Dynamics of nuclear pore density and distribution patterns within developing pollen: Implications for a functional relationship between the vegetative nucleus and the generative cell

Quantitative transmission electron microscopy was used to study pore density (mean pore number±standard deviation/nuclear envelope area) on developing pollen nuclei of alfalfa. We found that pore density is essentially uniform around the vegetative nucleus (VN) at an early developmental stage (29±6 pores μm^-2), but later, when the VN forms a close physical association with the generative cell (GC), nuclear pore density is 69% higher on the surface of the VN facing the GC (27±12 pores μm^-2) compared to the surface away from the GC (16±9 pores μm^-2). The surface area of the VN does not change significantly during the stages of this study. Pore density is nearly equal on vegetative and generative nuclei in young pollen, but at pollen maturity the VN has a mean pore density 3.5 times greater than that of the generative nucleus. Our results are consistent with those of other studies comparing pore densities on developing and mature pollen nuclei. However, this is the first study, to our knowledge, that has followed vegetative nuclear pore density and distribution as it relates to the formation of a close physical association between the VN and the GC. Taken together with biochemical studies on RNA and protein synthesis during pollen development, and studies on nuclear pore function, these results support the notion that even though mean pollen nuclear activity may decrease during pollen maturation, the potential for nucleocytoplasmic exchange is not diminished appreciably in the area of the VN-GC association. This suggests that there is a direct functional relationship between the VN and the GC, and that gene expression may be not only temporally but also spatially separated within the VN during pollen development.