Abstract
Rhizospheric soils from cultivated olive (Olea europaea) trees of Chemlali, Chetoui, Quaissi, and Djalat cultivars were assessed for their bacterial abundance and diversity and were further screened for production of exopolysaccharides and exoenzymes (cellulase, chitinase, amylase, protease, lipase, and peroxidase). The results of the present study indicate that Chetoui cultivar revealed higher diversity, followed by Chemlali > Quaissi > Djalat, wherein, bacilli, enteric bacteria, and pseudomonads were abundantly present as specific bacterial groups associated with the Chetoui rhizosphere. Moreover, the exopolysaccharide (EPS)-producing bacteria of Chetoui cultivar (68.4%) presented the highest efficiency, followed by Djalat (23.5%) > Chemlali (7 %) > Quaissi (1%). These results revealed that the Chetoui cultivar presented highest enzyme activities, followed by Chemlali > Djalat > Quaissi, with a distinct abundance of peroxidase- and chitinase-producing bacteria, which may play a pivotal role in adapting olives to the environmental stresses. From this preliminary study, we confirmed that olive rhizosphere microbial diversity is essentially driven by the geographical origin and genotype of olive cultivars. Furthermore, we recommended the Chetoui olive cultivar rhizosphere as a potential reservoir for exoenzyme- and EPS-producing bacteria useful for future biotechnological applications.
Highlights
Olea europaea L., is one of the oldest domesticated oil trees in the Mediterranean area since several millennia and has acquired undisputable ecological and socioeconomic importance, over its longevity and genetic diversity
The results of the present study indicate that Chetoui cultivar revealed higher diversity, followed by Chemlali > Quaissi > Djalat, wherein, bacilli, enteric bacteria, and pseudomonads were abundantly present as specific bacterial groups associated with the Chetoui rhizosphere
Total viable counts (TVC) of bacteria and fungi were assessed with the dilution plate method by suspending the rhizospheric soil samples in a sterile saline solution; the suspensions were serially diluted, and triplicates of 100 μl samples were plated on nutrient agar (NA) and potato dextrose agar (PDA)
Summary
Olea europaea L. (olive tree), is one of the oldest domesticated oil trees in the Mediterranean area since several millennia and has acquired undisputable ecological and socioeconomic importance, over its longevity and genetic diversity. Limited research is available on olive rhizosphere associated microbiota due to the lack of robust assessments methods; several studies were conducted to explore the olive rhizosphere microbial diversity, structure, and complexity[2,3,4]. Various techniques have been used to investigate both endophytic and rhizospheric olive microbiota, including media culture, fatty acid methyl ester (FAME) or phospholipid fatty acid analysis (PLFA)[5], denaturing gradient gel electrophoresis (DGGE)[6], fluorescent terminal restriction fragment length polymorphism (FTRFLP)[2], and high-throughput sequencing[7]. Microbial communities inhabiting the interior of olive root (endophytic) are less diverse than those living in the vicinity of their roots (rhizospheric) and are mainly shaped by the olive cultivar genotype; the environmental, pedological, and agricultural practices such as tillage, irrigation, fertilization, and pesticide application influence the microbial presence. Information regarding the microbial communities associated with organic rhizospheres will help farmers in selecting efficient and sustainable agropractices and management[8]
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