Figure 1
Schematic diagram of the role of calcium ions in self-incompatibility
(A) The role of Ca2+ in gametophytic self-incompatibility (GSI) in Papaveraceae. The incompatible pollen tube influxes a lot of extracellular Ca2+, which destroys the calcium ion gradient at the tip of the pollen tube. However, the Ca2+ gradient at the tip of the compatible pollen tube was not affected. The green solid arrow is Ca2+ flow direction. PT – pollen tube; PrpS – P. rhoeas pollen S determinant; PrsS – P. rhoeas stigma S. (B) The role of Ca2+ in the sporophytic self-incompatibility (SSI) in the Brassicaceae. ACA13 promotes extracellular Ca2+ into the compatible pollen tube. When incompatible pollination occurs, GLR regulates extra-cellular Ca2+ into stigma papilla cells. ACA13 – autoinhibited Ca2+-ATPase13; GLR – glutamate receptor-like channel
Figure 2
Schematic diagram of calcium ion signal transduction process in S-RNase-based gametophytic self-incompatibility response
(A) When compatible pollination occurs, the Ca2+ gradient in the pollen tube is not affected and promotes the transport of the vesicles to the tip. These vesicles contain S-RNase, and when the vesicles are fused with the tip cell membrane, the S-RNase is discharged outside the pollen tube so that the RNA cannot be degraded. (B) When incompatible pollination occurs, the activity of the tip Ca2+ channel in the pollen tube is inhibited and the tip Ca2+ gradient is destroyed. When the Ca2+ gradient is disrupted, the vesicles cannot be transported to the tip and rupture within the pollen tube, releasing S-RNase, which degrades the RNA when the concentration of S-RNase accumulates in the pollen tube to a certain level
| ABP | actin binding protein |
| ACA13 | autoinhibited Ca2+-ATPase13 |
| CaM | calmodulin |
| CBL | calcineurin B-like protein |
| CDPK | calcium-dependent protein kinase |
| CIPK | CBL-interacting protein kinase |
| CML | CaM-like |
| CRT | calreticulin gene |
| GLR | glutamate receptor-like channel |
| GSI | gametophytic self-incompatibility |
| MAPK | mitogen-activated protein kinase |
| PrpS | Papaver rhoeas stigma S determinant |
| PrsS | Papaver rhoeas stigma S |
| ROS | reactive oxygen species |
| SCR | S-locus cysteine-rich protein |
| SFB/SLF | S-haplotype-specific F-box/S-locus F-box |
| SI | self-incompatibility |
| SP11 | S-locus protein 11 |
| SRK | S-locus receptor kinase |
| SSI | sporophytic self-incompatibility |