Hairy root biotechnology—indicative timeline to understand missing links and future outlook

Abstract

Agrobacterium rhizogenes-mediated hairy roots (HR) were developed in the laboratory to mimic the natural phenomenon of bacterial gene transfer and occurrence of disease syndrome. The timeline analysis revealed that during 90 s, the research expanded to the hairy root-based secondary metabolite production and different yield enhancement strategies like media optimization, up-scaling, metabolic engineering etc. An outlook indicates that much emphasis has been given to the strategies that are helpful in making this technology more practical in terms of high productivity at low cost. However, a sequential analysis of literature shows that this technique is upgraded to a biotechnology platform where different intra- and interdisciplinary work areas were established, progressed, and diverged to provide scientific benefits of various hairy root-based applications like phytoremediation, molecular farming, biotransformation, etc. In the present scenario, this biotechnology research platform includes (a) elemental research like hairy root-mediated secondary metabolite production coupled with productivity enhancement strategies and (b) HR-based functional research. The latter comprised of hairy root-based applied aspects such as generation of agro-economical traits in plants, production of high value as well as less hazardous molecules through biotransformation/farming and remediation, respectively. This review presents an indicative timeline portrayal of hairy root research reflected by a chronology of research outputs. The timeline also reveals a progressive trend in the state-of-art global advances in hairy root biotechnology. Furthermore, the review also discusses ideas to explore missing links and to deal with the challenges in future progression and prospects of research in all related fields of this important area of plant biotechnology.

Ex vitro hairy root induction in detached peanut leaves for plant–nematode interaction studies

“…The A. rhizogenes strain type is strongly associated with the undesirable morphological variability of hairy roots, which comprises one of the main constraints to its use for functional gene studies. Hairy root phenotype (plagiotropic and fast growth, high degree of lateral branching, and profusion of root hairs) is associated with the imbalance in phytohormone levels promoted by the expression of T-DNA oncogenes and might hinder the analysis of transgene pleiotropic effects [12]. In the present study, the ‘K599’ strain induced hairy roots in peanut that displayed an attenuated phenotype (Fig.…”

“…As a result, transgenic hairy roots have been largely applied to molecular farming, phytoremediation, and biotransformation as well as for studying the function of genes [11, 12]. In the hairy roots, the effect of gene overexpression or silencing can be further assessed for changes in response to external stimuli, such as parasites or symbionts [13, 14].…”

Combination of Plant Metabolic Modules Yields Synthetic Synergies

“…By feeding these cells with the precursor, nicotine, this bottleneck could be removed suggesting substrate availability and compartmentalisation as two limiting factors for the production of the final product by these engineered cells. Although precursor feeding is an alternative way to improve productivity, it is not cost-effective in most cases [48]. As alternative, we designed a strategy, where the precursor was supplied "in a natural way" by secretion from one metabolic module.…”

“…We were able to release this metabolic potential in the Ntom CYP82E4 overexpressor by feeding nicotine (Fig 5b). Although this strategy requires only small amounts of the precursor, because nicotine seems to act as a signal rather than as a substrate, this approach is still costly, which holds true for precursor feeding in general [48]. In the search for cost-effective alternatives, we tested a strategy, where the nicotine would come from the conditioned medium of other cells.…”

Transgenesis as a Tool for the Efficient Production of Selected Secondary Metabolites from Plant in Vitro Cultures

“…For many reasons, this system of obtaining valuable biologically active compounds is very convenient. Hairy roots are fast growing, require no plant growth regulators, are highly genetically stable and are able to grow on a large scale [58]. Considering all the advantages of hairy roots over other types of in vitro plant culture systems, they seem to be the best transgenic alternative to medicinal plants occurring in the natural environment.…”