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regulated under stress conditions [85]. In addition, TIR1 function is also
repressed by DELLA proteins, which are regulated by the phytohormone
gibberellin [87, 88]. Restoration of DELLA-mediated TIR1 arrest resulted in
better growth under salt stress; however, the growth restraint conferred by
DELLA proteins is beneficial and promotes survival in adverse conditions
[87].
Recently, Borsani
et al.
[86] reported that the
Arabidopsis
P5CDH
(Δ
1
-
pyrroline-5-carboxylate dehydrogenase) and
SRO5
, an overlapping gene of
unknown function in the antisense orientation, produced two types of siRNAs:
a 24-nt siRNA and a 21-nt siRNA. Salt and H
2
O
2
treatments triggered
SRO5
expression, leading to dsRNA formation and consequently the generation of
siRNAs, which then down-regulated
P5CDH
transcript levels through mRNA
cleavage. Down-regulation of
P5CDH
reduces proline degradation, thereby
enhancing proline accumulation, which is beneficial for enhancing salt
tolerance. However, reduced P5CDH activity also leads to the accumulation of
the toxic metabolic intermediate P5C and ROS accumulation, which is likely
counteracted by SRO5. These results imply the important role of increased
ROS activity in the regulation of salinity-induced SRO5 expression and
consequent siRNA formation.
Transcription factors control the rate of transcription by promoting (as an
activator) or blocking (as a repressor) the recruitment of RNA polymerase to
specific gene promoter sequences [89]. Hiratsu
et al.
[90] reported that
chimeric transcription factor proteins that contained the EAR (ethylene-
responsive element-binding factor (ERF)-associated amphiphilic repression)
motif acted as dominant repressors and effectively suppressed their target
genes in transgenic
Arabidopsis.
This repressive effect was termed chimeric
repressor silencing technology (CRES-T).
Arabidopsis
plants with enhanced
salt tolerance were successfully screened from transgenic seed pools
transformed with chimeric repressors of 1600
Arabidopsis
transcription factors
(Tada
et al.
, unpublished data). This result suggests that suppression of
putative negative regulators of salt tolerance could confer enhanced salt
tolerance to plants.
Although there has been no report of a relationship between the relief of
gene suppression and salt tolerance in mangroves, it is possible that salt
tolerance mechanisms similar to those reported in glycophytes are also
functional in mangroves and other halophytes. Furthermore, one can speculate
that halophyte plants have acquired a comprehensive regulatory system based
on or similar to the ancestral salt tolerance mechanisms, which plants might
silence when they move inland during evolution. This could explain why
