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.

Genetic Parameters in Mesocotyl Elongation and Principal Components for Corn in High Valleys, Mexico

“…Likewise, the study allowed the observation of a positive correspondence between the two principal components (PC1 and PC2) and the GCA of the parents; that is, with a higher GCA of the parents in F 1 and F 2 , the association of the parents regarding the two principal components (PC1 and PC2) was higher and is similar to those obtained by other studies, where positive correlations were observed with the 1197AM (Pionner) corn in the field, OH, USA, for grain weight when a depth of 51 and 76 mm was established [52]. Therefore, the adjustments in the depth of sowing can improve the PCA of some genotypes, although the responses of grain yield can differ according to the soil type [56], as is the case of H-48, HS-2 and Promesa, ST34, ST11 and ST112 of Chalqueño race [57], IBM Syn10, B73, Mo17 [58], US13, Hopi, Navajo [59] and P11974M [47], which show a different response to the depth of sowing and soil, where the GCA of the LM-LC of some genotypes were associated with the speed and percentage of emergence and these were associated successively with the two principal components (PC1 and PC2) in the agricultural practice known as deep sowing, which is still used in semi-arid regions of Mexico [60] and in southwest USA [13]. Therefore, the parents (H-48, HS-2 and Promesa) with a higher GCA in LM-LC presented a greater relation with the two principal components, which suggests that LM-LC could be a response trait to stress from deep sowing [13].…”

“…Therefore, the adjustments in the depth of sowing can improve the PCA of some genotypes, although the responses of grain yield can differ according to the soil type [56], as is the case of H-48, HS-2 and Promesa, ST34, ST11 and ST112 of Chalqueño race [57], IBM Syn10, B73, Mo17 [58], US13, Hopi, Navajo [59] and P11974M [47], which show a different response to the depth of sowing and soil, where the GCA of the LM-LC of some genotypes were associated with the speed and percentage of emergence and these were associated successively with the two principal components (PC1 and PC2) in the agricultural practice known as deep sowing, which is still used in semi-arid regions of Mexico [60] and in southwest USA [13]. Therefore, the parents (H-48, HS-2 and Promesa) with a higher GCA in LM-LC presented a greater relation with the two principal components, which suggests that LM-LC could be a response trait to stress from deep sowing [13].…”

“…To have an idea of the magnitude of the aforementioned research, the diallel designs proposed by Griffing can be used [8], which constitute a useful tool to characterize and estimate the capacity of GCA and SCA of parents and their crosses [9], as well as to define the most adequate method of genetic breeding to predict superior crosses and combine the best traits of the parents [10]. In this regard, various results have confirmed the utility of these designs in corn to estimate GCA and SCA, the GCA/SCA relation and the heterosis of the parents [2] and to classify them into heterotic groups based on the genetic expression of the traits studied [11], such as mesocotyl-coleoptile elongation [12], which carries out an important role in the emergence and development of a normal seedling [13], and the components of grain yield [3], which are crucial in corn yield.…”

New insights into light spectral quality inhibits the plasticity elongation of maize mesocotyl and coleoptile during seed germination

“…Generally, soon after the maize seeds germinated in the soil, the elongated MES (an embryonic structure between COL node and basal part of seminal root in seedlings) pushes the COL (a conical structure that sheaths the shoot apex) through the soil surface. At the soil surface, incident light represses MES and COL elongation, induces leaf expansion, and promotes root formation ( Liu et al., 2017 ; Rodríguez and Cassab, 2021 ; Zhao and Zhong, 2021 ). As cells are recruited to the emerging leaf primordia, proplastids differentiate into the dimorphic bundle sheath and mesophyll cell chloroplasts, and the photoautotrophic phase of sporophytic development is initiated ( Markelz et al., 2003 ).…”

Integrated QTL Mapping, Meta-Analysis, and RNA-Sequencing Reveal Candidate Genes for Maize Deep-Sowing Tolerance

“…Different crop species have evolved diverse organs to adapt to deep-sowing stressors, including elongated mesocotyl in rice ( Oryza sativa L.) [ 10 ] and elongated coleoptile in wheat ( Triticum aestivum L.) [ 11 ] and barley ( Hordeum vulgare L.) [ 12 ]. These structures help the emergence of the shoot tips and the first internode from the embryo to the soil surface; the co-elongated mesocotyl and coleoptile in maize send the buds to the soil surface [ 13 , 14 ], where natural light represses the continued elongation of mesocotyl and coleoptile and induces leaf expansion [ 7 , 15 ]. However, different crops have different degrees of deep-sowing tolerance due to genetic variations.…”