Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

PROGRAMMED CELL DEATH DURING PLANT SENESCENCE 99
5.7.4 Elements of the upstream senescence-signaling PCD pathway
Receptor-like protein kinases have been implicated in senescence signaling (Hajouj et al.,
2000; Robatzek and Somssich, 2002; Arora et al., 2006). It is known that receptor-like
kinase serves as receivers and transducers of external and internal stimuli, acting through
phosphorylation/dephosphorylation cascades that eventually lead to changes in gene expression.
The senescence-associated kinase receptor gene (SARK) behaves as a typical
SAG, which is induced by senescence-inducing factors (ethylene; jasmonate) and repressed
by senescence-delaying factors (cytokinin, light). Both transcript and protein appear prior to
the onset of senescence (Hajouj et al., 2000). Another receptor kinase, senescence-induced
receptor kinase (SIRK), was identified by Robatzek and Somssich (2002) as one of the candidate
targets of the senescence transcription factor AtWRKY6. This transcription factor is
able to respond to senescence and other external stimuli often associated with senescence
and plant defense (Robatzek and Somssich, 2001). As this factor does not require de novo
synthesis for its activation, it can be considered an early-type element and together with
other WRKY, they are likely substrate for kinase or phosphatases (Eulgem et al., 2000).
In contrast to SARK that is expressed in roots and during senescence, SIRK is the only
identified plant receptor kinase developmentally expressed solely during senescence. In
addition to senescence, SIRK and WRKY6 participate in pathogen defense pathway. A
dual function for SIRK/WRKY6 has been proposed. On the one hand, senescence would
be initiated by the binding of a senescence-triggered signal to SIRK, which would lead
to the expression of WRKY6. On the other hand, SIRK would activate a kinase cascade
that would modify WRKY6 protein, resulting in the induction of several genes, including
SIRK.
MAP kinase cascade seems to be part of the signal transduction pathway linking the
senescence developmental signal and the downstream elements. Thus, MAPKK in tomato
and MAPK in maize are both identified by the increase in mRNA levels and by activity
gels during senescence. Interestingly, neither PGRs (like ethylene, cytokinin, GA, ABA) nor
nutrient starvation has affected either the kinase activity or the transcript levels of this natural
senescence-inducible MAPK (Berberich et al., 1999). The importance of MAP kinase in the
transcriptional activation cascade is consistent with the constitutive expression of a set of
potential WRKY effector genes observed in the MAP kinase mutant of Arabidopsis mpk4
(Petersen et al., 2002).
Other posttranslational modifications seem to be involved in the regulation of senescence.
The finding that the delayed senescence mutation ORE9 encodes a mutated form of
an F-box protein suggested that ubiquitin tagging and proteolysis of a repressor could be
an upstream regulatory component of the PCD/senescence pathways (Woo et al., 2001).
ORE9 is able to physically interact with ASKI (an Arabidopsis Skyline protein), which is a
component of the SCF type of E3 ubiquitin ligase complex. As F-box proteins appear to be
responsible for target specificity through protein–protein interaction motifs such as WD-40
(Callis and Vierstra, 2000); elucidation of the proteins interacting with ORE9 will be very
important in understanding which factors are targeted for degradation that are important
for senescence. Furthermore, this proteolytic pathway seems to be activated during senescence:
the E2-ubiquitin carrier protein shows increased expression during leaf senescence
(Nicotiana sylvestris; Genschik et al., 1994). Increased expression of a polyubiquitin gene,
SEN3, has also been detected in senescent leaves of Arabidopsis (Park et al., 1998).