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 39

Thirty-four late genes (if LLA23 is taken into account as a control) were
identified from lily maturing anthers. Some of the identified desiccation-
associated genes were identical or similar to those cold-storage-induced
genes identified by Wang et al. (2004) . These identified sequences are
categorized into several subgroups based on the known functions of gene
products and ontology annotations derived from similarities. It includes
proteins associated with membrane/cell wall synthesis, regulators/signal-
ing, transporter, cytoskeleton-associated proteins, and so on ( Table 2.1 ).
Among these functional categories, the genes involved in membrane/cell
wall synthesis represent the largest gene subgroup, which suggests that they
are most preferentially expressed in mature anther/pollen. This observation
is in agreement with the huge increase in cell wall proteins when the num-
ber of different genes expressed is changed into relative expression levels
in Arabidopsis pollen ( Honys and Twell, 2003 ). Pollen tube elongation is
associated with the dynamic changes in the destruction and reconstruction
of the cell wall structure and continuous cell-cell interactions in the pistil.
Therefore, the genes that encode regulator/signaling and transporter pro-
teins unsurprisingly represent the second largest subgroup of products in
mature anther/pollen. An apparent investment in such genes involved in
cell wall synthesis, cytoskeleton, and signaling pathways meets the func-
tional requirements needed for pollen germination and tube growth.
During the desiccation of developing anthers, pollen genes are
established through three distinct signaling pathways ( Fig. 2.1 ). According
to Hsu et al. (2007) , signaling pathway I is for the group of genes induced
by dehydration, either from environmental water stress or from natural
desiccation within the anther. These dehydration-inducible genes require
ABA for signaling. About half of the identified genes follow pathway I sig-
naling, among which LLA23 and LLP-Rop1 (Rho GTPase of plants) are
the two representatives. Expression of LLA23 is upregulated by ABA upon
desiccation, a part of the developmental program that occurs naturally in
the anther; thus, its transcript and protein abundantly accumulate in dried
pollen ( Huang et al., 2000 ; Wang et al., 1998 ). The LLP-Rop1 gene, on
the other hand, is downregulated by ABA upon desiccation, whereas the
gene is upregulated by other developmental cues. Considering LLP-Rop1
is not pollen-specific, it is not included in the list in Table 2.1 . This unique
desiccation-associated ABA signal transduction pathway has recently been
reported, through which the Rop gene is regulated during pollen matura-
tion ( Hsu et al., 2010 ). However, the lily Asr associated with ABA signaling
upon desiccation is the focus of this study.