MadSci Network: Botany
Query:

Re: How does an apple seed know when to start growing?

Date: Mon Sep 8 15:10:34 2008
Posted By: Susan Letcher, Grad student, Ecology and Evolutionary Biology, University of Connecticut
Area of science: Botany
ID: 1220743206.Bt
Message:

Dear Dave,

This is a good question, and a good hypothesis—but the truth, as far as scientists can tell, is a bit more complicated. It’s true that apples need to be exposed to cold in order to germinate, and the theory is that this prevents them from germinating in the wrong season (Halińska and Lewak 1987, Ranjan et al. 2006). But the mechanisms involved are a good deal more complicated.

Many species of plants undergo what is known as after-ripening, where even after the fruit is mature, the seeds need some kind of treatment in order to germinate. Some species need to be exposed to water; others (for instance, many pine cones) need the smoke and heat of a fire; some species need to pass through the digestive tracts of animals, and some need cold (Larcher 2001).

Because apples are a domestic, economically important species, there is a lot of information on the changes that their seeds undergo during after-ripening. As with most plants’ delayed germination, the mechanisms that govern the sprouting of apple seeds are very complex. Enzymes and plant hormones play large roles. After cold exposure, enzymes in the apple seed break down the hormone auxin, which can suppress root growth in high concentrations (Dziewanowska and Lewak 1987). Chilling also triggers a rapid decline in the levels of abscisic acid in the seed (Subbaiah and Powell 1992). This hormone is involved in tissue dormancy, so once it is broken down, the embryo can begin to grow. Jasmonic acid (Ranjan et al. 2006) is also implicated in the dormancy and eventual growth of the embryo. Finally, changes in the fruit around the seeds may also signal when to germinate. Protein expression in the flesh of the apple undergoes shifts after cold exposure (Read and Ross 2006). This is certainly not the full story. Usually, when scientists begin to examine a phenomenon that seems simple at the outset, like delayed germination, we find hidden layers of complexity. Understanding this complexity and its evolutionary origins can be an engaging life’s work.

Dziewanowska, K., and S. Lewak. 1987. Non-decarboxylating transformation of indol-3-ylacetic acid in apple seeds. Biologia Plantarum 29: 110-117.

Halińska, A. and Lewak, St. 1987. Free and conjugated gibberellins in dormancy and germination of apple seeds. Physiologia Plantarum 69: 523-530.

Larcher, W. 2001. Physiological Plant Ecology: Ecophysiology and Stress Physiology of Functional Groups. Springer, Berlin.

Ranjan, R., O. Meirsch, G. Sembdner, and S. Lewak. 2006. Presence and role of jasmonate in apple embryos. Physiologia Plantarum 90: 548-552.

Read, S.J., and G.S. Ross. 2006. Up-regulation of two cDNA clones encoding metallothionein-like proteins in apple fruit during cool storage. Physiologia Plantarum 100: 183-189.


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