Annexin5 Plays a Vital Role in Arabidopsis Pollen Development via Ca2+-Dependent Membrane Trafficking

Abstract

The regulation of pollen development and pollen tube growth is a complicated biological process that is crucial for sexual reproduction in flowering plants. Annexins are widely distributed from protists to higher eukaryotes and play multiple roles in numerous cellular events by acting as a putative “linker” between Ca2+ signaling, the actin cytoskeleton and the membrane, which are required for pollen development and pollen tube growth. Our recent report suggested that downregulation of the function of Arabidopsis annexin 5 (Ann5) in transgenic Ann5-RNAi lines caused severely sterile pollen grains. However, little is known about the underlying mechanisms of the function of Ann5 in pollen. This study demonstrated that Ann5 associates with phospholipid membrane and this association is stimulated by Ca2+ in vitro. Brefeldin A (BFA) interferes with endomembrane trafficking and inhibits pollen germination and pollen tube growth. Both pollen germination and pollen tube growth of Ann5-overexpressing plants showed increased resistance to BFA treatment, and this effect was regulated by calcium. Overexpression of Ann5 promoted Ca2+-dependent cytoplasmic streaming in pollen tubes in vivo in response to BFA. Lactrunculin (LatB) significantly prohibited pollen germination and tube growth by binding with high affinity to monomeric actin and preferentially targeting dynamic actin filament arrays and preventing actin polymerization. Overexpression of Ann5 did not affect pollen germination or pollen tube growth in response to LatB compared with wild-type, although Ann5 interacts with actin filaments in a manner similar to some animal annexins. In addition, the sterile pollen phenotype could be only partially rescued by Ann5 mutants at Ca2+-binding sites when compared to the complete recovery by wild-type Ann5. These data demonstrated that Ann5 is involved in pollen development, germination and pollen tube growth through the promotion of endomembrane trafficking modulated by calcium. Our results provide reliable molecular mechanisms that underlie the function of Ann5 in pollen.

Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

“…When bound to calcium, hydrophobic residues are accessible on the surface of annexin, enabling interaction with phospholipids at the membrane interface [26]. ANN5 was also shown to possess the ability to bind lipids in a calcium-dependent manner [13]. Gradual increases in calcium concentration up to 200 μM enhanced ANN5 binding to liposomes in vitro [13].…”

iTRAQ-facilitated proteomic profiling of anthers from a photosensitive male sterile mutant and wild-type cotton (Gossypium hirsutum L.)

“…The cml24 mutant displayed delayed pollen germination and inhibited pollen tube elongation, resulting in decreased seed set [69]. Other Ca 2+ binding proteins, like calreticulin [71] and annexin [72,73], participated in pollen development as well. Surprisingly, all these proteins were significantly up-regulated in MT plants ( Table 4), suggesting that Ca 2+ homeostasis was disturbed during MT pollen development.…”

Calcium is an organizer of cell polarity in plants

“…These are Ca 2+ -dependent phospholipid binding proteins that regulate secretion in animals, and possibly also in plants [30]. Consistently, overexpression of ANNEXIN5 (AnnAt5) rendered Arabidopsis pollen partially resistant to the exocytosis-inhibiting toxin brefeldin A (BFA) [31], and root hair growth is reduced in annexin1 (AnnAt1) mutants [32]. However, the function of annexins in regulating Ca 2+ -dependent secretion in plants remains to be demonstrated unequivocally.…”