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 37

Abstract
LLA23,amemberoftheabscisicacid-,stress-,andripening-induced(ASR)proteinfam-
ily,waspreviously isolated from lily ( Lilium longiflorum )pollen.The lilyASR is induced
through desiccation-associated ABA signaling transduction in the pollen. ASRs are
highlyhydrophilicandintrinsicallyunstructuredproteinswithmolecularmassesgener-
allylessthan18 kDa.LLA23isabundantinthecytoplasmandnucleiofbothvegetative
andgenerativecellsofpollengrains.Theproteininthenucleusandinthecytoplasmis
partlyregulatedbydehydration.AdualroleisproposedforLLA23,asaregulatoranda
protectivemolecule,uponexposuretowaterdeficits.Thischapterreviewsthecurrent
stateofliteratureon Asr genes,proteinstructure,function,andtheirresponsestovari-
ousstresses.Inastudy,agenome-widemicroarraywasusedtomonitortheexpression
of LLA23-regulated genes, focusing on the relationship betweenASR-, glucose-, and
drought-induciblegenes,andoutlinedthedifferenceandcrosstalkofgeneexpression
amongthesesignalingnetworks.Astrongassociationwasobservedintheexpression
of stress-responsive genes and found 25 genes that respond to all three treatments.
Highly inducible genes were also found in each specific stress treatment. Promoter
sequence analysisof LLA23-induciblegenes enabledusnotonly to identifypossible
known cis -actingelementsinthepromoterregionsbutalsotoexpecttheexistenceof
novel cis -actingelementsinvolvedinASR-responsivegeneexpression.ASRcanbeused
to improve crops and economically important plants against various environmental
stresses.
1. INTRODUCTION
Pollen development is a postmeiotic process that produces immature
pollen grains from microspores. The maturation process can be dissected
into various stages. During microspore mitosis, the pollen first develops a
large vacuole that mediates nuclear migration to produce polarized cells.
Concomitantly, starch begins to accumulate and the vacuolar volume
decreases in the maturing pollen. Continued intrinsic polarization of the
pollen grains must be maintained before germination. During the final
stage of maturation, pollen grains may exhibit developmental arrest, which
involves programmed dehydration ( Franchi et al., 2011 ). This dehydration
is required for maximum maintenance of pollen viability against various
environmental stresses ( Twell, 2002 ). Once fully mature, pollen grains are
released from the dehiscent anther for dispersal.
As a consequence of dehydration, the water content of dried pollen
becomes severely limited. Aside from dehydration, the induced oxidative
stress accumulates reactive oxygen species (ROS) that further damage mem-
brane lipids, proteins, and nucleic acids ( Mittler et al., 2004 ). Thus, the for-
mation of protective molecules during the final stage of pollen maturation
is important in ensuring that the pollen withstands drying. The pollen that