Primary Root and Mesocotyl Elongation in Maize Seedlings: Two Organs with Antagonistic Growth below the Soil Surface

Abstract

Maize illustrates one of the most complex cases of embryogenesis in higher plants that results in the development of early embryo with distinctive organs such as the mesocotyl, seminal and primary roots, coleoptile, and plumule. After seed germination, the elongation of root and mesocotyl follows opposite directions in response to specific tropisms (positive and negative gravitropism and hydrotropism). Tropisms represent the differential growth of an organ directed toward several stimuli. Although the life cycle of roots and mesocotyl takes place in darkness, their growth and functions are controlled by different mechanisms. Roots ramify through the soil following the direction of the gravity vector, spreading their tips into new territories looking for water; when water availability is low, the root hydrotropic response is triggered toward the zone with higher moisture. Nonetheless, there is a high range of hydrotropic curvatures (angles) in maize. The processes that control root hydrotropism and mesocotyl elongation remain unclear; however, they are influenced by genetic and environmental cues to guide their growth for optimizing early seedling vigor. Roots and mesocotyls are crucial for the establishment, growth, and development of the plant since both help to forage water in the soil. Mesocotyl elongation is associated with an ancient agriculture practice known as deep planting. This tradition takes advantage of residual soil humidity and continues to be used in semiarid regions of Mexico and USA. Due to the genetic diversity of maize, some lines have developed long mesocotyls capable of deep planting while others are unable to do it. Hence, the genetic and phenetic interaction of maize lines with a robust hydrotropic response and higher mesocotyl elongation in response to water scarcity in time of global heating might be used for developing more resilient maize plants.

Keywords: deep planting; drought; early vigor; hydrotropism; mesocotyl elongation; primary root.

Figure 1

Establishment of maize seedlings when utilizing deep planting as an agricultural practice. Elongation of the roots (positive gravitropic and hydrotropic response) and mesocotyl allows the exploration of a wide area of soil by helping to forage water, which ultimately favors early seedling vigor (g: gravity; LenR: root length of 80 and 120 mm, ME: mesocotyl elongation of 5 mm and 10 mm).

Figure 2

Maize kernel structure. The mature maize kernel consists of multiple tissues and organs within the embryo and endosperm, in addition to maternally derived structures. © 2013 Encyclopedia Britannica, Inc., modified image. (RAM: root apical meristem; SAM: shoot apical meristem).

Figure 3

Cross-sections of primary roots of maize breed B73 at 7 days post germination. (a) Anatomy of primary root (10×); (b) anatomy of mesocotyl (10×). The lines indicate major structures. (TCA: total cortex area, RootScan®).

Figure 4

Establishment of maize seedlings. The transition from the skotomorphogenic or etiolation (heterotrophic growth) period to the photomorphogenic period (phototropic growth) is primarily regulated by growth of both the roots (PR, LR, and SR) and the mesocotyl. Growth of the roots and mesocotyl is controlled by several regulatory mechanisms, mainly tropisms and hormone signaling (M: mesocotyl, PR: primary root, SR: seminal root, LR: lateral root; LenR: root length of 80 and 120 mm; ME: mesocotyl elongation of 50 mm). Blue line: water requirements throughout phenological cycle of maize; yellow line: energy consumption of the seed.

Figure 5

Diagram that shows the main effects of hormones on seedlings traits. (IAA: indole-3-acetic acid, ABA: abscisic acid, GA: gibberellins, E: ethylene, STR: strigolactones, JAS: jasmonates, BR: brassinosteroids, KIT: cytokinins). Their function in relation to growth and development of both the roots (PR: primary root, SR: seminal root, LR: lateral roots) and the mesocotyl (M). (S: seed, LE: leaves, CO₂: carbon dioxide, D: dark conditions, L: light). Red circles with lines indicate a positive stimulus and its direction; blue circles with lines indicate a negative stimulus and its direction; nude circles indicate a negative stimulus.