Endophytic and rhizospheric bacteria protect plants from water stress

Agriculture is facing an emerging request of food to satisfy the needs of an increasing world population. Moreover, the increasing frequency of dry periods in many world regions, together with the problems associated with soil salinity and phytopatogen attacks, result in a reduction of crop productivity. A low impact strategy to improve crop productivity without extending agriculture surfaces and using agro-chemicals, is the association of plants with Plant Growth Promoting Bacteria (PGPB) able to stimulate plant growth and to reduce plant disease incidence. Here we show that PGPB isolated from the phytosphere in arid ecosystems can couteract water sress in plants in vivo. PGPB were isolated from olive tree, grapevine and pepper plants from arid and semi-arid regions of the Mediterranean basin. Testing in vitro the isolates for their plant growth promotion (PGP) activities under different stress conditions allowed to identify polyvalent isolates, i.e. those capable of expressing multiple activities against drought and salinity stresses. When water stress was imposed to pepper test plants, they showed a similar behaviour to unchallenged plants only when they were pre-inoculated with the PGPBs, while those uninoculated showed water stress symptoms already after 8 day of drought. The increment of the leaf photosynthetic level and the root and shoot length and mass confirmed that the bacterial activity improved the overall plant health. Several activities in the tested PGPB have been associated to the improvement of plant health under water stress, among which several linked to plant hormone metabolism and including ACC deaminase and volatile compound production and auxin synthesis. The selected PGPB were also capable of and efficient colonization of plant root tissues. Transformation with plasmid encoding a green fluorescent protein and recolonization experiments of Arabidopsis thaliana roots indicated that the selected PGPB were able to colonize and efficiently enter the root tissues .