How rice spots its relatives underground: Kin recognition and productivity

4 minute read.

If you think that life in the city is crowded, you have never been a root. The world beneath the soil surface is busier than any metropolis. It is a place in which a root can find anything, from life-long mycorrhizal friendships, to pathogens waiting in dark alleys.

Roots also meet other roots, from the same species and from different ones, growing all around, pushing in a hurry to get resources. In this situation, how might a root tell which neighbouring roots are from relatives and which are from competitors?

In the wild, plants can discriminate between close relatives (kin) and unrelated plants from the same species (non-kin), and act differently in response, even if the mechanism they use is not entirely clear yet. This behaviour is called kin recognition, and has been studied in natural ecosystems, but things work quite differently for crops.

Thousands of years of agricultural domestication have selected for plants that are weakly competitive and strongly productive. On top of that, the genetic differences present between individual plants in a field are minimal, and can be non-existent in some monocultures. For these reasons many crops might have lost ability to recognise their neighbours, but not rice!

Xue-Fang Yang and co-authors, from China Agricultural University in Beijing, recently published a study in New Phytologist describing kin recognition in rice. They started from the assumption that “[In the case of crops] it is reasonable to look for kin recognition across cultivars rather than as a contrast between siblings and nonsiblings, as is done in wild populations”. Then they tested the hypothesis that rice plants growing with kin cultivars would shift resources from competitive root growth to reproduction, increasing grain yield.

Experimental setting used in the study: Window Rhizobox (Courtesy of Prof. Chuihua Kong)

So Yang et al. selected two rice cultivars and planted them together with some close and some distantly related cultivars, showing that roots grew in a more vigorous and intrusive way in the presence of a competitor – non-kin – root system than in presence of kin. Roots from unrelated plants would grow facing each other, while related roots would steer clear of each other, minimising competition. This could be observed by intruding on the plants’ privacy, by growing them in Window Rhizoboxes, like the one above.

The kin response could be still measured when a nylon mesh was placed between the two root systems, but disappeared when the plants were separated by a plastic film below ground. Therefore, the recognition happens at the root level, but does not require direct root contact. This was confirmed by the fact that plants grown in presence of root secretions from non-kin still boost their root biomass in readiness to hold their own against the competition. Yang et al. concluded that “active secretion is needed, involving soluble chemicals.”

Root morphology in the presence of the same cultivar, a closely related cultivar and distantly related cultivars in agar media. Figure S2 from Yang et al. 2018.

Unfortunately, all this underground jostling comes with a cost: plants growing with non-kin have a lower shoot biomass, which could reduce grain yield. To check if this was the case, Yang and their co-authors went out of the lab and into the field to plant some rice. They discovered that plants growing together with unrelated cultivars had a reduced yield, but the ones that were sharing the plot with kin were more productive than the ones raised in a monoculture, surrounded by clones.

This was great news for the authors, as “such cultivar mixtures that increase grain yield are easily transferable to the low input management system common throughout most Asian and African crop systems”. This implies that kin recognition could be a new mechanism to improve grain yield, but studies in other crops will be needed.

Now we know that rice plants can recognise each other and are more productive when surrounded by family then when alone in a monoculture or, worse, fighting their way through rivals.

Zoe Nemec Venza

References:

Yang, X. , Li, L. , Xu, Y. and Kong, C. (2018) Kin recognition in rice (Oryza sativa) lines. New Phytologist. doi: 10.1111/nph.15296

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Zoe Nemec Venza

Zoe Nemec Venza is a Sainsbury PhD student at the University of Bristol, in Dr Jill Harrison’s lab, funded by the Gatsby Charitable Foundation. She graduated from her Masters degree at the Universita’ di Pisa, with Prof Francesco Licausi. Her research interests are mostly located in the broad field of plant development. She is particularly fascinated by how cell identity is established and by how developmental pathways changed during evolution. Zoe is doing an internship at the New Phytologist Trust during summer 2018.