Deep roots helped corn conquer Mesoamerica

Deeper, more efficient roots emerged during corn domestication in Mexico's Tehuacán Valley, according to a study by researchers at Penn State University.

The research combined genetic analyses of archaeological samples, paleoclimate data, and digital simulations of root growth to understand how corn adapted to changing agricultural environments over the past 18 years.

Domestication influenced by the environment

O corn (zea mays) was domesticated from teosinte around 9 years ago. During this process, changes in the plant's roots were fundamental to its adaptation.

Scientists observed three main modifications: an increase in the number of seminal roots (SRN), a reduction in the number of nodal roots (NRN) and the development of multiseriate cortical sclerenchyma (MCS), a structure that reinforces the roots.

Using the OpenSimRoot model, researchers simulated root growth across different historical periods, taking into account changes in climate, soil composition, and agricultural practices such as irrigation. The modeling helped estimate when and why these changes occurred.

Environmental changes

Between 12 and 8 years ago, carbon dioxide levels increased. This favored the selection of roots with lower NRN and MCS, promoting greater root depth.

Around 6 years ago, with the introduction of irrigation in the Tehuacán Valley, the distribution of nitrogen in the soil shifted from the top to deeper layers. This process coincided with the emergence of soils degraded by intensive cultivation and erosion.

Simulations demonstrated that roots with lower NRN and higher MCS explore deeper soil better. The combination of these two characteristics began to provide competitive advantages for maize as early as 8 years ago.

Increased seminal roots

Although the weight of corn seeds was already sufficient to support additional seminal roots 6 years ago, genetic data indicate that the increase in SRN occurred around 3.500 years ago.

The period coincides with population growth in the region, increased dependence on agriculture, and soil degradation. Seminal roots, which use seed reserves to grow, are advantageous in soils poor in nitrogen and phosphorus.

The study analyzed the frequency of genetic variants associated with SRN in ancient maize samples. The genes Zm00001d021572 and BIGE1, for example, showed variations typical of modern maize only in samples from later than 3.500 years ago.

Past and future scenarios

The team simulated the root development of teosinte, modern maize, and intermediate hybrids in different environments in the past and also in future scenarios.

In the past, teosinte performed best between 18 and 10 years ago. From 8 years onward, deep-rooted corn became more efficient, especially in irrigated environments and degraded soils.

In the future scenario, corn roots tend to grow even deeper, about 30 cm, in response to the greater scarcity of water in the soil.

Evolutionary sequence confirmed

The simulations indicated the following likely sequence of root evolution: first, the reduction in the number of NRN and the emergence of MCS (between 12 and 8 years), then the increase in seed weight (around 6 years), and finally, the increase in SRN (3.500 years).

Nitrogen has been identified as the most important environmental factor for the success of modern roots. Root adaptations have allowed corn to better utilize nitrogen at depth, especially in soils depleted by ancient agricultural practices.

Methodological contribution

This is one of the first studies to use three-dimensional functional-structural modeling to reconstruct the evolution of crop plant roots. The approach allowed us to simulate ancient environments and test the performance of different root combinations that no longer exist in living populations.

Drawing on ancient DNA data, soil profiles, archaeological records, and reconstructed climate, the work shows how root innovations emerged in response to early agriculture and environmental changes in Mesoamerica.

Further information at doi.org/10.1111/nph.70245