Can plants recognize kin?
This week's paper is "Kin recognition in an annual plant", by Susan Dudley and Amanda File of McMaster University, just published online in Biology Letters.
Researchers in several countries have recently shown that roots respond differently to another root from the same plant than they do to a root from a different plant. Typically, they grow more aggressively towards a neighbor's root than towards one of their own. But what if the neighbor is a close relative?
Dudley and File compared root growth when four related plants (kin) were grown together in a pot, versus four unrelated plants. As the left side of the figure shows, roots grew less when surrounded by kin. In the "solitary" treatment, a single plant was grown in a pot one-fourth as big. Root growth in these plants was not affected by above-ground interactions with kin vs. nonkin.
Kin selection theory predicts that plants (or animals) should compete less severely with relatives than with nonrelatives. For example, Hamilton's rule states that a gene for altruism (helping another at some cost to oneself) will spread if the cost of helping is less than the benefit to the one helped, times that individual's genetic relatedness to the helper.
Strictly speaking, relatedness should be measured relative to one's usual competitors. If a plant drops 1000 seeds (half-sibs, since they all have the same mother), and there's only enough light or water for a few to live, don't expect a lot of altruism. But a plant with a sibling on one side and a stranger on the other might be expected to behave more aggressively (in terms of root growth, etc.) towards the stranger. This assumes that plants can distinguish kin from nonkin. I would have thought two roots bumping into each other would have a hard time measuring relatedness, but this week's paper suggests that they do, somehow.
I have suggested previously (see Darwinian Agriculture) that reducing aggressive root interactions could be a key to increasing efficient use of water and other soil resources by crops. Some experimental papers that appeared to support this result may have resulted from a tendency of plants to put more roots in larger pots, even apart from nutrient levels or the presence of another plant. See Hess and de Kroon (2007) "Effects of rooting volume and nutrient availability as an alternative explanation for root self/nonself discrimination" (Journal of Ecology 95:241).
This week's paper doesn't have that problem, as the number of plants per pot was the same in the kin vs. stranger treatments. On the other hand, the plants were sampled early enough that it's not clear whether there was a benefit to plants sharing pots with kin rather than with strangers. I hope other people will repeat this work with different species, so we can see how common an effect of kinship on rooting pattern is, and whether it often affects overall plant growth.