AUTHOR=Rat Angélique , Naranjo Henry D. , Krigas Nikos , Grigoriadou Katerina , Maloupa Eleni , Alonso Alicia Varela , Schneider Carolin , Papageorgiou Vassilios P. , Assimopoulou Andreana N. , Tsafantakis Nikolaos , Fokialakis Nikolas , Willems Anne 

TITLE=Endophytic Bacteria From the Roots of the Medicinal Plant Alkanna tinctoria Tausch (Boraginaceae): Exploration of Plant Growth Promoting Properties and Potential Role in the Production of Plant Secondary Metabolites

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.633488

DOI=10.3389/fmicb.2021.633488

ISSN=1664-302X

ABSTRACT=<p>Alkannin and shikonin (A/S) are enantiomeric naphthoquinones produced in the roots of certain plants from the Boraginaceae family such as <italic>Lithospermum</italic> spp. and <italic>Alkanna</italic> spp. They possess antimicrobial, anti-tumoral and wound healing properties. The production of secondary metabolites by <italic>Alkanna tinctoria</italic> might be influenced by its endomicrobiome. To study the interaction between this medicinal plant and its bacterial endophytes, we isolated bacteria from the roots of wild growing <italic>Alkanna tinctoria</italic> collected near to Athens and Thessaloniki in Greece. Representative strains selected by MALDI-TOF mass spectrometry were identified by partial 16S rRNA gene sequence analysis. In total, 197 distinct phylotypes of endophytic bacteria were detected. The most abundant genera recovered were <italic>Pseudomonas</italic>, <italic>Xanthomonas</italic>, <italic>Variovorax</italic>, <italic>Bacillus</italic>, <italic>Inquilinus</italic>, <italic>Pantoea</italic>, and <italic>Stenotrophomonas</italic>. Several bacteria were then tested <italic>in vitro</italic> for their plant growth promoting activity and the production of cell-wall degrading enzymes. Strains of <italic>Pseudomonas</italic>, <italic>Pantoea</italic>, <italic>Bacillus</italic> and <italic>Inquilinus</italic> showed positive plant growth properties whereas those of Bacteroidetes and <italic>Rhizobiaceae</italic> showed pectinase and cellulase activity <italic>in vitro</italic>. In addition, bacterial responses to alkannin and shikonin were investigated through resistance assays. Gram negative bacteria were found to be resistant to the antimicrobial properties of A/S, whereas the Gram positives were sensitive. A selection of bacteria was then tested for the ability to induce A/S production in hairy roots culture of <italic>A. tinctoria</italic>. Four strains belonging to <italic>Chitinophaga</italic> sp., <italic>Allorhizobium</italic> sp., <italic>Duganella</italic> sp., and <italic>Micromonospora</italic> sp., resulted in significantly more A/S in the hairy roots than the uninoculated control. As these bacteria can produce cell-wall degrading enzymes, we hypothesize that the A/S induction may be related with the plant-bacteria interaction during colonization.</p>