To be a male sex cell or not to be, that is the question

What decides the fate of a plant cell? What will it be, a male germ-line cell or a somatic cell? Unlike animals, where the male germ-line is set aside during early development, in plants the male germ-line cell arises from a previous cell of somatic lineage.

So what happens in the mundane life cycle of this ever growing and dividing somatic cell that causes a switch to the stud that it will become? Haerizadeh et al. (Science. 2006. 313: 496) have shed light on the molecular processes that result in the determination of the male germ-line cells.

In an earlier work by Xu and colleagues (PNAS. 1999.96: 2554) a LILY GENERATIVE CELL SPECIFIC-1 (LGC-1) gene has been identified to be a male-gamete specific gene from Lily. This male-gamete specific LGC-1 gene has a silencer region in its promoter which can direct the expression of a reporter gene in the male germline cells of a transgenic plant. A promoter is a specific region in a gene where the code transfer from the gene to the transcript (called a messenger RNA or mRNA) initiates. Like all headquarters, it has different control points. A silencer region is one such regulatory point comprising a DNA stretch where certain transcription factors (proteins) can bind and prevent the gene to produce a transcript.

It is the regulation of this silencer region in the LGC-1 gene which seems to control the fate of a somatic cell. In the non-male germline cells, such as the leaf and petal tissues, the researchers found a novel 24kD protein called the Germline Restrictive Silencing factor (GRSF) which could bind to the silencer region in the LGC-1 gene and prevent its expression. Interestingly, moderate levels of GRSF proteins are found in the uninucleate microspores but are absent from the generative cell.

I will briefly refresh the pollen development sequence in order to better appreciate this last observation. The development of male gametes in flowering plants involves meiosis of the microspore mother cells located along the inner edge of the anther sac. The microspores or the pollen grains thus produced comprise of a two layered wall (exine which is sometimes intricately sculptured and an intine). Inside the pollen grain, the nucleus divides by mitosis and one of the nuclei develops a cell membrane around it. This forms the generative cell that further undergoes mitosis to give rise to two sperm cells. The other cell inside the pollen grain, called the pollen tube cell, grows by the extension of the intine after pollination in flowering plants and carries the generative cell that divides to form the two sperm cells one of which fertilizes the egg cell and the other fuses with the polar nuclei which form the endosperm.

It is the generative cell which lacks any trace of the GRSF whereas the tube cell shows moderate levels of GRSF protein. The absence of GRSF in addition to the activation of male-germline specific transcription in the generative cells suggests that the release from GRSF imposed repression is the decisive event in the sperm cell developmental process of the flowering plants. GRSF may function as a key regulatory element in the network of regulatory controls for gamete development. The study also highlights the evolutionary conservation of this repressor system and suggests it to be a key process in the regulation of the male germline development in flowering plants.

The authors have also compared this repressor system to that of a neuron restrictive silencing factor (known as REST). REST is also evolutionarily conserved and is shown to repress transcription from numerous neuron specific genes in neural precursors and non-neuronal cells. The silencing activity of the neural specific cells is mediated via the recruitment of a co-repressor (COREST). This parallel from animal kingdom has prompted the authors to question if the GRSF induced silencing of the male germline specific genes in plants also mediated by a co-repressor.

The presence of the GRSF in ovary tissues has also been observed and raises questions for their purpose in this tissue. Along with this line of investigation, further investigations towards nature of silencing mechanisms for long term repression would make an interesting study.