Organogenesis and Somatic embryogenesis in Plant Tissue Culture

Organogenesis in Plant Tissue Culture. Before we understand what is Organogenesis we must understand about what is meant by plant tissue culture. Plant Tissue Culture is a technique of growing a plant asexually with the help of artificial media in a laboratory under sterile conditions. unlike a normal plant, which is grown in the soil this plant is grown with the help of artificial media such as Murashige and Skoog medium(MS), whites medium, Gamborg medium also called as B5 medium, Nitsch and Nitsch medium also can be written in short form as NN. Now that we have understood the meaning of Plant Tissue Culture it will be easy to understand what is Organogenesis. Organogenesis is simply the formation of plant organs in plant tissue culture and Somatic Embryogenesis is the formation of embryo like structures from somatic cells.

Organogenesis in Plant Tissue Culture

What is Organogenesis? Does it have to do anything to do with organs of human?

No it does not have anything to do with the organs of humans!!! let us break the term to understand its meaning organogenesis can be broken down in to two terms as organ and genesis.

Organ- Organ does not mean normal organs, in plants case, they also have organs just as humans... wait wait wait I am not saying similar organs as humans like heart, kidney, pancreas, liver, spleen, its just that plant organs are quite simple than us human organs, less complex than us, such as stem, roots, flowers, leaves, etc. for your knowledge you will be fascinated to hear that fruits and seeds are also considered as plant organs!!!

Now first word was broken down now let's understand or break down our second word ie. genesis.

Genesis- Don't worry it does not have a complex meaning, the word genesis simply means the origin of something, the formation of something new. 

Now that we have understood the meaning of both the words Organ and Genesis now lets put all of them together - The formation of plant organs while carrying out or growing plants via plant tissue culture.

 Organogenesis

Definition-

Organogenesis is the process of morphogenesis involving the formation of plant organs such as shoots, roots, flowers, and buds from explant or cultured plant tissues. 

 There are Two types of organogenesis in plant tissue culture depending on from where does the organs like stem or root formation take place from  - 

a) Direct organogenesis

b) Indirect organogenesis

a) Direct organogenesis = In this, the tissue undergoes morphogenesis without going through the callus formation or suspension cell culture stage. The term direct adventitious organ formation is also used for the direct organogenesis. in this organogenesis the organ formation takes place directly from explants.

For the appropriate organogenesis in the culture system, exogenous addition of growth regulators such as auxin and cytokinin is required.

b) Indirect organogenesis =  When the organogenesis occurs through callus or suspension cell culture formation, it is said as indirect organogenesis. It means that the organ starts to arise from the callus and not directly from the explant. so in this organogenesis the first callus formation takes place from explant and then from that callus organs start to arise 

Hormones for Organogenesis

The amount/concentration of growth regulators in the medium is critical for organogenesis. By varying the concentrations of auxins and cytokinins in the culture, in vitro organogenesis can be manipulated.

1) Low auxin and low cytokinin concentration will induce the callus formation. Although in certain books it is mentioned that adding auxin for callus formation.

2) Low auxin and high cytokinin concentrations helps promote shoot organogenesis from callus.

3) High auxin and low cytokinin concentration will induce the root formation.

Somatic embryogenesis in Plant Tissue Culture

Somatic embryogenesis is the formation of embryo like structures from the callus or explant.

Now let us understand or break down the word somatic embryogenesis like as we did organogenesis.

Somatic embryogenesis can be broken down in to three words such as somatic - embryo -   genesis

Now let's understand .....

Somatic - Somatic over here means we use plant somatic cells other than the plant germ line cells ie . we grow plants by using somatic cells.

Embryo - Embryo means over here since we are not using germ line cells of the plant still we can see an embryo like structure formation from the used somatic cells as an explant.

Genesis - Don't worry it does not have a complex meaning the word genesis simply means the origin of something the formation of something new. 

Now that we have understood the meaning of all the 3 words somatic - embryo - genesis so in short it simply means the formation of embryo like structures from explants or callus of somatic cells

Good now we are good to go with the topic of somatic embryogenesis

Somatic embryogenesis

The process of regeneration of embryos from somatic cells, tissues, or organs is regarded as somatic embryogenesis. 

Somatic embryogenesis may result in the formation of non-zygotic embryos or somatic embryos (directly formed from somatic organs), or androgenic embryos (formed from male gametophyte).

In general, when the term somatic embryo is used it implies that it is formed from somatic tissues under in-vitro conditions. Somatic embryos are structurally similar to the zygotic (sexually formed) embryos.

(NOTE- In short Somatic Embryogenesis means the formation of embryo-like structures (not actual embryos) from callus or explants, ie. embryo-like structures formed from somatic cells.)

Somatic embryos can be developed using somatic cells, particularly epidermis, parenchymatous cells of petioles, or secondary root phloem.

Two routes of somatic embryogenesis are known:- 

A) Direct Somatic Embryogenesis

B) Indirect Somatic Embryogenesis

A) Direct somatic embryogenesis = When the somatic embryos develop directly on the excised plant (explant) without undergoing callus formation, it is referred to as direct somatic embryogenesis This is possible due to the presence of Pre-embryonic determined cells (PEDC) which are found in certain tissues of plants

B) Indirect somatic embryogenesis =In this Embryogenesis the cells from explant (excised plant tissues) are made to proliferate, form callus, and then form an embryo. This is possible due to the presence of Induced embryogenic determined cells (IEDC). (In Indirect Somatic Embryogenesis the explant first has to go through the callus formation stage and then the embryo formation takes place.)

Factors influencing somatic embryogenesis

The addition of the amino acid L-glutamine helps promote embryogenesis 

The presence of auxin such as 2, 4-dichloro-phenoxy acetic acid is essential for embryo initiation in cultures. 

On the low auxin or no auxin medium, the embryogenic clumps develop into mature embryos.

Conclusion

We have covered up what is meant by -
Organogenesis - Organ formation via plant tissue culture.
Somatic Embryogenesis -  Embryo like structure formation from cultured somatic cells while carrying out plant tissue culture.
We also have covered different types of the direct and indirect paths the cells choose to attain Organogenesis and Somatic embryogenesis. So while we are carrying out our plant tissue culture with the help of micropropagation steps the plant will go through organogenesis and somatic embryogenesis. Thank you.

Reference:-

Biotechnology by U Stayanarayana.