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 43

Figure 2.1 Schematic diagram of the proposed signaling pathways from the percep-
tion of dehydration via abiotic stress and developmental cues to gene expression in lily
pollen. At least three signal transduction pathways exist: pathway I is for the group of
genes induced by environmental dehydration stress and is ABA-dependent. The other
two pathways are for the dehydration-irrelevant genes triggered by developmental
desiccation and other cues in the anther, through which pathway II is ABA-dependent
and pathway III is ABA-independent . (From Hsu et al. (2007) with permission from Springer
Science and Business Media).
The dehydration-inducible genes in pathway I require ABA for signaling.
However, the induction of dehydration-induced genes can be independent
of ABA ( Shinozaki and Yamaguchi-Shinozaki, 1997 ). Pathway II includes
dehydration-irrelevant genes, and their induction is ABA-dependent. Path-
way III of signal transduction is for dehydration-irrelevant genes, but its
signaling is ABA-independent. For instance, polygalacturonase accumulates
in mature pollen through developmental cues, but it is not induced by
desiccation and exogenous ABA ( Chiang et al., 2006 ; Hsu et al., 2007 ).
Although it cannot be induced by dehydration and ABA, its expression
is influenced by environmental stresses such as heat, high CO 2 , low tem-
perature, and ethylene ( Dopico et al., 1993 ; Hiwasa et al., 2003 ; Kagan-Zur
et al., 1995 ; Rothan et al., 1997 ). The genes in pathways II and III, triggered
by developmental cues other than desiccation in the anther, are intriguing
because little is known about the regulation of such genes.
2.2. Proteins Encoded by Desiccation-Associated Genes in
Pollen
Pollen contains large amounts of presynthesized mRNAs and proteins
( Mascarenhas, 1993 ; Taylor and Hepler, 1997 ; Twell, 2002 ). However, the
presence of presynthesized mRNAs and proteins needed for pollen ger-
mination and tube growth should adapt to the highly dehydrated state of
the pollen. Of the 34 identified proteins encoded by desiccation-associated