206

Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

phenotype, yield penalty on normal plant development and growth

(Liu et al., 1998; Kasuga et al., 1999, 2004; Ito et al., 2006; Nakashima

et al., 2006; Gao et al., 2011; Conant et al., 2014; Reis et al., 2014). For

example, researchers have noticed growth retardation in transgenic plants

overexpressing DREB1A/CBF TF (Ito et al., 2006). These undesirable and

other pleiotropic effects of TF overexpression have been attributed to the

sensitivity of certain cell types to the overexpression. On the other hand,

studies have shown that overexpression of TFs regulates the expression of

several downstream genes, may lead to different phenotypic effects resulting

in undesirable effects. Several studies have reported pleiotropic effects under

circumstances when constitutively overexpressed TF resulted in altered

expression of downstream target genes/stress-related genes which led to an

undesirable altered phenotype of transgenic plants. Similarly, stunted growth

is the most common undesirable effect in transgenic plants overexpressing

TFs (Kasuga et al., 2004; Ito et al., 2006; Wu et al., 2009). However, pleio­

tropic effects are not always present in transgenic plants overexpressing TFs.

For example, transgenic rice plants overexpressing DREB1A/CBF3 and

OsDREB1F showed no stunted growth or any visible phenotypic alterations

(Oh et al., 2005; Wang et al., 2008). Similarly, Oh et al. (2005) reported

no pleiotropic effects in transgenic rice overexpressing basic leucine zipper

(bZIP) transcription factor (ABF3).

Similarly, no pleiotropic effects were also observed for the bZIP tran­

scription factor ABF3 when ectopically expressed in rice (Oh et al., 2005).

Liu et al. (1998) reported an interesting observation that overexpression of

certain TFs such as DREB2 was unable to induce stress tolerance (drought) in

transgenic plants. DREB2 protein contains repressor domain which prevent

the activation of downstream stress related genes. Sakuma et al. (2006)

addressed this problem and showed that deletion of this repressor domain

(deletion of a region between residues 136 and 165) stores the active form of

AtDREB2A. Furthermore, overexpression of the active form of AtDREB2A

in transgenic plants resulted up-regulation of several drought inducible

downstream genes, which led to improved drought stress tolerance of trans­

genic Arabidopsis. Yoshida et al. (2008) demonstrated that overexpression

of modified DREB2A (Sakuma et al., 2006) rendered transgenic plants

tolerant to drought and salt stress but also led to enhanced heat tolerance of

transgenic plants. This indicates the existence of crosstalk between drought

and heat and that DREB2A controls the expression of HsfA3 (heat stress TF)

(Schramm et al., 2008). Similarly, pleiotropic problems can be easily over­

come by making a smart choice of either stress-inducible promoter (RD29A)