Agriculture Reference
In-Depth Information
(transcriptional activation) program for senescence (Bleecker, 1998). A number of potential
transcription factors are now known to be associated with senescence (Hinderhofer and
Zentgraf, 2001; Robatzek and Somssich, 2001; Yang et al., 2001; Zentgraf and Kolb, 2002).
Furthermore, a comprehensive analysis of the promoter region from 23 genes that are
induced during leaf senescence indicates that the consensus W region for binding of WRKY
proteins is highly enriched in those genes (Zhu et al., 2001).
A recombinant form of one of those factors, WRKY53, is able to bind to W boxes present
in the promoters of a number of genes, some of which happen to be transcriptional factors
themselves (like other WRKY members), or genes whose mutation alters the senescence
program (like the F-box protein ORE9) or other SAGs (like SAG12 , CATs , etc.). This suggests
the possibility (a) that WRKY53 is part of a transcriptional activation cascade and (b) that
some of the SAGs are to be the targets of WRKY53, and therefore responsible for its
activation during senescence. Interestingly, both age of the leaf and age of the plant drive
the same set of SAG genes including a set of transcription factors (Hinderhofer and Zentgraf,
2001).
Another study identified a number of transcription factors that are shared between senes-
cence and pathogen defense programs. Indeed, the WRKY genes are upregulated during
pathogen infection, wounding, and senescence, and some of them are shared between dif-
ferent programs (Eulgem et al., 2000; Chen et al., 2002). More specifically, AtWRKY4, 6,
and 7 are highly induced during senescence. Transcription factors induced during senes-
cence are not restricted to the WRKY but include other families such as AP2/ErebpS, Myb,
bZIPs, and homeobox (Chen et al., 2002).
The complexity of the transcriptional activation network can be illustrated with the can-
didate cis -acting elements present in the promoter in any one of the senescence-inducible
WRKYs. WRKY6 promoter contains tissue-specific motives, W boxes that have been found
to negatively influence the expression of AtPRl gene, an asl-like element and MYB recogni-
tion region that have been shown to bind tga factors. These interactions between mis -acting
elements and trans -acting factors are likely to mediate AtWRKY6 gene expression in re-
sponse to SA, JA, and auxin (Robatzek and Somssich, 2001).
When AGLI5, a member of the plant MADS domain family of regulatory factors, was
expressed in Arabidopsis under the control of a 35S promoter or under glucocorticoid-
inducible promoter, an important increase in the longevity of both sepals and petals was
observed (Fernandez et al., 2000; Fang and Fernandez, 2002). AGL15 is normally expressed
in juvenile tissues. Moreover, it was found that overexpression only affects longevity if the
transgenic is switched on around the time of flower opening (before senescence starts).
Together, these observations suggest that AGL15 contributes to maintain the juvenile non-
senescent stage probably by repressing the senescence program (Fang and Fernandez, 2002).
Similarly, transgenic tobacco plants expressing the maize homeo box gene knotted1 under
the control of the senescence-activated promoter SAG12 resulted in increased cytokinin
content and delayed senescence (Ori et al., 1999). It is proposed that ectopic expression
of Kn1 , a gene whose function in wt meristems seems to maintain a population of inde-
terminate cells in which differentiation is delayed or inhibited, would block developmental
progression to senescence in fully mature leaves of SAG12::Kn1 plants. These results re-
veal the existence of both positive and negative transcription signaling to the developmental
senescence program, and that probably the establishment of the senescence program may
require first switching-off juvenile factors such as AGL15 or Kn1.
Search WWH ::




Custom Search