Pores in cell walls allow movement of cell products between cells. These pores are filled with an extension of the endoplasmic reticulum, the membrane-like filaments within a cell. This extension called the plasmodesmata is essential to plants because of the cell walls. Animal cells do not have cell walls. These typically are large to move small molecules such a glucose but often need some energy dependent assistant to move larger ones such as proteins. It is hypothesized that when some corn varieties are chilled the movement of the compounds from the chloroplasts in the leaf cells to the bundle cells adjacent to the veins for the final production of glucose is inhibited (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958785).
Larger molecules such as RNA and proteins also move via plasmodesmata between cells with some specific chaperones’ molecules assistance. This same type of assistance allows viruses to move between cells as well, eventually reaching the growing point to cause a systemic disease.
Movement of carbohydrates from leaves to growing points, roots and corn stalks is facilitated with the plasmodesmata between cells of the phloem. Hormones are also moved in the phloem to other plant parts by the same system. Root cells also have plasmodesmata for hormone attraction of carbohydrates and proteins for growth.
Plasmodesmata represent another unique feature of plants, hidden from most of us, that is affecting the performance of the corn hybrid each season in our fields.
Part of the glucose use in the corn leaf cell is respiration, providing energy for the many physiology processes in the cell. However, much of the glucose becomes integrated into building cell walls. Polysaccharides, such as cellulose, hemicellulose and pectin are major components, several other molecules that include proteins, phenols and acids join in forming the cell walls.
As a newly formed cell develops a thin, flexible layer forms outside of the plasma membrane of the cell. This first layer formed is nearly gelatinous, composed of carbohydrates, magnesium and calcium pectates. Next the cell forms a layer called the primary cell wall. This is composed of carbohydrates with pectin holding it together. The primary cell wall is flexible, allowing cell expansion. As the cell matures, expanding to its maximum, a thicker layer, called the secondary cell wall forms outside the primary wall. This has the effect of limiting the swelling of the cell as water is imbibed into the cell.
The middle lamella allows for gluing cells together, the primary wall allows some expansion of the cell and the secondary cell wall provides a stronger boundary, limiting cell expansion and offering resistance of invasion by potential pathogens and insects.
Cell walls in each part of the corn plant has its own specific arrangement of these cell walls. The Primary cell walls of the leaf epidermis cells produces a waxy outer layer limiting water absorption and further protection against invasion by pathogens. Cells of the xylem, that essentially form tubes for the transport of water from the roots to the upper plant parts have exceptionally thick secondary layers forming ridged tubes with little obstruction for the movement of water and solutes.
Cell walls have small holes that allow movement of molecules between cells. Regulation of movement into the individual cell is dependent on the living functions of the cellular membrane and often this requires a complex interaction with specific proteins within the cell.
There is a lot going on in these smallest structural components of a corn plant that ultimately contribute to the final product of our interest.
Chloroplasts in cells within mesophyll and vascular bundle cells convert the light energy into chemical energy locked up in sugar molecules. These cell walls, although ridged giving strength, have pores (plasmadesmata) through which the proteins and sugars can be moved from the chloroplast laden cells. Most of the sugars move to the phloem portion of the vascular system. Phloem cells are living cells that are surrounded by companion cells that are the first site of sugar-protein molecules entrance into the phloem tissue. The remaining phloem is called a sieve-tube in which they have reduced cytoplasmic contents and sieve tube openings that connect them with sieve-tubes on above and below. This openings allow the transport of sugars in either direction.
As the concentration of sugars in these sieve tube portions of the phloem increases, water from the xylem moves, from the high concentration of water to the lower concentration because of the high concentration of the sugars. This adds pressure for for movement of the solution from that cell. The diffusion principle continues to guide the movement of the sugars within the phloem. This is basically a source-sink model. Pre-flowering, sugar movement to the growing points at the apical meristem and the root tips is caused by tying up by cellular respiration as well as tying up the sugars into more complex molecules such as cellulose for new cell walls. These sites become the ‘sink’ for diffusion of sugars from the ‘source’, the leaves.
This translocation process from leaves to vascular tissue continues as a source-sink phenomenon after flowering, as the growing point in the kernels become new sinks. These new sinks now compete with the root sinks. Genetics affect the strength of the sugar demand of each kernel and the number of kernels affect the total draw of sugars. Strength of the pull of sugars per kernel also involves the efficiency of the kernel removing sugars to more complex molecules such as starch. The movement of the sugars in phloem is directed by the pressure-driven bulk flow principle of moving from high concentration to lower concentration.
Corn vascular system not only transports leaf products to other parts of the plant, and water and minerals from the roots but has specialized cells that contribute to corn’s advantage as in photosynthesis. In most plant species, chloroplasts in the mesophyll cells capture CO2 and using light energy and a series of enzymes to produce for C3 molecules that are fused to make sugar (C6H12O6.). The potential weakness comes when the stomates are closed at night or during moisture stress causing a process called photorespiration in which instead of utilizing CO2, the system starts consuming oxygen. This not only needlessly uses energy it limits the capacity of the chloroplasts to maximize the light energy capture. Most plant species belong to this C3 photosynthesis system.
