|Ph.D Student||Glassner Hanoch|
|Subject||Characterization of Endophytic Bacteria with Agricultural|
Traits in Melon
|Department||Department of Biotechnology and Food Engineering||Supervisors||Professor Sima Yaron|
|Dr. Einat Zchori-Fein|
|Full Thesis text|
Endophytes are microorganisms that colonize internal plant tissues. They are mainly found in vegetative parts, but may also colonize reproductive organs. The hypothesis underlining my thesis was that reproductive organs of plants are colonized by endophytic bacteria that possess characteristics beneficial to agricultural. The main goal of the thesis was to isolate and characterize endophytic bacteria from wild and cultivated cucurbit fruits with potential benefits to agriculture in melons (Cucumis melo L.). To achieve this goal, the following immediate objectives have been set: 1) Identify and quantify endophytes from various fruits of several cucurbit genotypes, under field and natural conditions; 2) Isolate culturable endophytes found in association with the fruits; 3) Analyze the localization and distribution patterns of endophytes in fruit tissues and seeds; 4) Identify endophytes with beneficial properties for improving melon agricultural practices.
To identify bacterial associates of the agriculturally important plant family Cucurbitacae, endophytes were isolated and characterized from fruits tissues of seven cucurbit taxa with diverse fruits characteristics. Bacteria were isolated from tissues of cultivated and wild field-grown fruits, and from wild fruits growing under natural conditions, and were identified by 16S rRNA gene analysis. Specific bacteria such as Firmicutes were localized inside the fruits tissue and seed endosphere of the melon cultivar ‘Dulce’ using DOPE-fluorescence in situ hybridization (FISH) coupled with a confocal microscopy, and their antagonistic effects on cucurbit disease agents were determined. A selected set of isolates was characterized for plant growth promotion (PGP) traits ACC deaminase activity, IAA production, phosphate solubilization; and their effects on their host germination under normal or alleviated salt stress condition was determined in the two C. melo cultivars: ‘Dulce’ and PI 414723. Relatively low densities of culturable bacteria were detected in the investigated cucurbit fruits, especially in four out of the five wild species, regardless of their growing environment. Substantial differences were observed between the wild and cultivated cucurbit taxa with regard to the number of colonized fruits as well as the taxa of endophyte colonizing fruits. Bacillus species were the most dominant bacterial endophytes across the Cucurbitaceae. Microscopy further showed the niches of colonization of Firmicutes among other bacteria. Furthermore, endophytic bacteria could be visualized in the outer and inner parts of the seeds, as well as inside the cotyledon, at the perisperm/endosperm envelope and inside the embryonic hypocotyl-root axis tissues. Several bacterial isolates exhibited antagonistic properties against tested plant pathogens (fungi and bacteria), and some were associated with PGP traits. However, no specific bacterial isolate stands out as having a significant influence on seedling development in planta. Endophytic bacteria had only a small contribution to plant growth under salt stress. PGP traits exhibited by certain isolates did not correlate with the performance in planta.
Fruit tissues are a specific niche for a relatively low number of bacterial endophytes and the seeds are inhabited by yet a smaller number. Although the role of seed-associated microorganisms, and especially seed endophytic bacteria, is still undetermined, our data showed these niches are occupied in a structured way.