Niches for bacterial endophytes in crop plants: a plant biologist's view

Currently proposed means of entry of bacterial endophytes into roots of field-grown crop plants, as well as niches available for their successful colonization of living tissues, are evaluated from a plant biology perspective and the following conclusions drawn. (1) Opportunities for passive 'crack entry' into healthy, undisturbed roots in the field may not be as available as studies with laboratory-grown plants have suggested. Consistent entry of endophytes into living root tissues in the field probably requires bacterial capability to hydrolyse the hydrophobic incrustations of the walls of epidermal, hypodermal, endodermal and other cortical cells. (2) Xylem lumen apoplast is an unsuitable niche for endophytes, especially in the grasses, because of the reduced fitness such colonization imposes on plants subjected to field stresses. (3) The intercellular space apoplast is the most suitable niche for endophytes. More data about the environment of this apoplast are urgently needed so that plant varieties can be tailored to provide an optimal environment in these spaces for particular endophytes. It is suggested that many bacterial 'endophytes' may not have colonized living tissues, but are living in protective niches in dead surface tissues or closely adhering soil of rhizosheaths. Selection of strains of beneficial bacteria adapted for colonizing these external niches may be desirable.