New Insights into Desiccation-Associated Gene Regulation by Lilium longiflorum ASR during Pollen Maturation and in Transgenic Arabidopsis - International Review of Cell and Molecular Biology - page 50

whether the dimerization of ASR, if present in the cytoplasm, regulates
the translocation of ASR into the nucleus is still unclear. Phosphorylation/
dephosphorylation reactions are considered the most common mechanism
involved in the regulation of the nucleocytoplasmic transport of proteins
( Jans et al., 2000 ; Yoneda, 2000 ). The yeast transcription factor SWI5 is
located in the cytoplasm when it is phosphorylated near its NLS, whereas
it accumulates in the nucleus when dephosphorylated ( Moll et al., 1991 ).
For ASR, no phosphorylation site is identified in the sequence based on the
motif analysis. Thus, the regulation of nucleocytoplasmic transport of ASR
needs further investigation.
3.3. A Dual Function of ASR
Yang et al. (2005) have proposed a dual function of the lily ASR protein
being a regulator in the nucleus and an osmoprotectant in the cytoplasm. As
an osmoprotectant, the unstructural molecules of ASR in the cytoplasm may
act as chaperones stabilizing other proteins and enzymes against denatur-
ation caused by heat, cold and freeze-thaw cycles ( Hsu et al. 2011 ; Konrad
and Bar-Zvi, 2008 ), as a water replacement molecule protecting membrane
integrity in a manner similar to sugars ( Clegg et al., 1982 ), and at least in
part as a water-holding molecule retaining water in the cell as observed in
Arabidopsis plant overexpressing LLA23 or MpASR ( Dai et al., 2011 ; Yang
et al., 2005 ). Like LLA23, other LEA proteins have been hypothesized to play
a similar manner of protective role under unfavorable environments based
on their high-average hydrophilicity ( Goyal et al., 2005 ; Reyes et al., 2008 ).
LEAs can act as protein stabilizers and help the plant cells to retain water
during periods of low water availability ( Garay-Arroyo et al., 2000 ; Kovacs
et al., 2008 ). At very low water content and high cytoplasmic viscosity, LEAs
and sugars have been proposed to form a tight hydrogen-bonding network
together in the dehydrated cytoplasm ( Wolkers et al., 2001 ). Constitutive
overexpression of LEAs reported by several studies exhibits a significant
increase in their tolerance to dehydration, salt, or other stress conditions
( Dalal et al., 2009 ; Lal et al., 2008 ; Olvera-Carrillo et al., 2010 ).
The secondary level of protection comes from the regulatory properties
of LLA23 proteins as described above. The expression of RD29b and KIN2
is upregulated in 35S::LLA23 plants, suggesting that LLA23 can modulate
stress-responsive ABA signaling ( Yang et al., 2005 ). The massive production
of other osmoprotectant molecules greatly enhances the stress tolerance
of 35S::LLA23 plants under unfavorable conditions. The osmoprotec-
tants, also known as osmolytes or compatible solutes, are highly hydrophilic