Plant Cell Wall Structure, Function, and Phloem Transport

Posted on Dec 8, 2024 in Biology

**Item 1: Membership of the Physical Cell Wall**

Regarding the composition, a physical wall is formed by different layers that vary in thickness, chemical composition, and direction of microfibrils. Starting from the outside of the plasma membrane, we find the middle lamella, an intercellular space of the plant cell, the primary wall, and finally, the secondary wall.

  • Middle Lamella: Formed principally by pectins without cellulose. The soft tissue is not lignified.
  • Primary Wall: It is a flexible structure capable of accommodating growth to changes in protoplasmic volume. It is formed by cellulose and an amorphous matrix with an abundance of pectins in the case of dicots and scanty in monocots. If secondary growth begins to develop, the primary wall becomes very lignified. The microfibrils of the primary cell wall of meristematic cells form a simple multinetwork within the amorphous matrix (various models, none demonstrated). In the primary wall, the discontinuities that allow direct communication between neighboring cells through plasmodesmata are called primary pit fields.
  • Secondary Wall: Has the same components as the primary wall but with a completely different disposition. It is composed of microfibrils of cellulose, which are more abundant than in the primary wall, and an amorphous matrix in which hemicelluloses predominate. It can be formed by 3 sublayers:
    • The S1: Includes microfibrils forming a helix at a 50° angle with respect to the longitudinal axis of the cell.
    • The S2: Is the thickest and is subdivided into several sublayers forming a 10° angle with respect to the longitudinal axis.
    • The S3: Now found in a single helix, the sublamellae of S2 form an 80° angle.

**Item 1: Role of the Cell Wall in the Plant’s Defense Mechanism**

The upper floors were not able to move, so they developed a chemical defense system. Two groups of compounds can be considered: the constitutive, always present in tissues, and the inducible, only present when the plant is attacked by a pathogen.

  • Constituents:
    • Physical barriers (wall components): lignin, cutin, suberin, waxes.
    • Chemical barriers (presence of inhibitors on the wall and in the vacuole): saponins, glucosinolates, etc.
  • Inducible:
    • Local and systemic response, fortification of cell walls (callose, lignin).
    • PR proteins (pathogenesis-related).
    • Phytoalexins (antibiotics).
    • Hypersensitive response (programmed cell death).

**Item 1: Features and Physiological Functions of Pectins**

(Extracted with boiling water) Acidic: Abundant in higher plants. It is characterized by the presence of polyuronic acids. One example is the rhamnogalacturonans, unbranched chains composed of alpha (1,4) D-galacturonic with the presence of alpha (1,2) L-rhamnose, which makes the molecule provide a zigzag shape. The negative charges of the carboxyl groups of pectins are coordinated with calcium, forming calcium pectates. Pectic substances are the basis of jams and are highly prized in the food industry as thickeners, stabilizers, and emulsifiers.

The case of rhamnogalacturonan II (RGII): Considered a baroque pectin in its complexity. Its synthesis involves 50-60 enzymes and has 12 different monosaccharides and 22 linkages. Some monosaccharides, such as apiose, have not been found in any other molecules so far. Its structure is well preserved and is very resistant to enzymatic degradation. Borate-diester forms dimers, which sets the two chains of RGII through their respective apiose residues. Boron is an essential element for plants. Studies in boron-deficient plants using microscopy showed a disruption of the walls and a reduction in size, effects that disappear when supplying the element to the cultivation.

**Item 2 and 3: Principal Mechanisms of Phloem Loading in Source Organs**

Source organs are organs where the synthesis of a particular compound is higher than its consumption. In the case of sucrose, the source organs are expanded leaves. Once synthesized in the cytoplasm of the mesophyll cell, sucrose moves toward the leaf vascular system. This movement is a short distance of 2 or 3 cells via the symplastic route. Two types of phloem loading can be considered:

  • Type I: Symplastic loading occurs (80% of tree species). These are plants with a very large number of plasmodesmata between source cells and companion cells. Some species have no differences with regard to both the osmotic potential and the [sucrose] between mesophyll cells and companion cells. In this case, one cannot speak of a burden because there is symplastic continuity from photosynthetic cells to the phloem sieve cells. In other species, molecular polymers are synthesized from sucrose in the companion cells (e.g., raffinose). The plasmodesmata of these cells tend to retain these compounds, thus creating a favorable gradient for the movement of sucrose into the sieve cells.
  • Type II: Apoplastic loading occurs in 80% of herbaceous species, which have a low presence of plasmodesmata between source cells and companion cells. There are species where the [sucrose] inside the phloem tube is higher than in the mesophyll. In these cases, the loading must be active, so it is necessary for sucrose to exit into the apoplastic space. In 1983, a hypothesis was presented that explains a possible mechanism of active phloem loading using the electrochemical gradient created by the H+-ATPase of the plasma membrane. Sucrose is transported through a symport with H+. The loading of sucrose by a secondary active transport mechanism takes place in the companion cells. The inhibition of glycolysis in companion cells blocks the loading mechanism. It was experimentally verified that pyrophosphatase inhibits both glycolysis and sucrose loading. The situation returned to normal by adding a cytosolic invertase.

