A simple screening method to detect berseem clover (Trifolium alexandrinum) endophytic bacteria for rice plant growth-promoting agents on the basis of a root colonization bioassay and a plant growth promoting trait is characterized. Firstly, 200 isolates (80 endophytes and 120 rhizospheric isolates) isolated from berseem clover were inoculated as 10 mixtures of 20 strains each on two rice varieties under gnotobiotic conditions. Then, the reisolated endophytic strains from two rice varieties were characterized for plant growth promoting (PGP) traits. Secondly, the colonization and growth promoting effects of endophytic strains were compared in inoculated rice plantlets as single-strain inoculants. A significant relationship among indole-3-acetic acid (IAA) producing isolates, the size of root colonization, and plant growth was observed. Our results suggest that the ability of IAA production by the endophytic bacteria which may have a stimulatory effect on plant development may be the first plant growth promoting trait for screening bacteria isolated from clover plant for rice plant growth promoting agents. In addition, this study indicates that the selected bacterial isolates based on their IAA producing trait have the potential for PGP and more colonization of rice plant. 1. Introduction Research into how plant growth can be promoted has mainly concentrated on rhizobacteria. More recently, however, attention has been focused on the plant growth-promoting capacity of endophytes [1]. The study of plant bacterial endophytes is important for understanding ecological interactions and to develop biotechnological applications. Endophytic bacteria can be defined as those bacteria that can colonize the internal tissue of the plant without showing negative symptoms on their host [2]. Endophytes promote the growth of plants in various ways, for example, through secretion of plant growth regulators, such as indole-3-acetic acid (IAA) [3], via phosphate solubilizing activity [4], by enhancing hyphal growth and mycorrhizal colonization [5], production of siderophores [6], and supplying biologically fixed nitrogen [7]. In addition, endophytic bacteria supply essential vitamins to plants [7]. The production of auxin-like compounds increases seed production and germination [8] along with increased shoot growth and tillering [9]. Other effects of endophyte infection on the host plant include osmotic adjustment, stomatal regulation, modification of root morphology, enhanced uptake of minerals, and alteration of nitrogen accumulation and metabolism [10]. During the last
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