75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

233 Photoperiod-Sensitive Phase for Time to Flowering in Arabidopsis thaliana Landsberg
erecta as Revealed by Reciprocal Transfer Experiments
Daniel Villegas, Jose Alcalde
Facultad de Agronomia e Ingenieria Forestal. Pontificia Universidad Catolica de Chile. Casilla 306-22 Santiago.
Chile.
Photoperiod (P) and temperature (T) are the most important environmental factors regulating time to flowering in most
annual and biennial plants. Among photoperiod sensitive plants some are induced by a single inductive day/night cycle
(e.g. Sinapis alba, Lolium temulentum, Pharbitis nil), and may not flower if they are not exposed to sufficiently long or
short photoperiods to induce flowering, depending on whether they are long or short day plants, respectively. Others have
a long photoperiod-sensitive phase, requiring several photoperiodic cycles to become induced. Ellis et al. [Ann. Bot. 70:
87-92, 1992] have proposed an analytical procedure to estimate the durations of P-sensitive and P-insensitive phases of
preflowering development in annual crop species (e.g. soybean, barley, lentil, maize) by reciprocally transferring plants
grown in long days (LD) to short days (SD) and vice versa at different times after sowing. By analyzing the durations from
sowing to flowering (f) of the whole dataset the relative durations of preflowering subphases are estimated.
The application of this approach to Arabidopsis thaliana Landsberg erecta indicated that: a) Plants grown under 10 h d -1
photoperiod (SD) flowered 32 days later than plants grown under 20 h d -1 , at a mean 21.0 °C temperature. b) A photoperiodsensitive
phase for time to flowering was detected, starting approximately at 10 days after sowing and lasting for 35 days
under either LD or SD. c) Plants transferred from LD to SD and vice versa at different times during this sensitive phase
modified their flowering times gradually, implying a quantitative modulation of time to flowering by photoperiod. d) A post
sensitive phase of variable length depending on photoperiod perceived during the previous sensitive phase accounted for
the remaining time to flowering (corolla color visible). This behavior is different from what was expected from plants tested
previously in which differences in flowering time were explained by differences in the duration of the photoperiod sensitive
phase, and not in the duration of the post-sensitive phase. Discussion is centered on whether described molecular signaling
systems are compatible with results obtained, and on whether these results could be paralleled by expression of candidate
genes modulating time to flowering in Arabidopsis.
Researched funded by FONDECYT, Chile. Project No 1040551. A CONICYT Scholarship for doctoral studies for D.V. is gratefully
acknowledged.
234 Whole Gene Family Expression and Drought Stress Regulation of Aquaporins
Erik Alexandersson, Vamsi Moparthi, Urban Johanson, Per Kjellbom
Department of Biochemistry, Lund University, Lund, Sweden
In Arabidopsis thaliana aquaporins form a large family of proteins with 35 members. These can be divided into
four subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like proteins
(NIPs) and small basic intrinsic proteins (SIPs). PIPs and TIPs have been shown to act as water channels, even though
some isoforms have also been shown to transport small solutes such as urea and ammonia. NIPs are believed to facilitate
glycerol transport in plants, whereas the substrate specificity for SIPs still is unknown. Since many aquaporins act as
water channels, they are thought to play an important role in plant water relations. It is thus of interest to study individual
expression patterns of aquaporin isoforms in order to further elucidate their involvement in plant water transport.
Earlier, we monitored the expression patterns of all 35 Arabidopsis aquaporins in leaves, roots and flowers by cDNA
microarrays, specifically designed to avoid cross-reaction of highly homologous aquaporin isoforms, and by quantitative
real-time reverse transcriptase PCR (Q-RT-PCR) 1 . In this way we could show that many aquaporins are pre-dominantly
expressed in root or flower organs, while none seem to be leaf specific. Looking at the subfamilies, most PIPs and some
TIPs have a high level of expression, while NIPs are present at a much lower level. Upon gradual drought stress, we
showed that PIP transcripts are generally down-regulated in leaves, with the exception of AtPIP1;4 and AtPIP2;5, which
are up-regulated. AtPIP2;6 and AtSIP1;1 are constitutively expressed throughout the drought stress.
In order to further study the PIP isoforms displaying different regulation during drought stress, we fused the
promoters of AtPIP1;4, AtPIP2;5 and AtPIP2;6 with a reporter gene (GUS) and were in this way able to establish distinct
expression patterns for the three gene transcripts. We will also localise AtPIP1;4 and AtPIP2;5 on the sub-cellular level
by GFP-fusions. Furthermore, we could demonstrate that the PIP isoforms show the same kind of pattern of up- and
down-regulation during drought stress in five Arabidopsis ecotypes with different water use efficiency.
1) Alexandersson et al. 2005, Plant Mol Biol 59:469-484.