AP1G mediates vacuolar acidification during synergid-controlled pollen tube reception

Abstract

Double fertilization in angiosperms requires the delivery of immotile sperm through pollen tubes, which enter embryo sacs to initiate synergid degeneration and to discharge. This fascinating process, called pollen tube reception, involves extensive communications between pollen tubes and synergids, within which few intracellular regulators involved have been revealed. Here, we report that vacuolar acidification in synergids mediated by AP1G and V-ATPases might be critical for pollen tube reception. Functional loss of AP1G or VHA-A, encoding the γ subunit of adaptor protein 1 or the shared component of two endomembrane V-ATPases, respectively, impaired synergid-controlled pollen tube reception and caused partial female sterility. AP1G works in parallel to the plasma membrane-associated receptor FERONIA in synergids, suggesting that synergid-mediated pollen tube reception requires proper sorting of vacuolar cargos by AP1G. Although AP1G did not mediate the targeting of V-ATPases, AP1G loss of function or the expression of AP1G-RNAi compromised vacuolar acidification mediated by V-ATPases, implying their genetic interaction. We propose that vacuolar acidification might represent a distinct cell-death mechanism specifically adopted by the plant phylum, which is critical for synergid degeneration during pollen tube reception. . The female gametophyte (FG), i.e., the embryo sac, is formed inside the ovules and contains an egg cell, a central cell, two synergid cells, and three antipodal cells (3). Upon landing on a receptive pistil, a pollen grain forms a long cylindrical extension, called a pollen tube, which extends inside female sporophytic tissues. To deliver immotile sperm, pollen tubes perceive attractive signals sent out by the FG, change their growth axis, and finally enter the embryo sacs through the micropyle (1-4). A process called "pollen tube reception" instructs the cessation and discharge of the penetrating pollen tube, leading to sperm release and fertilization (1-4).Studies have uncovered key female factors controlling pollen tube reception, including FERONIA (FER) (5-7), LORELEI (LRE) (8, 9), and early nodulin-like proteins (ENODLs) (10) and NORTIA (NTA) (11). These synergid receptors likely operate in the same genetic pathway (10-12) whose functional loss caused the failure of pollen tube discharge and led to reduced female fertility (5-9, 11). Male ligands are yet to be identified. However, recent studies showed that the production of reactive oxygen species (ROS) and Ca 2+ spiking are downstream events of FER-controlled pollen tube reception (13-16).Synergid degeneration, a form of programmed cell death (PCD), is a key step during pollen tube reception. Between the two synergid cells, a receptive synergid cell is the one that succeeds in interacting with the pollen tube and induces it to burst and undergoes cell death first. The other synergid cell that continues to persist and then undergoes cell death orchestrated by the fertilized egg cell and the central cell is the persistent synergid (3,11,1...

Arabidopsis adaptor protein 1G is critical for pollen development

“…Adaptor protein‐1 (AP‐1) is a heterotetrameric protein complex regulating protein sorting at the trans ‐Golgi network/early endosome (TGN/EE) (Park et al ; Teh et al ; Wang et al ), the convergent endomembrane compartment for anterograde and retrograde trafficking pathways (Viotti et al ). We recently reported that the ϒ subunit of AP‐1 (AP1G) is critical for synergid‐controlled pollen tube reception (Wang et al ). AP1G is encoded by two homologous genes, AP1G1 and AP1G2 , which are expressed constitutively, including male and female reproductive cells (Wang et al ).…”

“…We recently reported that the ϒ subunit of AP‐1 (AP1G) is critical for synergid‐controlled pollen tube reception (Wang et al ). AP1G is encoded by two homologous genes, AP1G1 and AP1G2 , which are expressed constitutively, including male and female reproductive cells (Wang et al ). Synergid‐expressed AP1G controls pollen tube reception by mediating the degeneration of the receptive synergid (Wang et al ).…”

“…AP1G is encoded by two homologous genes, AP1G1 and AP1G2 , which are expressed constitutively, including male and female reproductive cells (Wang et al ). Synergid‐expressed AP1G controls pollen tube reception by mediating the degeneration of the receptive synergid (Wang et al ). AP1G loss‐of‐function also resulted in complete male gametophytic lethality (Wang et al ), the molecular mechanism underlying of which is obscure.…”

“…Synergid‐expressed AP1G controls pollen tube reception by mediating the degeneration of the receptive synergid (Wang et al ). AP1G loss‐of‐function also resulted in complete male gametophytic lethality (Wang et al ), the molecular mechanism underlying of which is obscure. Here, we demonstrated that male lethality of AP1G loss‐of‐function accompanies defective vacuolar dynamics and acidification during pollen development.…”

“…To determine the cause of male gametophytic lethality of ap1g (Wang et al ), we first analyzed pollen development of the trans‐heterozygous ap1g1 /+ g2 mutant, which did not show vegetative growth defects as ap1g1 g2 /+ (Wang et al ). By scanning electron microscopy (SEM), half the pollen produced from ap1g1 /+ g2 was shown to be either wrinkled or irregularly shaped (Figure C, D, M), in contrast to those of wild type (Figure A, B).…”