be effective in alleviating a portion of the stress effect, improving plant performance and consequently enhancing yields [Beltrano et al., 1997]. Another mechanism to overcome water stress is the production of exopolysaccharides (EPS). In fact, many studies (on wheat and sun-flower) have shown the importance of natural populations of EPS producing bacteria in improving soil macroaggregation and by doing so in the regulation of rhizosphere soil water content, counteracting the negative effect of a water deficit on plant growth [Kaci et al., 2005].

Fungal mycorrhization is important for the plants to overcome low water availability due to the large contact area between fungal hyphae and soil particles [Shi et al., 2002].

The Italian Partner is, now, collaborating with El Shams University of Cairo on a project for the identification of microorganisms that can improve plant growth in arid soil.