response in many plant species further leading to the idea thatrolgenes might act
synergistically. This natural phenomenon could be exploited in biotechnology to
generate the so called transformed root cultures. Hairy roots are readily induced
from many plant crops and grow profusely on hormone-free media with pla-
giotropic growth, abundant lateral branches and a high root hair density, charac-
teristics which define the hairy root phenotype (Zhou et al. 2011 ). Today, improved
techniques forAgrobacterium-mediated transformation allow inducing hairy roots
from a high number of plant species including rare or endangered medicinal plants,
contributing to global biodiversity preservation (Mehrotra et al. 2015 ). After a
period of stabilization in solid and/or liquid growth media, hairy root cultures are
established, which can be used for multiple purposes. Two types of transformed
roots can be distinguished: (i) wild type hairy roots harboring the complete set of
genes in T-DNA from the corresponding wild type Ri plasmid, and (ii) transgenic
hairy roots harboringrolgenes alone, combinations of them and foreing genes of
interest transferred byA. rhizogenesstrains from T-DNAs in binary vectors (Ono
and Tian 2011 ; Zhou et al. 2011 ; Mehrotra et al. 2015 ).
By three decades ago several investigations showed the capacity of transformed
root system for the synthesis of biologically active substances, especially alkaloids.
Today, the high genetic and biochemical stability (compared to undifferentiated
cultures) and the high growth rates (compared to non-transformed roots) attract high
interest on these systems as biological matrices for the production of high-value
secondary metabolites. Further, its phytohormone-independent growth is also of
significance given that some growth regulators are toxic and thus its presence in
final products is unacceptable (Georgiev et al. 2007 ; Mehrotra et al. 2015 ). Since
time agoA. rhizogenes rolgenes have been considered to affect transformed root
growth and development, but a new function became apparent with the discovery
that these genes could potentially activate secondary metabolism upregulating some
defense genes by a yet unknown mechanism (Bulgakow 2008 ; Bulgakow et al.
2013 ). Even alone,rolgenes can induce amounts of secondary metabolites higher
than those obtained in plant cell cultures. IndividualrolA,rolBandrolCgenes
increased biosynthesis of anthraquinones in transformed calli ofRubia cordifoliaby
increasing transcription of a key gene in this metabolic pathway, the isochorismate
synthase gene (Kiselev et al. 2007 ; Shkryl et al. 2011 ). Whether or not therolgenes
effects on plant secondary metabolism could be synergistic would, however, depend
on plant species among other factors.
At present, the list of natural products of industrial or pharmaceutical value
obtained by mean of this technology is considerably long (see Pistelli et al. 2010 ;
Matveeva et al. 2015 ; Zhou et al. 2011 , forfigure) as well as that of plants of origin.
Together with standard metabolites already present in mother plants, hairy roots
could also be considered as potential sources for new natural products.
Transformation itself might some way affect root secondary metabolism expression
so that secondary compounds, other than those normally found in untransformed
tissues, were synthesized (Berkov et al. 2003 ; Hu and Du 2006 ). Further treatment
of hairy root cultures with certain chemical agents, a process called elicitation, may
drastically alter the metabolite profile (Kawauchi et al. 2010 ).
14 Hairy Root Culture as a Biotechnological Tool inC. sativa 301