The mesocotyl is a temporary embryonic stem structure found in the seedlings of germinating monocots, such as maize, rice, and other grasses. It forms a connecting stalk between the seed and the shoot of the young plant, serving a specialized function during the initial stages of growth beneath the soil surface. This structure is the first internode of the stem, and its ability to adjust its length based on environmental conditions governs successful seedling establishment. The mesocotyl’s transient existence ensures the growing tissues of the shoot are safely delivered to the light.
Structure and Anatomical Development
The mesocotyl is the internode located between the scutellar node, which connects to the seed’s food reserves, and the coleoptilar node, the base of the protective shoot sheath. It contains vascular tissues that transport stored energy from the endosperm to the rapidly growing shoot. This temporary organ is white and tubular, composed of an epidermis, a cortex, and a central vascular cylinder (stele).
Elongation Process
Elongation is achieved through cell division and significant cell expansion. This process is often linked to somatic polyploidy, which allows for the development of fewer, larger cells advantageous for rapid growth in dark conditions. The growth is hormone-regulated, with auxins and gibberellins promoting the cell elongation needed to push the shoot upwards. Once the seedling emerges and encounters light, this developmental process is abruptly halted.
Role in Monocot Seedling Emergence
The primary purpose of the mesocotyl is to act as an underground extension that elevates the shoot to the soil surface. It serves as a spacer, pushing the coleoptile, a protective sheath that encases the first true leaves, up through the soil column. This mechanism allows the growing point of the stem, known as the crown, to be positioned consistently at a shallow depth, regardless of the initial planting depth.
This emergence strategy contrasts with the emergence patterns of many dicots. In dicots, the hypocotyl often elongates and forms a hook that pulls the cotyledons and shoot through the soil, a process known as epigeal emergence. By using the mesocotyl to push the tough coleoptile, monocots ensure their tender plumule remains protected until emergence. This protection shields the leaves from abrasive soil particles and mechanical damage during the upward journey.
How Planting Depth and Light Affect Mesocotyl Growth
Mesocotyl elongation is highly sensitive to environmental cues, particularly the absence of light, which is a classic example of photomorphogenesis. The growth of the mesocotyl is a skotomorphogenic response, meaning it is specifically triggered by darkness and is directly related to the depth of the seed. Deeper planting results in a longer mesocotyl as the structure extends itself to ensure the shoot reaches the surface.
The elongation mechanism stops immediately when the coleoptile tip is exposed to light, specifically the red wavelengths of solar radiation. This light exposure reduces the production of the growth-promoting hormone indoleacetic acid (IAA) in the coleoptile tip, which in turn halts cell expansion in the mesocotyl. This precise control mechanism is agriculturally significant because it ensures the crown and developing nodal roots are consistently established at a shallow, fixed depth, allowing for optimal root development. If the seed is planted too deep, the seedling may exhaust its limited energy reserves from the endosperm before the mesocotyl can push the shoot to the surface. Conversely, planting too shallowly may lead to suboptimal mesocotyl development, potentially exposing the young nodal roots to harsh surface conditions.