Ivan Lopez-Valdivia, Alden C. Perkins, Hannah M. Schneider, Miguel Vallebueno-Estrada, James D. Burridge, Eduardo González – Orozco,  Aurora Montufar, Rafael Montiel, Jonathan P. Lynch, and Jean-Philippe Vielle-Calzada

PNAS April 18, 2022 | 119 (17) e2110245119.

Significance

Despite their importance in supplying nutrients, root traits related to maize domestication are scarce. We used laser ablation tomography to characterize the root architecture and anatomy of 5,300-y-old maize specimens recovered from San Marcos (Tehuacán, Mexico), revealing exquisite preservation of their cellular organization. Outer cortical cells contained thick and lignified walls typical of extant maize adapted to hard soils. By contrast, the absence of seminal roots is only found in the maize ancestor, teosinte. Two genes important for seminal root development had mutations that could relate to their absence. Our results indicate that some traits related to drought adaptation were not fully present in the earliest maize from Tehuacán, providing clues to conditions prevailing during early maize cultivation.

Abstract

Efforts to understand the phenotypic transition that gave rise to maize from teosinte have mainly focused on the analysis of aerial organs, with little insights into possible domestication traits affecting the root system. Archeological excavations in San Marcos cave (Tehuacán, Mexico) yielded two well-preserved 5,300 to 4,970 calibrated y B.P. specimens (SM3 and SM11) corresponding to root stalks composed of at least five nodes with multiple nodal roots and, in case, a complete embryonic root system. To characterize in detail their architecture and anatomy, we used laser ablation tomography to reconstruct a three-dimensional segment of their nodal roots and a scutellar node, revealing exquisite preservation of the inner tissue and cell organization and providing reliable morphometric parameters for cellular characteristics of the stele and cortex. Whereas SM3 showed multiple cortical sclerenchyma typical of extant maize, the scutellar node of the SM11 embryonic root system completely lacked seminal roots, an attribute found in extant teosinte and in two specific maize mutants: root with undetectable meristem1 (rum1) and rootless concerning crown and seminal roots (rtcs). Ancient DNA sequences of SM10—a third San Marcos specimen of equivalent age to SM3 and SM11—revealed the presence of mutations in the transcribed sequence of both genes, offering the possibility for some of these mutations to be involved in the lack of seminal roots of the ancient specimens. Our results indicate that the root system of the earliest maize from Tehuacán resembled teosinte in traits important for maize drought adaptation.

See: https://www.pnas.org/doi/10.1073/pnas.2110245119

Figure 1: Ancient maize root specimens from San Marcos cave. (A) Maize root stalk corresponding to specimen SM3 and dating 5,280 to 4,970 y B.P. (B) Transversal section (10 µm thick) of a nodal root belonging to specimen SM3. (C) Maize root stalk corresponding to specimen SM11 and dating 5,280 to 4,880 y B.P. (D) Transversal section of a second node root from SM3 used for lignin content determination. (E) Red staining of the outer cortical cells by phloroglucinol-HCl, indicating presence of lignin, a distinctive component of the MCS phenotype.  Abbreviations: EP, epidermis; C, cortex; EN, endodermis; PE, pericycle; P, phloem; LMX, late metaxylem; EMX, early metaxylem. (Scale bars: A, 1 cm; B, 500 µm; C and D, 500 µm.)