1-Aminociclopropane-1-carboxylate (ACC)-degrading bacteria having been widely studied for their use in alleviating abiotic stresses in plants. In the present study, we isolated and characterized ACC-degrading bacteria from the rhizosphere, phyllosphere, and endosphere of the Antarctic vascular plants Deschampsia antarctica and Colobanthus quitensis. One hundred and eighty of the 578 isolates (31%) were able to grow on minimal medium containing ACC, with 101 isolates (23, 37, and 41 endosphere-, phyllosphere- and rhizosphere-associated isolates, respectively) identified as being genetically unique by enterobacterial repetitive intergenic consensus (ERIC)-PCR. Subsequently, freeze/thaw treatments and ice-recrystallization-inhibition (IRI) activity assays were performed, the results of which revealed that 77 (13%) of cold-tolerant isolates exhibited putative ACC deaminase activity. Significant (p ≤ 0.05) differences in IRI activity were also observed between the studied plant niches. Surprisingly, all the cold-tolerant isolates showed ACC deaminase activity, independent of the plant niches, with 12 isolates showing the highest ACC deaminase activities of 13.21–39.56 mmol α KB mg protein−1 h−1. These isolates were categorized as ‘cold-tolerant hyper-ACC-degrading bacteria’, and identified as members of Pseudomonas, Serratia, and Staphylococcus genera. The results revealed the occurrence of cold-tolerant hyper-ACC-degrading bacteria in diverse plant niches of Antarctic vascular plants, that could be investigated as novel microbial inoculants to alleviate abiotic stresses in plants.

The use of high-throughput DNA sequencing (HTS) has revealed the great diversity of rhizobacteria in plant rhizospheres; however, only a minor portion (≤ 1%) of rhizobacteria belonging to few taxa can be cultured under laboratory conditions. In recent years, in situ cultivation has opened a window to explore a greater diversity of bacterial taxa in the environment. Here, we explored the total and culturable rhizobacterial communities associated with the rhizosphere of wheat plants by using 16S rRNAbased HTS and in situ cultivation with microwell chambers (MWCs), respectively. Results by HTS revealed to phyla Proteobacteria (29-39%), Acidobacteria (17%), Actinobacteria (11-15%), and Bacteroidetes (5-12%) as the most abundant rhizobacterial taxa in rhizosphere samples. A total of 206 isolates (26 genera) were obtained with MWCs, where coincidentally with HTS, the most abundant phyla were Proteobacteria (70.4%), Firmicutes (24%), Actinobacteria (4%), and Bacteroidetes (1.5%). At the genus level, the most of isolates (72%) belonged to Pseudomonas, followed by Bacillus, Stenotrophomonas, Delftia, and Herbaspirillum. Members of rare taxa (Lelliottia, Rhodococcus, Micrococcus, Variovorax, and Bosea) also were isolated by MWCs. In addition, a high proportion (82%) of isolates showed high similarity with plant beneficial and environmental non-pathogenic bacteria whereas a minor proportion (18%) of isolates showed high similarity to human and plant pathogenic bacteria. This study demonstrates that in situ cultivation represents a useful tool to isolate a greater number of rhizobacterial taxa, which can be investigated under laboratory conditions in bioprospecting (e.g., plant growth-promoting bacteria) and public health (e.g., human opportunist and plant pathogens) studies.