Some species, including corn, have evolved a system to avoid this wasteful system. Chloroplasts in the corn leaves make normal photosynthesis process but then break down the C3 molecules, have them transfer them to the specialized, vascular bundle cells surrounding the vascular system that are loaded with special chloroplast for C4 molecules. These molecules are then enzymatically combined to make sugar which is moved elsewhere in the leaves and other parts of the plant. This system occurs in species that are native to dry, hot environments such as that of corn’s central America origin. The ultimate advantage is that corn can continue to produce carbohydrates despite environments that cause stomates to close. Whereas most C3 plants such as soybeans, wheat and rice do not utilize light intensity greater than 3000 foot-candles, corn photosynthesis rate keeps increasing with light intensities to our maximum sun brightness of 10000 foot-candles. Those few cells surrounding the xylem and phloem of a corn leaf vein have a special role in allowing the photosynthetic efficiency of maize.
Those folks in Mexico that selections from Teosinte did not know about C4 photosynthesis but we have benefited from 10000 years of selections leading to what we call corn (or maize).
Corn leaves, as well with as other monocots, have multiple veins located parallel to each other within the leaves and ultimately connecting with vascular system of the stalk. They are critical to the distribution of the cell products and ultimately the grain. Structure and function of this system is summarized in Corn Journal blog of 5/17/2016.
Corn leaf veins run parallel to each other to the length of the leaf. There are primary veins easily seen on each side of the leaf midrib and smaller secondary ones as seen in the photo. Basic function is to move water and minerals from the roots to the leaves and distribute photosynthesis products to other parts of the plant. It is complicated, of course. A single row of thin-walled cells surround the vascular bundle. These cells function to regulate and transform products into and from the other parts of the vein. In C4 plants like corn, special chloroplasts in the bundle cells perform the final stages of photosynthesis making sucrose and finally starch. At night the starch is broken down into smaller molecules, allowing it to move into the phloem components of the vascular bundle.
Phloem tissue includes two cell types. Companion cells are the immediate recipients of materials from the bundle sheath cells. Chemical processes change the carbohydrates from the molecules as they arrive into forms that allows continual input. More changes allow the movement of sugars and proteins to be moved to the second cell component of the phloem tissue, the sieve cells. These living cells share cytoplasm with adjacent sieve cells above and below through pores, and thus allow the movement of carbohydrates to sink as dictated by plant hormones. The phloem thus becomes the means of moving carbohydrates from the leaf to meristem for more growth, to roots for growth and metabolism and to the grain. Phloem tissue is living, requiring energy to function. Death of phloem tissue stops translocation of carbohydrates. Phloem tissue also can be an avenue for viruses to spread through the sieve cell pores.
The xylem portion of vascular bundles is composed of dead cells that function as a tube allowing water and minerals to move by capillary action from the roots to leaves and other parts of the plant. Water molecules consumed by photosynthesis or passing through stomata by evaporation (transpiration) are replaced because of the cohesion character of water. Fungal spores and bacteria can be moved through xylem cells as well, although xylem arrangements at the stem nodes can foil movements of larger particles such as these.
A significant cellular component in corn leaves are the chloroplasts. Cells in the mesophyll receive the light and CO2 and thru multiple steps captures the light energy, changing it to a form of chemical energy as it locks carbon, hydrogen and oxygen molecules together. Cells in the bundle sheath surrounding the leaf vascular tissue finish the job, converting this energy into sugars. Blog from 4/26/2016 summarizes the role of these organelles.
Plant cells have plastids, distinguishing them from animal cells. They are believed to have been derived from cyanobacteria, such as single cell blue green alga, when a symbiotic relationship with a single celled organism merged with it, perhaps a billion years ago. Like any symbiosis between organisms, each one benefits, and they often become interdependent. Plastids, like bacteria, have two surrounding membranes and DNA organized in a circular manner as opposed to the chromosomal arrangement in all other organisms with a membrane-bound nucleus. Plastids multiply by division independent of host cell division but are carried along with new cells. Consequently, in corn, they are present in the female egg cell. After pollination, as the fertilized egg cell divides and ultimately forms meristems, each cell includes the plastids. These are called protoplastids because they are not fully developed. Those in the cells reaching the light quickly are transformed into chloroplasts. Although plastids have their own DNA and capability to produce the many enzymes and other components of chlorophyll, as in other cases of symbiosis, they are also dependent upon the host cell to provide some proteins and plant hormones such as cytokinins needed for proper development.
