Biology Reference
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6.3.3 Genes Involved in Abiotic Stress Responses
6.3.3.1 Heat Shock Proteins
Heat shock proteins (HSPs) are well represented in plants. They have been
divided into six different classes on the basis of their subcellular localization,
immunological cross-reactivity and sequence similarity (Scharf et al. 2001).
Small heat shock proteins are not only induced after heat shock, but also by
other abiotic stimuli, such as cold, water deficit, heavy metals, ozone, UV
radiation and
-radiation (Barcala et al. 2008). In sunflower, three different
cDNAs were cloned and sequenced from seed-stored mRNA by Almoguera
and Jordano (1992). Sequence similarities and response to heat shock
identified one of the cDNAs as low-molecular weight heat shock protein
(lmw-HSP or small heat shock proteins, sHSP), whereas the two other clones
showed significant sequence homology to late embryogenesis abundant
(Lea) proteins from cotton and carrots, called D113 and Emb1, respectively.
Applying osmotic stress, mRNAs of lmw-HSPs were induced earlier than
Lea mRNAs. The coordinate accumulation of Lea and lmw-HSP transcripts
during embryogenesis and in response to stress and ABA suggests the
existence of common regulatory elements for Lea and lmw-HSP genes, and
supports the assumption that HSPs might have alternative functions in the
plant cell (Almoguera and Jordano 1992). Apart from this cloned cDNA Ha
hsp17.6 , a second cDNA was isolated and sequenced coding for the gene Ha
hsp17.9 (Coca et al. 1994). The two lmw-HSPs, HSP17.6 and HSP17.9, belong
to two different families of cytoplasmic LMW HSPs. Using specific
antibodies, Coca et al. (1994) observed differential expression of both proteins
during zygotic embryogenesis under controlled environment, and a
remarkable persistence of these LMW HSPs during germination. Tissue-
print immunolocalization experiments showed that HSP17.6 and HSP17.9
were homogeneously distributed in every tissue of desiccation-tolerant dry
seeds and young seedlings under non-stressed conditions (Coca et al. 1994).
In 1996, Coca et al. reported the cloning of two additional sunflower genes
coding for the small heat shock proteins (sHSPs) Ha hsp18.6 G2 and Ha
hsp17.7 G4. In sunflower, Ha hsp17.7 G4 mRNAs accumulated during zygotic
embryogenesis at 25°C and in vegetative tissues either in response to heat
shock (42°C), abscisic acid (ABA), or mild water stress treatments. In contrast,
Ha hsp18.6 G2 transcripts accumulated under heat shock conditions. In
transgenic tobacco, using a fusion construct G4::GUS the developmental
induction of Ha hsp17.7 G4 during zygotic embryogenesis was confirmed.
The proximal sequences (from -83 to +163) conferred most of the
developmental regulation to the chimeric gene construct, as well as the
responses to ABA and heat shock. However, the water stress response of
this gene was not reproduced in transgenic tobacco, which indicates that
 
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