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Why is holly prickly?

Why is holly only prickly some of the time?

Ilex Aquifolium_Garden World Images ltd_gencontWe are all used to seeing holly used as a Christmas decoration – but why are only some of its leaves prickly? Scientists at the National Research Council of Spain in Seville (CSIC) appear to have the answer. As they outline in the Botanical Journal of the Linnean Society, variations within a single tree are the combined result of herbivore activity and molecular responses to environmental change.

Dr Herrera and Ms Pilar Bazaga from CSIC studied the European holly (Ilex aquifolium) – a species particularly able to adapt to changing conditions. The species is well known for its variation in leaf forms in a single tree, known as heterophylly.

“Heterophylly is often witnessed in holly trees, where some leaves are prickly, a defense against herbivores, while others are non-prickly, with smooth margins and no defense,” said Dr Herrera. “We wanted to find out if this variation was a response to environmental changes and if this took place without wider genetic change, that is, without alteration of the organism’s DNA sequence.”

Carrying out their studies in a forest in South Eastern Spain, the team used a method of chemical modification of DNA (methylation), which does not alter the DNA sequence of an organism. On comparing the DNA methylation profiles in 40 trees the scientists found that 39 were heterophyllous (with both prickly and non-prickly leaves) in neighboring positions. This correlated with the grazing patterns of herbivores, including deer and goats, to show a significant relationship between recent feeding and the growth of prickly leaves. A higher number of prickly leaves under the height of 2.5 meters: the average reach of an adult red deer.

“An increasing number of studies support the idea that the presence of spines and prickles in plants is a response to herbivore activity, and our research suggests this is the case with holly,” concluded Dr Herrera. “The ability of plants to respond to environmental changes through quick epigenetic modifications makes also one to feel a bit more optimistic about plant survival in a quickly changing world.”

Read the abstract for this study in the Botanical Journal of the Linnean Society.

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Full citation: Herrera. C, Bazaga. P, ‘Epigenetic correlates of plant phenotypic plasticity: DNA methylation differs between prickly and nonprickly leaves in heterophyllous Ilex aquifolium (Aquifoliaceae) trees’, Botanical Journal of the Linnean Society, Wiley, December 2012, DOI: 10.1111/boj.12006

The Botanical Journal of the Linnean Society publishes original papers on systematic and evolutionary botany and comparative studies of both living and fossil plants. Review papers are also welcomed which integrate fields such as cytology, morphogenesis, palynology and phytochemistry into a taxonomic framework. The Journal will only publish new taxa in exceptional circumstances or as part of larger monographic or phylogenetic revisions. Also published on behalf of the Linnean Society: Biological Journal of the Linnean Society and Zoological Journal of the Linnean Society.

Image above (Ilex aquifolium): Garden World Images Ltd