7.5. Aquaporins and plant resistance to abiotic stresses
As discussed above, exposure of plants to abiotic constraints as
diverse as soil water deficit or dry air, heat or cold stress, ionic stress
or nutrient deprivation, or changes in irradiance challenge the plant
water status and trigger highly specific hydraulic responses [27,56].
Molecular analyses on regulation of the whole aquaporin family in
these contexts have often revealed complex transcriptional and posttranslational
response patterns, with sometimes opposite profiles
between isoforms. Genetic approaches have been developed, as a
complement of these expression studies. However, they also led to
somewhat contrasting results, depending on the plant species or stress
conditions investigated. For instance, antisense inhibition of PIP1s in
transgenic tobacco plants reduced Lpr and leaf water potential, and
enhanced plant sensitivity to drought stress [137]. In contrast, inhibition
of PIP1s and/or PIP2s using a similar antisense approach in Arabidopsis
did not modify leafwater potential and hydraulic conductivity in plants
under normal or water deficit conditions. After rewatering, however,
the recovery of leaf water potential and plant hydraulic conductivity
was significantly delayed in antisense as compared to wild-type plants
[138]. These data indicate that PIPs can play an important role during
the early phase of water stress, by acting on root water transport
(a transient ABA-mediated increase in Lpr can be observed before a
longer term inhibition), or during recovery from water stress,
by favoring water mobilization in dehydrated leaves.
Overexpression of aquaporin genes has become a widely used
strategy to understand and possibly engineer plant water relations
under stress. Numerous studies have shown that enhancing aquaporin
expression can confer on transgenic plants either a higher resistance
[116,139–143], or a higher sensitivity [117,144,145] to stresses.
Remarkably, negative effects on stress resistance were rather seen
when an aquaporin of interest was over-expressed in a heterologous
plant species [117,144,145]. We speculate that the foreign
aquaporin may not be properly recognized by the endogenous stress
response machinery. Among the few success stories, we may cite the
case of OsPIP1;3, which is induced by water-deficit in a droughtresistant
rice cultivar. Its expression in a lowland drought-sensitive
rice cultivar, using a stress-induced promoter, significantly enhanced
plant water stress resistance [142]. Spectacular results were also
obtained with SlTIP2;2, a stress-induced aquaporin of tomato [146]. Its
over-expression in the same species dramatically altered plant water
relations, enhancing transpiration and modifying leaf water potential
maintenance under drought. Nevertheless, the transgene had beneficial
effects on plant growth and fruit yield under both control and water
stress conditions.
7.5. Aquaporins and plant resistance to abiotic stressesAs discussed above, exposure of plants to abiotic constraints asdiverse as soil water deficit or dry air, heat or cold stress, ionic stressor nutrient deprivation, or changes in irradiance challenge the plantwater status and trigger highly specific hydraulic responses [27,56].Molecular analyses on regulation of the whole aquaporin family inthese contexts have often revealed complex transcriptional and posttranslationalresponse patterns, with sometimes opposite profilesbetween isoforms. Genetic approaches have been developed, as acomplement of these expression studies. However, they also led tosomewhat contrasting results, depending on the plant species or stressconditions investigated. For instance, antisense inhibition of PIP1s intransgenic tobacco plants reduced Lpr and leaf water potential, andenhanced plant sensitivity to drought stress [137]. In contrast, inhibitionof PIP1s and/or PIP2s using a similar antisense approach in Arabidopsisdid not modify leafwater potential and hydraulic conductivity in plantsunder normal or water deficit conditions. After rewatering, however,the recovery of leaf water potential and plant hydraulic conductivitywas significantly delayed in antisense as compared to wild-type plants[138]. These data indicate that PIPs can play an important role duringthe early phase of water stress, by acting on root water transport(a transient ABA-mediated increase in Lpr can be observed before alonger term inhibition), or during recovery from water stress,by favoring water mobilization in dehydrated leaves.Overexpression of aquaporin genes has become a widely usedstrategy to understand and possibly engineer plant water relationsunder stress. Numerous studies have shown that enhancing aquaporinexpression can confer on transgenic plants either a higher resistance[116,139–143], or a higher sensitivity [117,144,145] to stresses.Remarkably, negative effects on stress resistance were rather seenwhen an aquaporin of interest was over-expressed in a heterologousplant species [117,144,145]. We speculate that the foreignaquaporin may not be properly recognized by the endogenous stressresponse machinery. Among the few success stories, we may cite thecase of OsPIP1;3, which is induced by water-deficit in a droughtresistantrice cultivar. Its expression in a lowland drought-sensitiverice cultivar, using a stress-induced promoter, significantly enhancedplant water stress resistance [142]. Spectacular results were alsoobtained with SlTIP2;2, a stress-induced aquaporin of tomato [146]. Itsover-expression in the same species dramatically altered plant waterrelations, enhancing transpiration and modifying leaf water potentialmaintenance under drought. Nevertheless, the transgene had beneficialeffects on plant growth and fruit yield under both control and waterstress conditions.
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