Gene Cloning and Characterization - Genetics, Genomics and Breeding of Crop Plants - page 206

been used to study the in vivo function of genes, which had been isolated
from sunflower, assuming that basic processes are conserved in dicotyledon
plants. The transcription factor Hahb-4, encoded by a sunflower homeobox-
leucine zipper gene, had been isolated from sunflower as it is upregulated
by drought and ABA in roots, stems and leaves (Gago et al. 2002). To analyze
its function Arabidopsis plants were transformed with Hahb-4 under the
control of the CaMV 35S promoter (Dezar et al. 2005a). Overexpression of
Hahb-4 in Arabidopsis resulted in transgenic plants that were more tolerant
to water stress conditions than the wild type plants. However, the transgenic
Arabidopsis plants showed additional distinct morphological changes and
a delay in development due to the constitutive expression of Hahb-4 . Plants
transformed with constructs using the Hahb-4 promoter sequences fused to
gusA show the expression of the reporter gene in defined cell-types and
developmental stages and are induced by drought and abscisic acid (Dezar
et al. 2005b). Transgenic Arabidopsis plants carrying Hahb-4 under control of
its own stress-inducible promoter also exhibit water-stress tolerance though
to a lower degree, but without any major changes in the phenotype
(Manavella et al. 2006). In addition, these plants show a marked delay in
senescence and were less sensitive to ethylene. Using microarrays and
quantitative RT-PCR, Manavella et al. (2006) could demonstrate that
expression of Hahb-4 has a major repressive effect on genes related to ethylene
synthesis and ethylene signaling. The transcription factor Hahb-4 seems to
mediate the cross-talk between ethylene and drought signaling pathways.
Several inducible promoters were used to control expression of Hahb - 4 in
transgenic Arabidopsis plants (Cabello et al. 2007). These plants showed a
normal phenotype and an enhanced drought tolerance but not as high as
the transgenic plants expressing Hahb-4 constitutively. Finally, a chimeric
construct between Hahb-4 promoter and the leader intron of the Arabidopsis
Cox5c gene was used either to express gus or Hahb-4 (Cabello et al. 2007).
Transgenic plants with the chimerical construct for Hahb-4 were
indistinguishable from the wild type plants under normal growth
conditions but water stress tolerance was achieved at a level as strong as
the plants constitutively expressing Hahb-4 .
Plant homeodomain-leucine zipper (HD-Zip) proteins, unlike many
animal homeodomains (HDs), are unable to bind DNA as monomers (Tron
et al. 2004). To investigate the molecular basis of their different behavior
chimeras between HD of the sunflower HD-Zip protein Hahb-4 and that of
Drosophila engrailed (EN) were constructed and overexpressed in E. coli .
Electrophoretic mobility shift assays were used to study the binding ability
to DNA (Tron et al. 2004). Another transcription factor of the same homeobox-
leucine zipper family, Hahb-10 , was also investigated for its function in
Arabidopsis . Hahb-10 , which is regulated by light, promotes early flowering
in transgenic Arabidopsis plants (Rueda et al. 2005).