Pathways of Androgenesis

Based on the studies in different plants, the five routes of androgenesis that have been identified are: (i) by repeated divisions of the vegetative cell, (ii) by repeated divisions of the generative cell, (iii) by repeated divisions of both, (iv) through symmetrical divisions in uninucleate microspore giving rise to two identical cells rather than unequal generative and vegetative cells (B-pathway) and (v) origin from fusion product of generative and vegetative. A species can exhibit predominance of one or the other pathway [27].

Thus, division of otherwise quiescent vegetative cell, more than one division in generative cell or formation of two-celled unit with identical cells from a uninucleate microspore can be taken as the first sign of deviation from gametophytic development.

Of the abovementioned pathways, the fourth pathway is the most widely studied and is considered to be the major pathway of androgenesis. This pathway involves symmetrical division of microspore. Stress serves as a major signal for this symmetrical division. Since colchicine treatment has been shown to increase the number of symmetrically dividing embryogenic pollen grains in cultures of Brassica napus [9, 10, 28], it has been suggested that symmetry during cytokinesis is an important factor in deflecting the gametophytic program of the pollen grain towards the androgenic one.

The significance of microspore division symmetry for vegetative cell-specific transcription and generative cell differentiation has been addressed in microspores of transgenic tobacco plants transformed with promotor of vegetative cell-specific tomato lat52 gene fused to reporter gus gene [29]. In vitro maturation, in the presence of high concentrations of colchicine, blocks the first pollen mitosis effectively, resulting in the formation of uninucleate pollen grains expressing both the abovementioned genes which are capable of germination and a pollen tube growth, despite the absence of a generative cell. Lower amounts of colchicine induced symmetric division producing two similar daughter cells, both expressing the gus gene. These results demonstrate that division asymmetry, at the first pollen mitosis, is essential for the correct generative cell differentiation. Moreover, the activation of vegetative cell-specific transcription and functional maturation may be uncoupled from cytokinesis [29].

Touraev et al. [17] on the other hand, have shown that cultivation of pollen grains containing even two equal sized cells under the maturation conditions, lead to the development of mature pollen grains. This indicates that rather than the symmetry of first pollen mitosis irreversible commitment to embryogenesis is essential. In an interesting study, Zonia and Tupy [30] have shown that lithium disrupts the partitioning of membrane-associated calcium, blocks polar nuclear migration and subsequently, induces a symmetrical mitosis in microspores of tobacco.

Eady et al. [29] proposed two models to explain the significance of first pollen mitosis for pollen determination and differentiation. According to the first model of passive repression, low levels of gametophytic expressed factors (which are the result of asymmetric division), are present in the generative cell. On the contrary, the symmetrically dividing cells, or in the case of colchicine-blocked uninucleate microspores, no such repression of the vegetative cell-specific genes occurs. According to the second model, there exists an active repressor, blocking transcription in the generative cell, which upon asymmetric division, is again selectively retained in the generative cell.

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