Enterobacter cloacae, an Obligatory Endophyte of Pollen Grains of Mediterranean Pines

Enterobacter cloacae was found to be associated with the pollen of several Mediterranean pines. The bacterium was detected only in mature pollen ofPinus halepensis, P. brutia, andP. pinea. E. cloacae is considered to be an obligatory endophyte based on its occurrence in disinfected male cones and the successful inoculation of seedlings of the above 3 species withE. cloacae AS1 isolated from pollen ofP. halepensis used as a model strain. Strain AS1 was able to produce indolyl-3-acetic acid (IAA) froml-tryptophan in culture, and this was probably the source of the increased IAA content in the germination medium of pollen. In addition, strain AS1 promoted adventitious root formation in mung bean (Vigna radiata) cuttings. However, it was not possible to obtain bacterium-free pollen to elucidate its role in pollen germination.