to PPT. Considerable results have been achieved
in the transformation experiment of S. polyrhiza
and W. arrhiza by adding 5 mg/L hygromycin
(Khvatkov et al. 2015a, b; Thu et al. 2015). The
conditions for regeneration and selection in plant
gene transformation were not immutable and the
optimal conditions should be evaluated with
specific explants and plant vectors involved.
15.4 Applications of Genetic
Transformation in Duckweeds
15.4.1 Fundamental Researches
The five genera (Spirodela, Landoltia, Lemna,
Wolffiella and Wolffia) of duckweed species
widely distributed in various freshwater habitats
which were easy to be harvested (Appenroth et al.
2013). These small duckweeds (0.5–15 mm)
propagated mostly or exclusively in a vegetative
manner via budding of daughter fronds which
arose from the primordia of mother fronds. Their
organ constitution evolved from thalloid fronds
and adventitious roots (Spirodela, Landoltia,
Lemna) to thalloid fronds (Wolffiella, Wolffia)
(Landolt 1986). The genome information of some
duckweed species has been surveyed such as S.
polyrhiza (Wang et al. 2011a, b), L. minor (Van
et al. 2015). Some are being sequenced, such as
Landoltia punctata, Wolffiella neotropica, Wolffia
brasiliensis and W. columbiana. However, the
genome sizes of duckweed species from S. polyrhiza (158 Mbp) to W. arrhiza (1881 Mbp) displayed a negative correlation with their body size
and morphological structures (Landolt 1986;
Wang et al. 2011a, b; Cao et al. 2015; Wang and
Messing 2015). Furthermore, different degrees of
interspecific genome size variation were also
observed in five genera from little variation in
Spirodela (150–167 Mbp) and Landoltia (372–
427 Mbp) to 1.6- or 2-fold in Wolffiella (623–973
Mbp) or Lemna (323–760 Mbp) and up to
5.3-fold in the genus Wolffia (357–1881 Mbp)
(Wang et al. 2011a, b; Bog et al. 2015). The above
progress and advantages of duckweeds made them
ideal to be used as a model plant for fundamental
researches. Therefore, efficient transgenic methods
of duckweed species are very important for future
research (Zhao et al. 2012; Lam et al. 2014).
There remain many questions to be answered in
this family of plants, such as the relationship
among duckweed genome, morphological structure analysis, evolution and development. More
effective methods of molecular biology are to be
developed.
15.4.2 Bioenergy and Wastewater
Treatment
Bioenergy such as bioethanol and biobutanol is
important energy alternative to reduce world
dependence on fossil-based fuels (Cui and Cheng
2015). The utilization and popularization of
bioenergy have created a large amount of economic, social and environmental benefits (Lynd
et al. 1991). Materials from corn grain containing
sugar, starch or cellulose are currently the dominant
feedstock for bioethanol production which inevitably raised environmental concerns as well as
competed for limited cropland (Pimentel 2003;
Endo et al. 2008; Cheng 2010). Biobutanol, mainly
produced by acetone-butanol-ethanol (ABE) fermentation, also needs new feedstock for the fermentation (Cheng 2010). Therefore, it is important
to explore new materials that do not necessarily
compete for cropland for production of bioenergy.
Duckweeds have the ability to double their
biomass in every 16–24 h under appropriate
conditions which is faster than most plants (Peng
et al. 2007). The starch contents of duckweeds
also varied from 3 to 75% (dry based) by
manipulating culture conditions, such as temperature, light, pH, phosphate concentration or other
nutrients (Reid and Bieleski 1970; McLaren and
Smith 1976; Landolt and Kandeler 1987). In
addition, duckweeds contain relatively low lignin
content in comparison with other crops which
enable us to utilize their feedstock more economically (Bai et al. 2008). The aquatic life of
duckweeds is also farmland-free. Furthermore,
duckweeds can convert nutrients to biomass by
absorbing and purifying wastewater. Therefore,
transgenic duckweeds by overexpressing the
genes related to the synthesis of starch, sugars and
15 Transformation Development in Duckweeds
151
in the transformation experiment of S. polyrhiza
and W. arrhiza by adding 5 mg/L hygromycin
(Khvatkov et al. 2015a, b; Thu et al. 2015). The
conditions for regeneration and selection in plant
gene transformation were not immutable and the
optimal conditions should be evaluated with
specific explants and plant vectors involved.
