Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

104 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Most interesting is the interaction between cytokinins and sugars and its relation with
senescence. Cytokinins have been found to increase extracellular invertase and sugar uptake
in different plant systems. One hypothesis is that this would lead to a delay in senescence
by increasing the sink strength of the organ. Indeed, contrary to what happens in wildtype
plants, transgenic sag12::ipt plants show an increase in invertases with the age of
the leaf that accompanies the delay of senescence. Furthermore, sag12::cin1 plants that
express invertases under the control of a senescence-inducible promoter show a delay in
senescence. And finally, transgenic plants carrying a cytokinin-inducible promoter cin6
linked to an invertase inhibitor gene show no delay in senescence. All these experiments
give further support to the importance of achieving high levels of invertase/sugars for a
delayed senescence. This is however far from being the complete history as high levels of
sugars are also inducers of senescence and, indeed, some of the sag12::cin1 lines show a
premature senescence rather than a delay, indicating that an optimum range of sugar levels
is important and that a fine control of senescence by sugar levels is in action.
Although analysis of promoters of different photosynthesis-related genes that showed
decreased expression during senescence did not reveal common regulatory elements for
sugar regulation (Sheen et al., 1999), specific regulatory elements have been detected in
the promoters of α-amylase, malate synthase, and RbcS genes that are required for sugar
repression. Interestingly, SPF1, a transcription activator that binds SP8 motif of sugarregulated
genes contains orthologs in Arabidopsis that belong to the WRKY family of
transcription factors. Interestingly, some of these factors, which are known to bind W boxes
found in promoters of α-amylase and defense-related genes (Du and Chen, 2000), are also
induced during senescence.
The sugar connection can be at the crossroads of the multiple environmental and endogenous
factors that affect senescence. This can be accomplished by the existence of
regulatory sequences present in the promoter regions. Thus, regulatory sequences present
in sugar-regulated genes include the G-box (Martinez-Garcia et al., 2000) and the related
ABA responsive elements (Pla et al., 1993), which could channel the input signals from
the environment through phytochrome/ABA signals (light/developmental or environmental
stress). Alternatively, the interaction can be at the level of elements of the signal transduction
pathway of senescence or any of the hormones that affect senescence. In this same
direction, the ethylene signal transduction element EIN2 contains a cytoplasmic carboxyl
terminus that is sufficient to activate downstream elements in the ethylene pathway and has
structural homology to the yeast glucose sensor Snf3 (Alonso et al., 1999). It is thus possible
that the ethylene pathway may integrate signals from sugar levels and senescence. The
existence of a network is highlighted by the several sugar-insensitive Arabidopsis mutants
(insensitive or uncoupled), which are affected in hormone action (Gazzarrini and McCourt,
2001). Unfortunately, most of what is known on the interaction of sugars and hormones has
been studied in Arabidopsis during early seedling growth and not in senescent organs. It
is tempting to speculate that the hypothetical model depicted for early seedling growth in
Arabidopsis can be extended to include senescent leaves (Gazzarrini and McCourt, 2001).
In this view, sugars obtained through hydrolysis of storage compounds from senescent
leaves (lipids, starch, fractions, etc.) would signal an increase in the levels of ABA. Contrary
to the effect on germinating seedlings, where sugar signaling causes growth arrest, the
senescent leaf will inhibit photosynthesis and activate the lipid/starch/fraction breakdown
pathway.