SOMATIC EMBRYOGENESIS

 Somatic embryogenesis This is the formation of embryos from somatic cells in vitro. It was first observed in suspension cultures (Stewart) of carrots.

 Somatic embryos are formed directly or indirectly in tissue culture. Somatic embryogenesis depends on totipotency and polarity. Totipotency is the ability to produce all types of body cells. Polarity is the condition where cells become polar, i.e. some cells are determined to form shoot and other cells are determined to form root. 

SOMATIC EMBRYOGENESIS: IN VITRO PRODUCTION OF SOMATIC EMBRYOS

Explants are root tissue in the case of carrot, mesophyll cells can also form somatic embryo callus cultured in vitro may also result in the formation of somatic embryogenesis. The scutellum is used for the production of the somatic embryo in the case of rice. Monocots are less viable for the formation of somatic embryos. In dicots variety of tissues are used for the formation of somatic embryos.

 Somatic embryogenesis can be divided into 3 steps first is Induction, the second is embryogenesis, and the third is the maturation of induction.

INDUCTION

 Induction requires the culture of the explant in auxin rich medium.  Embryonic induction is actually the determination of cells to produce embryos. Embryonic induction can occur in auxin free medium. These PEMs are formed in auxin free medium if the cell can synthesize its own auxin.

FORMATION OF SOMATIC EMBRYOS

The induced cells form pre-embryonic masses. These pre-embryonic masses are then cultured in an auxin-free medium so that the cell can form somatic embryos.

 In the auxin-free medium, the pre-determined cells divide and form somatic embryos. These cells undergo globular, heart shape, and torpedo stages of embryonic development. Delaying the culture of callus in auxin free medium removes the induction of cells. The cells will now be used for the formation of plantlets through organogenesis.

 Maturation of embryos

It is the development of a complete embryo through somatic embryogenesis in a culture medium.  The torpedo stage or cotyledonary stage is achieved in somatic embryogenesis.

These embryos are used for Artificial seeds production, or they may be allowed to develop into complete plantlets in a culture medium.

Types of embryos

Embryos can be zygotic embryos or non-zygotic embryos. 

Zygotic embryogenesis occurs naturally after the fertilization of gametes. Where the zygotic cell is formed which is a totipotent cell, it became polar, it develops into an embryo. Parthenogenesis is non-zygotic embryogenesis using female gametes.

 Somatic embryogenesis on the other hand involves the formation of embryos from a somatic cell. Somatic embryogenesis can be direct or indirect. Direct somatic embryogenesis occurs directly from the cultured tissue.

Indirect somatic embryogenesis involved the formation of embryos after the formation of the callus.

 Non-zygotic embryogenesis involved the formation of embryos from non-zygotic cells or any of the body cells except the zygote.

 Non-zygotic embryogenesis can occur through parthenogenesis where unfertilized eggs develop into embryos. 

Polyembryony which is observed in citrus plants, where suspenser cells can develop into multiple embryos, this is also non-zygotic embryogenesis.

 Embryogenesis can also occur through androgenesis where microspores develop into embryos.

  Stages of development of embryos

Somatic embryogenesis involved all stages of development that are taken up by zygotic embryogenesis, it involves a globular, heart shape, torpedo stages, and cotyledonary stages of embryogenesis. From torpedo stages/cotyledonary stage of embryogenesis, can be utilized to form plants. SEs do not involve the formation of cotyledons that are formed in the case of zygotic embryogenesis.

 Limitation of somatic embryogenesis

 Somatic embryogenesis can be formed only in the case of some specific plant.

 Only specific tissues of a plant respond to the formation of somatic embryos.

 Cotyledons are not formed in somatic embryos, so not a regular seed is formed in somatic embryogenesis. Somatic embryos are very fragile. It is impossible to store them as we can store seeds.

 The uses of somatic embryogenesis

 It utilizes suspension culture well somatic embryos produced in suspension culture.

 The plants can be produced through somatic embryogenesis.

 Distant species can be hybridized and somatic embryos can be produced that are otherwise fail to fertilize.

 Artificial seeds are produced through somatic embryogenesis. This process can be scaled up to produce synthetic seeds.