Androgenesis is the phenomenon that provides a possibility of raising the haploid plants which has an application in plant breeding. The process of androgenesis forms an experimental system for reading the sequence of events that are involved in transition from gametophytic phase to sporophytic phase. In Angiosperms or the flowering plants, the gametophytic stage is short-lived and is dependent on the sporophyte. The male gametophytes in this group are represented by pollen grains which are haploid. Pollen grains develop into sporophytes through a process called as androgenesis. Several studies using biochemical, cytochemical, molecular and electron microscopic techniques have revealed that biochemical and structural alterations occur in microspores, when they get transformed into a form in the sporophytic pathway.
The initiation of divisions resulting in sporophytic phase of the microspore are preceded by the biochemical and molecular level changes. These changes may be the cause for the beginning of androgenesis. Some scientists identified that a few factors present in the anther are responsible for the development of angiosperm pollen. Experiments were conducted to see whether the culture medium in the In Vitro conditions, is triggering the sporophytic development of pollen. It is found that culture medium need not be the reason for controlling the gametophytic phase and development of sporophytic phase.
Stress as a trigger for androgenic induction
It is understood from certain studies that stress or sub optimal conditions can change the developmental processes. Stress is considered as a vital constituent that induces the androgen. There are two theories hypothesized to explain the involvement of stress in androgenic induction. One of them is the enhancement of the expression or stabilization of the target gene due to the developmental defects caused by stress. The second one is the repression of the critical genes lower than the threshold level. The heat treatment is found as the common pre-treatment initiating androgenesis in Brassica napus. The temperature that is around 32 degree centigrade is considered optimum for the androgenesis. High temperatures will not help the microspores to develop but die. Therefore, stress in combination with the culture conditions are considered as appropriate for the initiation of androgenesis. Temperature treatment along with gamma irradiation has shown just stimulatory influence on the androgenesis induction.
In another sample study, androgenesis was initiated in bi-cellular pollen grains of tobacco. They were previously cultured in glutamine rich and sugar deficient medium and were further transferred to the medium containing glucose. The molecular processes that were going on in the pollen grains, prior to the induction of androgenesis by stress was appearance of heat shock protein transcripts generated as a stress response. The same stress response was observed when the stress stimuli like colchicine and gamma irradiation were used. The temperature below 25 degree C could not trigger the stress response. The heat shock protein transcripts were used as molecular markers to study whether the microspores have responded to the stress stimulation.
Developmental stage of Microspore
It is studied that the appropriate time of androgenesis can be between before or after the first pollen mitosis. It is observed that the sporophytic specific gene products are eliminated from the cytoplasm prior to meiosis. The gametophytic specific gene products are usually transcribed before the first pollen mitosis. After the first mitosis, the gametophytic related information will be established in the cytoplasm and it helps in the development of male gametophyte. Several external stimuli and microspore culture media are used to mask the gametophytic programs and stimulate the expression of sporophytic specific genes. These genes drive the microspore to follow sporophytic mode of development.
As per the studies done in various plants, the five ways of androgenesis processes that were recognized are repeated divisions of generative cell, repeated divisions of vegetative cell, repeated divisions of both of the cells, two identical cell formations from the uninucleate microspore, and origin from the fusion of vegetative and generative cells.
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