Kimberley J. Simpson, Ruth N. Wade, Mark Rees, Colin P. Osborne & Sue E. Hartley
When humans domesticated wild plants thousands of years ago to produce the crops we rely upon today, they caused substantial changes in these plants. Some of these changes are obvious and desirable, such as increased yield and faster growth, but others are less noticeable and even unwanted. One such undesirable change may be a reduction in plants’ defences against herbivores. With more of the plant’s resources being channelled into producing more or bigger seeds and faster growth, less can be invested into their costly defensive armoury.
The main anti-herbivore defence in cereals, grasses that include many of our most important crops such as maize, rice and wheat, is silicon. Grasses often take up silicon from the soil in unusually high concentrations, and deposit it in their leaves in the form of spines and sharp granules. This creates an effective defence against a range of herbivores, by making the leaves harder to chew and digest. We explored how this key defence had been altered through domestication and modern crop breeding by comparing silicon levels in domesticated cereals with their wild relatives and with the most modern varieties. To establish whether any changes in defence levels were due to a reallocation of resources to other traits, we measured the growth rate of each species too.
Our results show that cereal domestication had caused a small but significant 10% reduction in leaf silicon concentration, but modern breeding hadn’t reduced levels any further. In addition, we measured the levels of distasteful phenolic compounds in leaves, another anti-herbivore defence, and found it was unchanged through domestication.
We found no evidence for crops growing faster than their wild counterparts, or that growth rate traded-off with silicon defences. We did however show, through modelling, that in plants with higher silicon levels, their growth rate slowed more as they grew resulting in them attaining a smaller final size than low silicon plants, implying a cost of this defence.
Our finding of a small decrease in silicon through domestication suggests that cereals haven’t been disarmed by domestication: our crops are as well defended as their wild ancestors.