Isolation Of Plant Growth-promoting Rhizobacteria In Alfalfa And Their Effect In Plant Growth Promotion

Alfalfa is a perennial leguminous grass, known as the "king of the grass", has beenwidely planted all over the world, but the cultivation of it in moderately severesaline-alkali land is still needed to improve. In this present study, eleven PGPR strainswere isolated from saline alkali soil of alfalfa rhizosphere by using efficient directionalseparation methods. Identification of strains isolated were investigated based on colonymorphology, physiological and biochemical, combined with16s rDNA sequenceanalysis. In-vitro screening of these PGPR were carried out to detect their ACCdeaminase activity, ability of synthesise indole acetic acid and produce siderophore,antagonism between the bacteria and fungi. Furthermore, the effects of PGPR on alfalfagrowth were examined under salinity stress, which woud provide advises for increasingability of alfalfa salt resistance. The findings of the study showed that:1. Eleven PGPR strains were isolated from saline alkali soil of alfalfa rhizosphere, andthey were identified belong to Pseudomonas sp., Enterobacter sp., Mesorhizobiumsp., Arthrobacter sp. and Acinetobacter sp. respectively.2. The results of the biological characteristics of these strains showed that ACCdeaminase activity of strains MJM-3, MJM-5, MJM-8, MJM-9, MJM-1andMJM-11achieves above27μmolα-KA·(mgPr·h)-1. Synthesise IAA content of thestrains tested were increased in five different mediums in different degree, with theincrease of L-Trp concentration. When there was no L-Trp in the medium, almostno IAA was tested. All of the eleven PGPR strains had the ability to synthesissiderophore. Strains MJM-3, MJM-5, MJM-9and MJM-11had remarkable effect toantagonize fungi.3. The results of the alfalfa pot experiments indicated that strains MJM-3, MJM-5,MJM-9and MJM-11showed a significantly effect role in the alfalfa growth.Compared with the control, plant heights had increased by24.5%,34.5%,20.7%and19.0%respectively; root lengths had increased by29.9%,55.0%,22.4%and20.7%respectively; fresh weight of aerial parts had increased by58.7%,71.5%,49.0%and41.2%respectively; fresh weight of underground parts had increased by 60.0%,66.5%,38.1%and33.5%respectively; dry weight of aerial parts hadincreased by50.9%,64.0%,46.5%and43.0%respectively; dry weight ofunderground parts had increased by96.7%,103%,90.0%and83.3%respectively.4. Alfalfa physiological measurement results showed that strains MJM-3, MJM-5MJM-9and MJM-11promote the activity of plant roots obviously. Strain MJM-5had the most significant effect of chlorophyll content up to27.8%, the treatment ofstrains MJM-9and MJM-11followed by a little increase by21.6%and23.8%, thetreatment of MJM-3strain up to19.1%. The proline content of the plants inoculatedwith strains reached a significant level（P<0.05）compared with control group.