A major structural feature of chloroplasts is formation of multiple layers of membranes (thylakoids) with the chlorophyll molecule and thereby enhancing the capacity for photosynthesis. The plant hormones classified as cytokinins, perhaps produced more by the host cell but some from the chloroplast itself, apparently affect the size and quantity of these layers. Host cell genetics, those inherited from both parents of a corn hybrid, thus influence the chloroplast development and function despite the fact that the protoplastids are carried along in only the female parent egg cells.
All protoplastids do not develop into chloroplasts. Those remaining below soil surface and some others do not become green and become sites for starch storage. Some others accumulate other pigments, contributing to other colors expressed in plants. Some chloroplasts located near the veins in plants develop slightly different carbon-fixing methods that allows corn’s photosynthesis to be among the most efficient of plants to convert light energy into carbohydrates.
Those corn seedlings, soon to be emerging in US fields, will have protoplastids in coleoptile and other new leaves with newly-formed chloroplasts converting of light energy into the chemical energy needed for growth of the plants.
Corn cells are the location of corn plant activity of its growth and its ultimate grain production. The cells include a nucleus with chromosomes with the DNA genetics. Leaf cells contain chloroplast for converting light into chemical energy, including the carbohydrates that eventually end up in grain. Mitochondria, another type of organelle in all living cells convert the carbohydrates into a form of energy (ATP) that can drive the multiple other biological processes for cell and plant growth. These organelles can barely be seen with 1000X power of a light microscope. The rest of the cytoplasm within the cell is even more difficult to distinguish at that magnification but the rest of the cell cytoplasm can only be distinguished with the power of electron microscopy.
The endoplasmic reticulum (ER) is a thin, tubular membrane that has multiple folds as it connects many of the other operating particles within the cell cytoplasm. Part of this membrane has a rough appearance because it is dominated with multiple ribosomes. These organelles are the sight of translating to mRNA code into proteins. The ER assists in movement of the mRNA to the ribosome and the proteins to other function particles in the cytoplasm. Part of the ER appears as smooth because it lacks the ribosomes but remain as the sites for the multiple products of the cells. Chemical products within the smooth ER are essential to most plant functions producing the lipids such as those needed for cell wall construction and anti -pathogen toxins. Folds in the smooth ER also commonly separate toxins from other potential detrimental organelles of the cell.
Endoplasmic reticulum also assists in the movement of products of chloroplasts to other cells such as carbohydrates moved through the phloem to the root cells. Endoplasmic reticulum is considered an organelle although it is not as easily distinguished as chloroplasts and mitochondria.
A lot is going on that living corn plant.
Chromosomal DNA in corn cells is located in the nucleus of the cell. Within the 10 chromosomes of corn are a total of about 40000 genes. Each gene consists of a specific string of nucleotides that ultimately gets translated into a string of specific amino acids resulting in specific functionality of the protein. Translation of a small portion of DNA in the cell nucleus results in specific coded RNA designated as mRNA. This RNA molecule must travel to a ribosome in the cytoplasm outside of the nucleus to produce the protein that it codes.
Transport of the mRNA through the nuclear membrane requires a special transport protein that attaches to the mRNA. This interaction allows movement through pores in the two layers of nuclear membrane into the cytoplasm of the cell. Multiple ribosomes are located on the strings of endoplasmic reticulum. When the mRNA is attached to the ribosome, another form of RNA called translation RNA (tRNA) attaches to one of the 20 amino acids as called for by the nucleoside code. This process occurring in the ribosome results in specific amino acids ordered by the nucleoside code.
While we appreciate the performance of a corn hybrid plant as we observe them in the field, it is amazing to think that in all those cells, individually only visible by microscope, that the real work is going continuously in the thousands of cells of the plant.
Chromosomal DNA located in the nucleus codes of about 40000 genes but it is not the only DNA in cells. Chloroplasts have their own DNA that codes for about 100 genes and mitochondria have DNA as well. DNA strands in these two organelles is single strand, unlike the two-stranded DNA in the plant nucleus. This feature, as well as the double-layered membrane of these two organelles, have contributed to the concept that they originated early in the evolutionary change to advanced forms of life. Chloroplasts are believed to happen 3-400 million years ago when a cyanobacterium (a blue green algae) became engulfed in a non-green single celled organism. The chloroplast self-duplicates in plant today, dividing much like bacteria. Its DNA gets translated into RNA, that moves to ribosomes within the chloroplast for production of the proteins needed as enzymes to convert light into chemical energy. Despite producing some of its own proteins, about 90% of the proteins in chloroplasts come from the cells’s nuclear DNA.
Mitochondria, the organelles that convert sugars into the chemical energy of ATP, also have single stranded DNA, double-layered outer membrane and divide like bacteria. Thus, the theory that they originated as bacterial. Like chloroplasts, they are also dependent upon the host cell’s DNA for part of their existence. Just as with chloroplasts, mitochondria have a symbiotic relationship with the host cells.
While the whole corn plant gets our attention, the real work is happening in its smallest components.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.