Companion cell
Companion Cell
A companion cell is a type of plant cell that is closely associated with the sieve tube elements of the phloem. These cells play a crucial role in the function and maintenance of the sieve tube elements, which are responsible for the transport of nutrients, particularly sugars, throughout the plant. Companion cells are unique to angiosperms (flowering plants) and are a key feature in the efficient transport system of these plants.
Structure
Companion cells are typically elongated and have a dense cytoplasm, which is rich in organelles such as mitochondria, ribosomes, and endoplasmic reticulum. This high density of organelles supports the metabolic activities necessary for the maintenance and function of the sieve tube elements. The cell walls of companion cells are thin but contain numerous plasmodesmata, which are microscopic channels that facilitate direct cytoplasmic connections with the sieve tube elements. These plasmodesmata allow for the transfer of essential molecules and signals between the companion cells and the sieve tube elements.
Function
The primary function of companion cells is to support the sieve tube elements in the phloem. They are involved in the loading and unloading of sugars and other nutrients into and out of the sieve tube elements. This process is essential for the distribution of photosynthates (products of photosynthesis) from the leaves, where they are produced, to other parts of the plant, such as roots, stems, and developing fruits and seeds.
Companion cells also play a role in the maintenance of the sieve tube elements. They provide metabolic support and help in the repair and maintenance of the sieve tube elements, which lack nuclei and are therefore unable to sustain themselves independently. The close association between companion cells and sieve tube elements is critical for the survival and function of the latter.
Types of Companion Cells
There are different types of companion cells, which can be classified based on their structure and function:
Ordinary Companion Cells
Ordinary companion cells are the most common type and are characterized by their dense cytoplasm and numerous plasmodesmata connections with the sieve tube elements. They are primarily involved in the loading and unloading of sugars.
Transfer Cells
Transfer cells have an increased surface area due to extensive cell wall ingrowths. These ingrowths facilitate the transfer of nutrients across the cell membrane, making transfer cells particularly efficient in nutrient transport.
Intermediary Cells
Intermediary cells are specialized companion cells found in some plants, such as cucurbits. They are involved in the symplastic loading of sugars, which involves the movement of sugars through the plasmodesmata directly from the mesophyll cells to the sieve tube elements.
Development
Companion cells and sieve tube elements develop from the same mother cell, a process known as symplastic development. During differentiation, the mother cell divides asymmetrically to form a larger sieve tube element and a smaller companion cell. This close developmental relationship ensures that the companion cell and sieve tube element are functionally integrated.
Role in Phloem Transport
The phloem transport system relies on the pressure-flow hypothesis, which describes the movement of sugars from sources (e.g., leaves) to sinks (e.g., roots, fruits). Companion cells are essential in this process as they actively transport sugars into the sieve tube elements, creating a high osmotic pressure that drives the flow of sap through the phloem. The active transport of sugars into the sieve tube elements is facilitated by proton pumps and sugar transporters located in the plasma membrane of the companion cells.
Interaction with Other Cells
Companion cells interact closely with various other cell types in the phloem, including sieve tube elements, parenchyma cells, and phloem fibers. The plasmodesmata connections between companion cells and sieve tube elements are particularly important for the coordination of phloem transport. Additionally, companion cells can communicate with adjacent parenchyma cells to regulate the flow of nutrients and signals within the phloem.
Pathways of Sugar Loading
There are two main pathways for sugar loading into the sieve tube elements: apoplastic and symplastic loading.
Apoplastic Loading
In apoplastic loading, sugars are transported across the cell wall (apoplast) and then imported into the sieve tube elements via specific sugar transporters. This pathway is energy-dependent and involves the active transport of sugars against a concentration gradient.
Symplastic Loading
In symplastic loading, sugars move directly from the mesophyll cells to the sieve tube elements through plasmodesmata. This pathway is passive and relies on the concentration gradient of sugars between the source cells and the sieve tube elements. Intermediary cells are particularly important in this type of loading.
Genetic Regulation
The development and function of companion cells are regulated by a complex network of genes and signaling pathways. Key regulatory genes include those involved in cell differentiation, plasmodesmata formation, and sugar transport. Mutations in these genes can lead to defects in phloem function and overall plant growth.
Research and Applications
Recent research has focused on understanding the molecular mechanisms underlying the function of companion cells and their role in phloem transport. Advances in genetic engineering and molecular biology have allowed scientists to manipulate the expression of specific genes in companion cells, providing insights into their function and potential applications in agriculture. For example, enhancing the efficiency of sugar loading in companion cells could improve crop yields and stress tolerance.