User:Na03moha/sandbox

Background
When a primary root begins to develop, it possesses three developmental regions that allow it to do so: the meristematic zone, the elongation zone, and the maturation zone. To increase the growing root's overall surface area, lateral roots must develop.

To do so, first primordia of the becoming lateral root are signaled from the terminal root's pericycle. This is achieved by constructing another growth axis within the plant through the creation of secondary meristems. Cell divisions continuously occur to allow for root expansion, ultimately producing a lateral root that begins to protrude from the epidermal cell layer. Because the lateral root buds from the zone of maturation within the terminal root, the former is composed of the very same cell types as its parent counterpart.

Lateral roots are vital to the success and survival of a plant. Anatomically, they are one of the primary leaders by which the main root system is structured. In addition to this, lateral roots act to absorb water and nutrients, as well as keep plants firmly planted within the surrounding soil.

Early morphological changes
The following description is for early events in lateral root formation of the model organism Arabidopsis thaliana, where lateral roots typically form when the plant is between seven and nine days old.


 * Stage I: The first morphologically identifiable stage is the asymmetric division of two cells of the pericycle, termed pericycle founder cells, which are adjacent to the protoxylem poles and from which the lateral roots are derived entirely. These cells then undergo further division, causing radial expansion.
 * Stage II: The small, central cells then divide periclinally (parallel to the surface of the plant body) in a series of transverse, asymmetric divisions such that the young primordium becomes visible as a projection made up of an inner layer and an outer layer.
 * Stages III and IV: At the third stage, the outer layer of cells divide so that the primordium is now made of three layers. The fourth stage is then characterized by the inner layer undergoing a similar division, such that four cell layers are visible.
 * Stages V to VIII: Expansion and further division of these four layers eventually results in the emergence of the young lateral root from the parent tissue (the overlying tissue of the primary root) at stage eight.

The number of lateral roots corresponds to the number of xylem bundles.

Arabidopsis Root Emergence via PIN-FORMED8 Intracellular Localization
The plant hormone auxin is responsible for generating concentration gradients to allow for proper plant development. As of 2020, one auxin transporter was identified as a means to flood the hormone into cells: AUXIN-RESISTANT1 (AUX1)/AUX1-LIKEs (LAXs). In addition, two transporters that allowed for the hormone to exit cells, PIN-FORMEDs (PINs) were established, as well as ATP-binding cassette Bs (ABCBs)/P-glycoproteins (PGPs). PIN proteins steer auxin to areas of necessity throughout plants. These proteins present in the apical meristem of the plant direct auxin downward through the plant, a process independent of gravity. Once in the vicinity of the root, vascular cylinder cells shuttle auxin toward the root cap's center. Lateral root cells then absorb the phytohormone through AUX1 permease. PIN proteins recirculate the auxin upward to the plant's shoots for direct access to the zone of elongation. Once there and utilized, the proteins are then finally shuttled back to the lateral roots and their corresponding root caps. This entire process is known as the foundation model.

In Arabidopsis thaliana, PIN proteins are localized in cells based on the size of their loop that connects the intercellular matrix to the extracellular matrix. Shorter PIN proteins (PINs 1, 2, 3, 4, 6, and 7) are found intracellularly as well as nearest to the plasma membrane, whereas the longer proteins (PINs 5 and 8) are found almost exclusively by the plasma membrane.

The protein PIN8 significantly influences the development of lateral roots in a plant. When a nonfunctional mutant of the protein, pin8, was inserted into a plasmid and into Arabidiopsis thaliana, lateral roots lost root density. It was shown that this mutant had no lingering effects on the development of the primary root. When further investigated, it was discovered that the pin8 mutant was significant only as the lateral root was beginning to appear in the plant, suggesting that a function PIN8 protein is responsible for this action. This was later corroborated with another experiment. This was done under the hypothesis that PIN proteins aided in the movement of auxin, which would increase lateral root development. The PIN8 protein was expressed in the pericycle of transgenic plants. It was found that these plants had significantly less density than the wildtype plants. This result was comparable to the result of the short PINS, PIN3 and PIN5. In a final experiment, PIN proteins were found to be localized in both the endomembrane system (within the plasma membrane) as well as the extracellular matrix.

Notes (Things to Fix/Add)
-Add to Background section with more information about eudicots and gymnosperms?

-Tweak Early Morphological Changes section (maybe combine with Background?)

-Add to Arabidiopsis section (more info in second paragraph), and add to methodology of third paragraph