Comparative Proteomic Analysis of Generative and Sperm Cells Reveals Molecular Characteristics Associated with Sperm Development and Function Specialization

In flowering plants, two sperm cells
(SCs) are generated from a
generative cell (GC) in the developing pollen grain or growing pollen
tube and are then delivered to the embryo sac to initiate double fertilization.
SC development and function specialization involve the strict control
of the protein (gene) expression program and coordination of diverse
cellular processes. However, because methods for collecting a large
amount of highly purified GCs and SCs for proteomic and transcriptomic
studies from a plant are not available, molecular information about
the program and the interconnections is lacking. Here, we describe
a method for obtaining a large quantity of highly purified GCs and
SCs from just-germinated lily pollen grains and growing pollen tubes
for proteomic analysis. Our observation showed that SCs had less condensed
chromatin and more vacuole-like structures than GCs and that mature
SCs were arrested at the G2 phase. Comparison of SC and GC proteomes
revealed 101 proteins differentially expressed in the two proteomes.
These proteins are involved in diverse cellular and metabolic processes,
with preferential involvement in metabolism, the cell cycle, signaling,
the ubiquitin/proteasome pathway, and chromatin remodeling. Impressively,
almost all proteins in SCF complex-mediated proteolysis and the cell
cycle were up-regulated in SCs, whereas those in chromatin remodeling
and stress response were down-regulated. Our data also reveal the
coordination of SCF complex-mediated proteolysis, cell cycle progression,
and DNA repair in SC development and function specialization. This
study revealed for the first time a difference in protein profiles
between GCs and SCs.