Extended Data Fig. 9: A summary of FER-regulated pollen tube–ovule interaction.

The diagram summarizes previously published results on how FER mediates pollen tube rupture and sperm release to enable fertilization⁴ (steps 2 and 3 in the scheme), and results reported here on how FER affects two interconnected conditions at the filiform apparatus to prevent the penetration of ovules by supernumerary pollen tubes (steps 4 and 5 in the scheme). Solid lines reflect information that comes directly from experimental data; dashed lines reflect extrapolations from experimental data and other relevant information. Results present in this Article show that FER maintains a pistillate environment enriched in de-esterified pectin (Fig. 3a) (step 1 in the scheme), including at the filiform apparatus of the female gametophyte (Fig. 1e, f). Pollen tubes are attracted by female guidance cues to depart from their main growth axis to approach ovules, and are then guided by synergid-cell-produced chemoattractants (for example, LUREs) to the ovules to penetrate the female gametophyte^1,5,6 (Fig. 1b,c, Extended Data Fig. 1b). In penetrating the filiform apparatus that is enriched in de-esterified pectin, the pollen tube that arrives first should continue to secrete cell-wall degradative enzymes—presumably producing pectic fragments in the vicinity, just as growth in the transmitting track produced these polymers (Fig. 3a, Extended Data Fig. 4). The application of pollinated pistillate exudate enriched in de-esterified pectin (Fig. 3b) and fragmented, commercially obtained de-esterified pectin (Fig. 3c, d) to ovules from unpollinated pistils, and the arrival of pollen tubes at ovules in pollinated pistils (Fig. 2a, b) (step 4 in the scheme), triggered NO accumulation at the filiform apparatus. Taken together with the observation that NO-deficient ovules also had elevated levels of multiple pollen tube entrance (Fig. 2e), these results are consistent with pectic fragments generated by pollen tube growth acting as a mediator to trigger NO accumulation to prevent the entrance of supernumerary pollen tubes (step 4 in the scheme). Our results demonstrate that NO does so by modifying LURE and disengaging LURE-mediated attraction of late-arriving pollen tubes to already penetrated ovules (steps 4 and 5 in the scheme) (Fig. 4, Extended Data Figs. 6, 7). Previously published results⁴, and the properties of de-esterified pectin, are consistent with an altered ovular environment before and after pollen tube rupture. This could be achieved (for example) by Ca²⁺ leakage from the degenerating synergid cell and Ca²⁺ released from the ruptured pollen tube. The elevated extracellular [Ca²⁺] would immediately mediate stiffening of the filiform apparatus that is rich in de-esterified pectin. The stiffened cell wall could then present a back-up blockade to prevent entrance by an errant, late-arriving pollen tube into an already penetrated female gametophyte (step 6 in the scheme).