2023-08-01-BIOL254-W3-Notes .docx

LECTURE 5: PRINCIPLES OF PLANT PHYSIOLOGY 1 – Cell (Embryo) Max auxin is one of the plant hormones. An example is IAA. After the first division of the zygote, there will be a deliberate deployment of one type of protein (PIN proteins) between the basal daughter cell and Apical daughter cell. The Au...

LECTURE 5: PRINCIPLES OF PLANT PHYSIOLOGY 1 – Cell (Embryo) Max auxin is one of the plant hormones. An example is IAA. After the first division of the zygote, there will be a deliberate deployment of one type of protein (PIN proteins) between the basal daughter cell and Apical daughter cell. The Auxin – IAA works in the Apical Daughter cell. After the first cell division, differences are seen in the hormonal content of the cells. Embryo Mutants Plant embryo development/ formation is genetically determined, this means the patterns observed can be lined to specific genes. This is known because we can generate mutants that can produce embryos which do not perform like the normal plants. In the normal plant (Pic a). it has the normal cotyledons, roots etc. but when the embryo is being treated with chemicals known as mutagens, they form mutants ( See figure b to e). they are called embryo mutants. They do not look anything as the original plants (also known as wild type). GNOM (gn) Gene The gnom mutant is linked to the gnom (gn) gene. They look gnom because of the function of the gnom (gn) gene. This gene codes for a guanine nucleotide exchange factor. This gene product moves things around. This gnon (gn) gene product is responsible for moving the PIN protein in a particular direction during embryogenesis. In the gnom (gn) mutant, you cannot see the cotyledon or proper roots as the normal plants. This occurs at the early stage; IAA moves in specific way in different direction but in the mutant this movement is all wrong resulting in the formation of a gnom mutant. Monopteros (mp) genes This mutant has genes that encodes for auxin response factors proteins. In the plants without treatment, plants will be normal. During embryogenesis, these genes function properly. This is essential for preferential accumulation of auxin in the hypophysis. Hypophysis is that cell that will develop into root axis or root apical meristem. When germination of normal embryo, you will observe the plant with the normal long root but then treated with a mutagen or chemical, during embryogenesis, IAA fail to accumulate in the hypophysis and do not produce roots. Root production in plants is genetically determined because of the expression of genes at certain stages of embryogenesis. Going Back to this Slide When a zygote is fertilized, it is developed in embryogenesis leading to the formation of mature embryo. When it is mature, you can see the root, shot, apical meristem, cotyledon. You have a miniature plant. Once the mature embryo stage is achieved, the embryo losses water and reaches the resting stage or storage stage without damaging the seeds. This mature embryo loses water because that is a signal to stop the end of embryogenesis. All the hormonal changes that occur in the embryo will stop. It is also switching or prepare for the next stage of the germination program as well. When the dried seed/embryo is provided water, they germinate giving rise to leaves and roots. After germination, the plant enters the post-embryonic growth and development stage. In this stage the plant height increases, roots become deeper (primary plant growth). After this stage, the plant begins to increase in diameter that is plant getting fatter and fatter (secondary plant growth), this stage is associated with maturity of the plant body after that it will start reproductive development, fertilization, production of seeds and the life cycle continues. Cells, Tissues, and Organs: What is a Meristem? Meristem is a region of unspecialized cells in the plant. The activity in the meristem is not finite that is no endpoint, also meaning the meristem keep on adding cells to the plant body and does not ever stop. It is said to be indeterminate that is no predetermine limit provided the growth conditions for the plant are optimum. The meristem adds new cells to whatever part of the plant needs it. This contrasts with the animal cells. Meristematic cells A meristematic cell has a prominent nucleus and no large vacuole or empty spaces inside the cells because of the fewer cytoplasm and very thin walls. Some meristems are considered primary while others are considered secondary. Apical Meristems – this is formed during embryogenesis also known as primary meristems. It is shoot, root and primary meristems. They are responsible for primary growth such as plant heigh, root length. They increase in the longitudinal direction. Apical meristems have undifferentiated cells called INITIALS (according to some textbooks). They are undifferentiated. You cannot tell whether they are vascular bundles, cambium etc but they are special because they can differentiate into these specialized cells. Apical meristem adds new cells by cell division (increase in cell number), Cell expansion (elongation). All the new cells expanding/ elongating will stop. When they stop, they become specialized and become other types of cells such as xylem, phloem, dermal tissue etc. The fact that shoots apical meristem and root apical meristem keep adding cells to the top and to the bottom is essential to maintain the architecture of the axial pattern. This is essential because plants do not move and live in a terrestrial environment and because of competition. Lateral Meristems – when plants reach maturity, new meristems begin to form in the sides. These are called secondary meristems or lateral meristems. They are responsible for secondary plant growth. What is the relationship between Apical meristem and length of root and plant height? The stem grows taller because the shoot apical meristem accumulates cells and keep on adding the cells in a longitudinal direction. Shoot Tip The cells below the shot apical meristem initially were small, then the get more elongated and more specialized. The darker cells in the leaves are the vascular tissues that is leaf trace, and this continue to the vascular tissue of the cells. Shoot Apical Meristem – Tunica and Corpus Meristem identity is maintained while nee daughter cells are added to increase the length of the shoot axis. When a meristematic cell undergoes cell division, one of the daughter cells remain meristematic while the other one gets elongated, so throughout plant life the number of meristematic cells always remain the same. The behaviour is genetically determined.