15.4 Applications of Genetic
Transformation in Duckweeds
15.4.1 Fundamental Researches
The five genera (Spirodela, Landoltia, Lemna,
Wolffiella and Wolffia) of duckweed species
widely distributed in various freshwater habitats
which were easy to be harvested (Appenroth et al.
2013). These small duckweeds (0.5–15 mm)
propagated mostly or exclusively in a vegetative
manner via budding of daughter fronds which
arose from the primordia of mother fronds. Their
organ constitution evolved from thalloid fronds
and adventitious roots (Spirodela, Landoltia,
Lemna) to thalloid fronds (Wolffiella, Wolffia)
(Landolt 1986). The genome information of some
duckweed species has been surveyed such as S.
polyrhiza (Wang et al. 2011a, b), L. minor (Van
et al. 2015). Some are being sequenced, such as
Landoltia punctata, Wolffiella neotropica, Wolffia
brasiliensis and W. columbiana. However, the
genome sizes of duckweed species from S. polyrhiza (158 Mbp) to W. arrhiza (1881 Mbp) displayed a negative correlation with their body size
and morphological structures (Landolt 1986;
Wang et al. 2011a, b; Cao et al. 2015; Wang and
Messing 2015). Furthermore, different degrees of
interspecific genome size variation were also
observed in five genera from little variation in
Spirodela (150–167 Mbp) and Landoltia (372–
427 Mbp) to 1.6- or 2-fold in Wolffiella (623–973
Mbp) or Lemna (323–760 Mbp) and up to
5.3-fold in the genus Wolffia (357–1881 Mbp)
(Wang et al. 2011a, b; Bog et al. 2015). The above
progress and advantages of duckweeds made them
ideal to be used as a model plant for fundamental
researches. Therefore, efficient transgenic methods
of duckweed species are very important for future
research (Zhao et al. 2012; Lam et al. 2014).
There remain many questions to be answered in
this family of plants, such as the relationship
among duckweed genome, morphological structure analysis, evolution and development. More
effective methods of molecular biology are to be
developed.
15.4.2 Bioenergy and Wastewater
Treatment
Bioenergy such as bioethanol and biobutanol is
important energy alternative to reduce world
dependence on fossil-based fuels (Cui and Cheng
2015). The utilization and popularization of
bioenergy have created a large amount of economic, social and environmental benefits (Lynd
et al. 1991). Materials from corn grain containing
sugar, starch or cellulose are currently the dominant
feedstock for bioethanol production which inevitably raised environmental concerns as well as
competed for limited cropland (Pimentel 2003;
Endo et al. 2008; Cheng 2010). Biobutanol, mainly
produced by acetone-butanol-ethanol (ABE) fermentation, also needs new feedstock for the fermentation (Cheng 2010). Therefore, it is important
to explore new materials that do not necessarily
compete for cropland for production of bioenergy.
Duckweeds have the ability to double their
biomass in every 16–24 h under appropriate
conditions which is faster than most plants (Peng
et al. 2007). The starch contents of duckweeds
also varied from 3 to 75% (dry based) by
manipulating culture conditions, such as temperature, light, pH, phosphate concentration or other
nutrients (Reid and Bieleski 1970; McLaren and
Smith 1976; Landolt and Kandeler 1987). In
addition, duckweeds contain relatively low lignin
content in comparison with other crops which
enable us to utilize their feedstock more economically (Bai et al. 2008). The aquatic life of
duckweeds is also farmland-free. Furthermore,
duckweeds can convert nutrients to biomass by
absorbing and purifying wastewater. Therefore,
transgenic duckweeds by overexpressing the
genes related to the synthesis of starch, sugars and
15 Transformation Development in Duckweeds
151