Abstract
Several unfruitful attempts to grow axenic shoot cultures of Smallanthus sonchifolius, also known as yacon, were made before healthy shoots grew in association with bacteria on half strength Murashigue and Skoog media supplemented with 2.2 µM benzylaminopurine. Twenty-one bacterial isolates were obtained from in vitro S. sonchifolius plantlets, eight of these isolates were identified as Flavimonas oryzihabitans, Curtobacterium pusillum, Sphingomonas paucimobilis, and Microbacterium imperiale. These microorganisms produced indole acetic acid (IAA) at amounts varying between 8.89 to 47.45 μg/mL, reason for being classified as plant growth promoting bacteria (PGPB). The results show that buds associated with bacteria cultured on sucrose free media produced 3.77 new roots measuring 18.33 cm in length after a 30-day growing period. In contrast, buds growing on sucrose supplemented media, the number of roots induced was higher (6.67 to 14 roots/explant) but shorter in length, 4.67 to 5.83 cm. During plant acclimatization to soil, photosynthesis and water efficiency were measured showing that the plants were healthy and vigorous. A slightly higher rate of photosynthesis and water use efficiency was recorded in the plants produced on heterotrophic conditions as compared to plants grown in sucrose free media. Plants adapted well in the soil demonstrating that the PGPB community associated to S. sonchifolius in shoot cultures was not harmful to plant production. The purpose of this study was to demonstrate that the bacteria associated with in vitro S. sonchifolius shoot cultures was not the result of microbial contamination, but rather from symbiotic associations that extended from cultivation in the greenhouse, to culture and back to soil. This is the first report to show that autotrophic cultures may represent a viable alternative to grow healthy plants without eliminating beneficial bacteria associated with the host.
Highlights
Micropropagation in the horticulture industry is a routine technique used to propagate healthy and elite germplasm
Several unfruitful attempts to grow axenic shoot cultures of Smallanthus sonchifolius, known as yacon, were made before healthy shoots grew in association with bacteria on half strength Murashigue and Skoog media supplemented with 2.2 μM benzylaminopurine
The results show that buds associated with bacteria cultured on sucrose free media produced 3.77 new roots measuring 18.33 cm in length after a 30-day growing period
Summary
Micropropagation in the horticulture industry is a routine technique used to propagate healthy and elite germplasm. One of the critical steps in micropropagation is the disinfection procedure. Published reports dedicate special attention to protocols used to disinfect different plant tissues and to avoid microbial contamination during the process of plant multiplication [1,2,3]. Leifert et al [4] suggest that the elimination of plant parts close to the soil could avoid microbial growth in tissue culture because disinfection of those parts is difficult. According to Hardoim et al [5], plant rhizospheres are the hot areas of microbial colonization. Miller et al [6] demonstrate that leaf and stem tissues of Traditional Chinese Medicine (TCM) herbs are host to diverse bacteria and fungi with the potential to synthesize secondary metabolites which contribute to the plant's chemical composition and therapeutic properties Our studies on Echinacea immune enhancing properties revealed that lipoproteins and lipopolyssaccharides of bacterial endophytes are the actives responsible for in vitro macrophage activation [7]
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