Cells, Genes & Molecules Ecosystems Growth & Development

Move over Factor 50! Can increasing their number of chromosomes help protect plants against UV?

Increased chromosome number due to UV is known to occur in lab experiments, but does it happen in real environments?

UV-B radiation is not just dangerous to us, but is also a threat to plants growing in a variety of environments through its damaging effects on photosynthetic machinery and DNA. Whilst we can escape UV-B through simply moving out of sunlight, plants have no such luck and must deal with whatever hits them in their environment. One of the methods that plants are well known to use to reduce damage by UV-B radiation is production of UV-absorbing molecules such as flavonoids. Another strategy that plants may use to protect against the damaging effects of UV-B is endopolyploidy – where genomes are duplicated without subsequent cell division resulting in cells with higher numbers of chromosomes.

Arabidopsis plants with increased ability to undergo endopolyploidy are more resistant to UV-B. However, these and other similar experiments were performed in laboratory conditions, and little is known about how much endopolyploidy may be used by a greater variety of plants in natural environments to protect against UV-B. In their recent paper in Annals of Botany, František Zedek and colleagues examine the concentration of UV-absorbing substances and rates of endopolyploidy in plant species growing naturally in both the shaded understories and open clearings of forests in the Czech Republic.

Zedek and colleagues find that all species had increased concentration of UV-absorbing substances when growing in forest clearings compared to when growing in the shaded understory. An index for endopolyploidy also significantly increased for some plants when exposed to higher solar radiation, but this did not occur in all species. Interestingly, this increase in endopolyploidy was found only in species with monocentric chromosomes. Monocentric chromosomes can only attach to the cell spindle via one point during cell division, whilst holocentric chromosomes can attach anywhere along their length to the cell spindle during cell division. In other words, it is probably much harder to undergo endopolyploidy when your chromosomes can attach to a spindle at any point along their length, as opposed to only via one point. In species with monocentric chromosomes, the index for endopolyploidy increased significantly with the presence of UV-absorbing compounds but did not in species with holocentric chromosomes.

Three plant species used by Zedek and colleagues. Left: Alliaria petiolata, monocentric chromosomes (Fabelfroh/Wikimedia Commons), Middle: Convallaria majalis, monocentric chromosomes (Wikimedia Commons), Right: Carex pilosa, holocentric chromosomes (Stefan Iefnaer/Wikimedia Commons).

The big question that this raises is why do some plant species respond to high UV-B levels by undergoing endopolyploidy? Zedek and colleagues point to previous work showing that cell size can go up with increased chromosome number, and suggest that endopolyploidy may be a way to maintain growth in conditions that are otherwise known to halt growth. Interestingly, although all plant species surveyed had increased concentrations of UV-absorbing substances when growing in higher sunlight, this increase was lesser in species with holocentric chromosomes. This supports previous work suggesting that plant species with holocentric chromosomes are in general less sensitive to UV-B radiation, although the exact reason for this remains unclear.

Monocentric vs. holocentric chromosomes. Red: DNA, Green: spindle attachment points. (Left: Neumann et al., 2012, Right: Wormbook/Wikimedia Commons)

Zedek and colleagues therefore demonstrate that endopolyploidy in response to increased UV-B radiation is likely also a phenomenon that exists in natural environments, in addition to experimental scenarios. They also provide further support for the belief that plant species with holocentric chromosomes are somehow less sensitive to high levels of UV-B radiation. Interestingly, this aligns well with the hypothesis that holocentric chromosomes of the algal ancestors of land plants may have been important to resisting the greater exposure to UV radiation that the earliest land plants presumably found themselves exposed to. Exactly why other plant species use endopolyploidy as a possible defence against UV-B radiation remains unclear, but it will be interesting to see what knowledge emerges about this in the future and how it may relate the variety of environments in which plants grow today.

Cover image from Josef Reischig/Wikimedia Commons.