**2.1 Plant-growth-promoting endophytes (PGPEs)**

The PGPEs as well as PGPRs (plant-growth-promoting rhizobacteria) promote plant growth by regulating plant hormones, improving nutrition acquisition, siderophore production, and enhancing the antioxidant system [13]. Bacterial endophytes associated with plants can be classified into three groups, based on the ecological interaction: beneficial, deleterious, and neutral. Various genera of *Pseudomonas*, *Enterobacter*, *Bacillus*, *Klebsiella*, and *Burkholderia*, (which are normally considered pathogenic), are also present as PGPEs, promoting plant growth and development under both normal and stress conditions [14]. In most cases, these are indirect mechanisms, such as preventing the deleterious effects of other phytopathogenic microorganisms, and this function is achieved by antibiosis, induction of systemic resistance (ISR), and competitive exclusion [13–15]. There are several mechanisms for plant growth stimulation by PGPEs, such as nitrogen fixation; synthesis of auxin, 1-aminocyclopropane-1-carboxylate (ACC)-deaminase, siderophores production, and phosphate solubilization, and most of them are well documented [16]. In the case of endophytic bacteria isolated in native or hybrid genotypes of maize, genera, such as *Pantoea*, *Bacillus*, *Burkholderia*, *Klebsiella*, and others were found [6, 8]. There are mechanisms by PGPE to mitigate stress responses, and these are described in the following sections [17].
