Electric Signaling and Pin2 Gene
Expression on Different Abiotic Stimuli Depend on a Distinct Threshold
Level of Endogenous Abscisic Acid in Several Abscisic Acid-Deficient
Tomato Mutants |
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Authors: | Oliver Herde Hugo Pe?a Cortés Claus Wasternack Lothar Willmitzer Joachim Fisahn |
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Institution: | Max Planck Institut für Molekulare Pflanzenphysiologie, Karl Liebknechtstrasse 25, D-14476 Golm, Germany (O.H., L.W., J.F.);Universidad de Santiago, Casilla 40, Correo 33, Santiago, Chile (H.P.C.);Institut für Pflanzenbiochemie, Weinberg 3, 06120 Halle/Saale, Germany (C.W.) |
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Abstract: | Experiments were performed on three
abscisic acid (ABA)-deficient tomato (Lycopersicon
esculentum Mill.) mutants, notabilis,
flacca, and sitiens, to investigate the
role of ABA and jasmonic acid (JA) in the generation of electrical
signals and Pin2 (proteinase
inhibitor II) gene expression. We selected
these mutants because they contain different levels of endogenous ABA.
ABA levels in the mutant sitiens were reduced to 8% of
the wild type, in notabilis they were reduced to 47%,
and in flacca they were reduced to 21%. In wild-type
and notabilis tomato plants the induction of
Pin2 gene expression could be elicited by heat
treatment, current application, or mechanical wounding. In
flacca and sitiens only heat stimulation
induced Pin2 gene expression. JA levels in
flacca and sitiens plants also
accumulated strongly upon heat stimulation but not upon mechanical
wounding or current application. Characteristic electrical signals
evolved in the wild type and in the notabilis and
flacca mutants consisting of a fast action potential and
a slow variation potential. However, in sitiens only
heat evoked electrical signals; mechanical wounding and current
application did not change the membrane potential. In addition,
exogenous application of ABA to wild-type tomato plants induced
transient changes in membrane potentials, indicating the involvement of
ABA in the generation of electrical signals. Our data strongly suggest
the presence of a minimum threshold value of ABA within the plant that
is essential for the early events in electrical signaling and mediation
of Pin2 gene expression upon wounding. In contrast,
heat-induced Pin2 gene expression and membrane potential
changes were not dependent on the ABA level but, rather, on the
accumulation of JA.The plant hormones ABA and JA play a predominant role in the
conversion of environmental signals into changes in plant gene
expression. An increase in endogenous ABA and JA levels precedes and is
involved in Pin2 (proteinase
inhibitor II) gene expression upon wounding
(Peña-Cortés et al., 1989, 1991, 1995, 1996; Farmer and
Ryan, 1992; Farmer et al., 1992). This increase in ABA and JA is not
restricted to the tissue damaged directly but can also be detected in
the nonwounded, systemically induced tissue
(Peña-Cortés et al., 1989; Peña-Cortés and
Willmitzer, 1995). The accumulation of ABA and JA have been described
for several plant species, including potato, tomato, and tobacco
(Sanchez-Serrano et al., 1991; Peña-Cortés and Willmitzer,
1995).Further evidence for the involvement of ABA and JA in wound-induced
Pin2 gene expression was provided by a series of experiments
in which potato plants were sprayed with ABA or JA and Pin2
mRNA accumulated in the absence of any wounding
(Peña-Cortés et al., 1989; Hildmann et al., 1992). Both
nonsprayed leaves and leaves that were sprayed directly showed
increased Pin2 mRNA levels with a pattern identical to the
one described for wounded plants (Peña-Cortés et al., 1988;
Peña-Cortés and Willmitzer, 1995). Conclusive evidence for
the involvement of ABA in wound-induced Pin2 activation was
obtained from mutants impaired in ABA biosynthesis. Consequently, wound
induction of Pin2 was not observed in the droopy
mutant of potato or the sitiens mutant of tomato
(Peña-Cortés et al., 1989). However, in these mutants
treatment with ABA caused a return of the accumulation of
Pin2 mRNA to levels normally found in wild-type plants upon
wounding (Peña-Cortés et al., 1991).Like wounding, the application of electrical current was able to
initiate ABA and JA accumulation in wild-type plants but not in
ABA-deficient plants (Herde et al., 1996). These results suggested
that, like wounding, electrical current requires the presence of ABA
for the induction of Pin2 gene expression (Herde et al.,
1996). In contrast to wounding and electrical current, burning of
leaves activated Pin2 gene expression in sitiens
mutants by directly triggering the biosynthesis of JA via an
alternative pathway that is independent of endogenous ABA levels (Herde
et al., 1996).To determine the endogenous levels of ABA that are sufficient to
mediate electrical current-, heat-, and wound- induced Pin2
gene expression via electrical signals, we used several tomato mutants
containing progressively reduced levels of ABA. The effects of these
attenuated ABA levels on JA content and membrane potentials and the
expression pattern of Pin2 genes were analyzed. Analysis of
JA content was conducted to confirm the existence of an alternative
pathway that is independent of endogenous ABA levels in the different
ABA-deficient mutants. |
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