Using plant growth-promoting microorganisms (PGPMs) to improve plant development under in vitro culture conditions

The use of beneficial microorganisms improves the performance of in vitro - cultured plants through the improvement of plant nutrition, the biological control of microbial pathogens or the production of phytohormones that promote plant growth and development. Plant in vitro culture techniques are highly useful to obtain significant amounts of true-to-type and disease-free plant materials. One of these techniques is clonal micropropagation which consists on the establishment of shoot tip cultures, shoot multiplication, in vitro rooting and acclimatization to ex vitro conditions. However, in some cases, the existence of recalcitrant genotypes, with a compromised multiplication and rooting ability, or the difficulties to overcome the overgrowth of endophytic contaminations might seriously limit its efficiency. In this sense, the establishment of beneficial interactions between plants and plant growth-promoting microorganisms (PGPMs) under in vitro culture conditions might represent a valuable approach to efficiently solve those restrictions. During the last years, significant evidence reporting the use of beneficial microorganisms to improve the yield of in vitro multiplication or rooting as well as their acclimatization to greenhouse or soil conditions have been provided. Most of these positive effects are strongly linked to the ability of these microorganisms to provide in vitro plants with nutrients such as nitrogen or phosphorous, to produce plant growth regulators, to control the growth of pathogens or to mitigate stress conditions. The culture of A. thaliana under aseptic conditions has provided high-quality knowledge on the root development signaling pathways, involving hormones, triggered in the presence of PGPMs. Overall, the present article offers a brief overview of the use of microorganisms to improve in vitro plant performance during the in vitro micropropagation stages, as well as the main mechanisms of plant growth promotion associated with these microorganisms.