Arabidopsis thaliana:
It is a little plant that gives flowers and belongs to the family of Brassicaceae. Cabbage and radish are their close relatives.
Though it is not an agriculturally important plant species, it is the model organism owing to its several features. It is named in honor of its discoverer Johannes Thal.
Reasons for this plant being the model organism are as follows-
Its seeds develop into a plant within a short period of about 8 weeks. It is small in size and forms a dense group of plants.
Easily cultivable.
Self-pollination and fertilization occur forming a pure breed and also generates several plants this way.
The genome size ranges from only 114 Mb to 125 Mb.
It is diploid making genetic studies easier.
Its genome is well-mapped.
Transformation experiments using Agrobacterium tumefaciens is effective.
Several mutants and genomic sources are available.
Highly researched by the academic institutions, government, industries, and several research laboratories.
The disadvantages of this plant being a model organism are, it does not produce any fruit and is a dicotyledonous plant. Hence, information regarding fruit production and monocotyledonous crops cannot be derived.
But most of the other information about plants can be derived from this model. Always some exceptions exist.
Transportation of nutrients in Arabidopsis thaliana:
The nutrients can move over small distances through active transport by expending energy or passively using the plasmodesmata.
By passive transport mechanism, the molecules of nutrients or solutes move from the higher concentration regions to the regions of lower concentration. This movement is the major means of gaseous flux in this plant.
Using facilitated diffusion the different substances that are fat-soluble or water-soluble, hydrophobic, or hydrophilic are transported using proteins. The lipid layers or some other membranes can be crossed using this selective method of uptake by nutrients. Example- Aquaporin is involved in the transportation of only water molecules.
This transportation method could be of- uniport, symport, or antiport mechanisms.
Using energy, active transportation moves the substances uphill, transporting them from regions of lower concentration to those of higher concentration. Example- ‘Pump’ is a membrane-associated protein that is involved in this kind of active transportation.
The nitrate, phosphate, sulfate are taken up by the cells, induces depolarization initially on the cell membranes and the cells become more positive. This is then followed by repolarization of the membrane which occurs due to the high concentration of hydrogen ions and the activity of ATPase at the plasma membrane.
The uptake of anions is regulated. In conditions of nutrient deprivation, high-affinity transporters of phosphate and sulfate get derepressed. But, nitrate induces its high-affinity transporters and the reduced and organic forms of nitrogen are associated with the feedback-inhibition.
The CHL1 gene belongs to the family of NRT1 genes and it codes for the transporters that are a part of the low-affinity system. Later, it was identified to be a major part of the high-affinity uptake in the presence of ammonia, and in its absence played a minor role.
The ATPase, channel, and co-transporters are involved in mediating the uptake of the nutrient potassium.
Translocation and water transport in Arabidopsis thaliana:
The nutrients that have entered the plant and those synthesized by photosynthesis have to be translocated to different parts of the plant.
Translocation occurs through the xylem and phloem vessels to reach the target sites. The young, maturing, and developing parts of the plant like the stem, fruits, flowers, and meristematic tissues are the sink of these nutrients.
The nutrients present in the dead and senile parts are drawn back from these sources and transferred to the young regions of the plant. The most essential macronutrients from the old parts like nitrogen, phosphorus, sulfur are removed from that site and sent. But, the structural plant components like the calcium do not undergo this transport.
Using the mass flow hypothesis the nutrients are translocated. This mechanism is based on building up pressure and translocating the nutrients.
AtSUCs and At sweets are the families of transporters that mediate sucrose transport in Arabidopsis thaliana. During phloem loading at the site of leaves, AtSUC2 and AtSWEET11 genes are involved.
Unloading of sucrose is mediated through the apoplast or symplast pathway. Its release at cells is associated with AtSWEET11 and AtSWEET12.
Water transport:
The dead cells like tracheids and vessels of the xylem transport water and the sieve elements of the phloem (live cells) transport other nutrients. The movement of water in this plant ranges only a few centimeters as it is small in size.
Its absorption from the soil is majorly performed by the roots. The thin walls of the underground root system increase the surface area and facilitate the absorption of water.
The water thus absorbed passes through the root by the apoplastic or symplastic pathway.
The root pressure also facilitates water absorption by serving as a lift to water molecules but only up to a certain height in the stem.
Several mechanisms of water uptake are carried out by the plants including the cohesion-tension-transpiration pull which helps in the absorption and transportation.
Aquaporins (depicted in the image), which is a tetramer protein are majorly associated with water transport. They are a part of the ‘major intrinsic protein (MIP)’ family. It is made up of six membrane-spanning domains. About 23 MIP have been found in this plant, but it’s not still sure how many are aquaporins.
Water molecules go in a single file in this pore.
