Paulownia - Bioenergy Crop Plants - page 681

programs are needed for reforestation and to improve existing forest tree species. Plant genetic
transformation techniques and gene isolation and characterization are no longer serious problems;
forest-tree species should be a major target for commercial genetic engineering and molecular
breeding. The introduction of cloned genes into plant cells and the recovery of stable fertile
transgenic plants can be used to make modifications in a plant and has created the potential for
genetic engineering of plants for crop improvement.
27.6.1 t iSSuE c ulturE
The use of an in vitro propagation technique provides a supply of healthy, homogenous planting
material. Micropropagation of tree species offers a rapid means of producing clonal planting stock for
afforestation, woody biomass production, and an effective way to capture genetic gains. Generally
Paulownia is propagated through seed or by root cuttings. A conventional method of propagation
through seed is unreliable because of disease and pest problems, poor germination, altered growth
habit, and slower growth than root cuttings (Bergmann 1998; Bergmann and Moon 1997). Most
of the tissue culture work conducted on Paulownia has used MS (Murashige and Skoog 1962) and
woody plant medium (Lloyd and McCown 1981) with considerable success. For optimal results, it is
customary to optimize the composition of growth medium for the variety and the stage of growth.
27.6.1.1 organogenesis
High-frequency plant regeneration and rapid multiplication are important aspects of plant tissue culture.
Plantlet formation from cultured cells and tissues can occur via one of two routes: organogenesis
or somatic embryogenesis. Micropropagation research on various species of Paulownia is available.
Reproducible shoot and then root induction protocols have been perfected for P. tomentosa (Burger
et al. 1985; Rao et al. 1996; Rout et al. 2001; Corredoira et al. 2008), P. elongata (Bergmann and
Whetten 1998), and P. fortunei (Rao et al. 1993; Venkateswarlu et al. 2001; Khan et al. 2003). A
preliminary study carried out on P. kawakamii reported the presence of a differentially expressed
cDNA encoding a putative bZIP transcription factor (Low et al. 2001). A 6-fold increased expression
of this gene in the shoot apex region suggests involvement of this gene during the adventitious shoot
regeneration process in Paulownia. A very interesting research study to investigate the effect of
magnetic field was carried out on P. tomentosa nodal cultures (Celik et al. 2008). The study revealed
that the magnetic field strength and exposure duration are important factors for rapid multiplication,
which is probably supported by a higher concentration of chlorophyll a and b and total chlorophyll in
treated explants. Most of the micropropagation protocols that have been developed for various species
of Paulownia to date have predominantly used nodal explants. Figure 27.2a presents a schematic
depiction of a nodal explant-based multiple shoot regeneration protocol developed for P. elongata .
Paulownia plants can be produced commercially and shipped to domestic and international destinations
(Figure 27.2b). In general, rooting in Paulownia is fairly easy (Figure 27.2c). Another explant that has
been used successfully to a lesser extent is petiole with cut leaf (Figure 27.2d). In our laboratory, we
have achieved considerable success in multiple shoot regenerations using shoot tip explants (Figure
27.2e). To assist commercial production by rapid multiplication, we have tried a liquid-culture-based
production system using a liquid laboratory rocker system (Caisson Labs, North Logan, UT). Insertion
of a filter paper as a substratum in the culture box helps multiplication a great deal. In our experience,
plants can be hardened in 7-10 days in a greenhouse under misting conditions to produce a uniform
planting stock (Figure 27.2f). There have been reports of vitrification during in vitro propagation of
many Paulownia species, but reduction of vitrified shoots was possible by adjusting culture medium
conditions, especially gelling substance and concentration of sugar (Ho and Jacobs 1995).
27.6.1.2 somatic embryogenesis
One of the earliest reports of somatic embryogenesis and plantlets from callus cultures was in P.
tomentosa (Radojevic 1979). The culture medium used was MS containing 0.7% agar, 200 mg/L