**Item 2: Regulation of Stomatal Opening/Closing by CO2 Absorption**

In any plant, a small amount of water becomes part of the molecular constituent, one part is the scattering medium where metabolic processes take place, and most of the water is lost as vapor in a process known as transpiration. Between 50-85% of rainwater returns to the atmosphere through the process of plant transpiration. Vegetation can lose up to 3 times more water than a cloud. In a cross-section of a leaf (C3 plant), we find:

  • Cuticle: Cutin, waterproof and of variable thickness.
  • Epidermis: One layer without chloroplasts. Stomata.
  • Mesophyll and lacunose mesophyll in palisade -> many chloroplasts.
  • System. Host of xylem and phloem.

Transpiration takes place in structures called stomata. The stomatal apparatus comprises:

  • The guard cells (or stomata), two with a linear distribution, both on the back and the front in monocots, and a bean shape and distribution, mostly on the underside, in dicots.
  • When the stoma is open, the guard cells are separated, leaving a space called the ostiole, beneath which there is an intercellular space, the substomatal chamber.
  • The guard cells are surrounded by subsidiary cells, which may or may not be morphologically different from the other epidermal cells.

Characteristics of Guard Cells:

  • Cellulose microfibrils of the wall arranged transversely to the longitudinal axis of the cell make it possible for them to deform longitudinally and separate.
  • No plasmodesmata.
  • They have chloroplasts without a functional Calvin cycle.
  • When the stoma is closed, the cell presents an increased starch content, an increase in osmotic potential (-1.9 MPa), and a decrease in [K+] (0.1 M), whereas when it is open -> low starch, low osmotic potential (-3.5 MPa), and increased [K+] (0.5 M), unlike the subsidiary cells.
  • When the stoma opens, following a gradient of osmotic potential, water enters the interior of the guard cells from the subsidiary cells.

Theory explaining stomatal opening and closing:

The factor that controls stomatal opening is the availability of CO2 in the mesophyll cells. The opening and closing mechanism is sensitive to pH changes. A possible reaction sequence is:

  1. If [CO2] in the cell is low, cytoplasmic pH rises, which activates the process of starch degradation, glycolysis that produces phosphoenolpyruvate (PEP).
  2. PEP carboxylase (PEPC), enabled by the rise in pH, catalyzes the formation of oxaloacetic acid (OAA) from PEP and bicarbonate. OAA is reduced using the NADPH created in the chloroplast electron transport chain, forming malic acid by malate dehydrogenase.
  3. At basic pH, malic acid is present in dissociated form, and H+ are used by the H+-ATPase of the plasma membrane to create an electrochemical gradient for the transport of K+ from the subsidiary cells to the guard cells. K+ and malate are transported and accumulated in vacuoles, which can produce a lowering of the osmotic potential of the guard cell.
  4. Following the osmotic potential gradient, water moves from the subsidiary cells to the guard cells. The turgidity of the guard cells increases and the subsidiary cells decrease. The guard cells change shape -> the stoma opens.
  5. If the [CO2] rises, the pH decreases, inhibiting enzymes involved in the opening process and initiating the reverse process that leads to stomatal closure.

**Item 3: A Theory to Explain a Possible Mechanism for Phloem Transport**

The mass flow hypothesis to explain the movement of solutes within the phloem tubes was introduced in 1930 by Münch. In this hypothesis, the phloem is considered an osmotic system where the movement of sucrose occurs passively, following an osmotic gradient and pressure between source and sink organs. The loading of sucrose in the sieve tubes of source organs produces a reduction of osmotic potential that attracts water. The entry of water into the phloem tube increases the pressure, which gives rise to a shift in the phloem sap to the sinks. Sink organs keep the pressure low due to the consumption of sucrose. The water that arrived with the sucrose moves to the xylem, following the pressure gradient. This theory is accepted for the movement of sucrose in angiosperms, where the sieve plate presents open perforations between cells.

  • -> In the case of gymnosperms, the connections between cells can be found to be more restricted, so the transport mechanism may be different, since the theory of mass flow is based on the fact that the sieve tubes have low resistance to movement.
  • -> Also invalidating the theory is the fact that bidirectional flow has been demonstrated in a single conducting tube, something that has not been demonstrated until now.
  • In favor -> A gradient of [sucrose] and pressure between sources and sinks has been found experimentally.
  • Direction of transport: It has been confirmed that the distribution always takes place towards the nearest sink located on the same side of the plant where the source is located. In the case of missing sinks, the system is shared, and the only source will supply sucrose to all sinks.
  • Regulation of the direction of transport; 2 types:
    • Passive: The organ with the greatest consumption of sucrose creates a greater gradient between it and the sources, promoting transport in its direction.
    • Active: Hormonal regulation, both of loading in sources and unloading in sinks, depending on the situation.
  • Limitations of sinks: During the vegetative growth phase, growth is limited by the capacity of young leaves to utilize assimilates. In general, mature photosynthetic leaves are not always at 100%.
  • Limitations of sources: The dominance of some sinks over others is according to the time cycle of the plant. Thus, during the reproductive stage, the fruits and seeds dominate over the vegetative organs. This competition is reflected in a decrease in the growth of less-favored organs. Distribution of assimilates: sink = young leaves / source = mature leaves.