Death Hormone Senescence Signal

As outlined above (Section IF), monocarpic senescence is under correlative controls, and usually, the developing fruits, particularly the seeds, exert some senescence-promoting influence. Thus, it makes sense to look for a simple solution like production of a senescence hormone (a positive signal) from the fruits (Wilson, 1997).

Except for soybean, there has not been much effort to characterize the in vivo behavior of the senescence signal (Nooden, 1988). In soybean, it seems to be exerted by the developing seeds on the leaves fairly late in seed development, i.e., at a time, when the seeds are able to live independently from the parent plant. The movement of the senescence signal is somewhat restricted by the vascular architecture (orthostichies), and it can act on the leaf nearest the pod via the xylem, i.e., it is not blocked by steam girdling of the petiole on that leaf (Figs. 15-1 and 15-2).

There is some direct evidence for an actual chemical factor that behaves like the senescence signal in soybean (Nooden et al., 1990). Basically, axenic leachate from seeds excised at their senescence-inducing stage induces senescence resembling the in vivo pattern when it is fed into an excised leaf through the base of the petiole. A wide range of known senescence-inducing compounds including abscisic acid have been tested, but none matches the senescence signal.

Another line of evidence warrants some discussion here, because it has been offered as a test of the senescence hormone hypothesis and implied evidence against the senescence hormone idea (Hamilton and Davies, 1988). 14C-Labeled CO2 was supplied to the pods of pea plants in an effort to determine if labeled metabolites are exported to the shoot apex where the pods cause senescence. There are a number of technical problems with these experiments, but the simplest and most relevant here is isotopic dilution. Given the extent of isotopic dilution (i.e., lowering the specific activity of the 14C-CO2) by the 12C-CO2 produced by the pods or from the atmosphere and then further dilution by metabolites within the source and target tissues, one cannot expect to get detectable levels of 14C into hormone-like molecules which function at very low concentrations (e.g., < 10-6 M). It should also be noted that much of the 14C from 14C-CO2 appeared in sucrose, which as noted above, can promote senescence; however, pods are unlikely to export the large amounts required even if they actually do export some small quantities of organic materials.

So, where does the senescence hormone idea stand? It remains a feasible option, and it would explain the behavior of the senescence signal in soybean at least. Nonetheless, the senescence signal hypothesis cannot be proven by eliminating the alternatives, and direct evidence remains very limited.

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