Recent Trends in Endophyte Research

Endophytes are endosymbiotic microorganisms colonizing the internal tissues of healthy host plants [1] and possess the ability to improve the quality and growth rate of their respective hosts [2]. Their colonization does not produce any disease symptoms or morphological changes like gall formation of plant tissues [3]. Most of the plants on earth are host to one or more types of endophytes [4]. These endophytes can be either bacteria or fungi [5,6]. Their population density in a host plant can vary from hundreds to more than 9 x 109 bacteria per gram of plant tissue [7,8, 9]. They can be either obligate or a facultative and the obligate types cannot be cultured due to their specificity of growth conditions. On the other hand, facultative endophytes can be cultured outside the plant tissue using artificial nutrient media [10, 11]. Endophytes form an important part of the micro-ecosystem inside plant tissues [12]. The most explored endophytes are non-pathogenic fungi that provide a number of useful characteristics to their host plant. However, bacterial endophytes remain an unexplored group [13]. Any bacteria which could be isolated from a surface-sterilized plant or extracted from its tissues can be called an endophyte if it does not affect the plant negatively. Bacteria can positively promote plant growth whereas studies show that plants are able to select these beneficial bacterial members in their microbiome including those inside the plant tissues [14, 15, 16]. There is no shred of evidence suggesting that these bacteria take advantage in this relationship [17], but certainly, they get protection from pathogens in adverse times. They could also communicate much better than the rhizospheric bacteria at times of stress [18,19].