The plant life cycle involves an alternation of generations between sporophyte and gametophyte. Currently, the genes and pathways involved in gametophytic development and function in flowering plants remain largely unknown. A large-scale mutant screen of Ds transposon insertion lines was employed to identify 130 mutants of Arabidopsis thaliana with defects in female gametophyte development and function. A wide variety of mutant phenotypes were observed, ranging from defects in different stages of early embryo sac development to mutants with apparently normal embryo sacs, but exhibiting defects in processes such as pollen tube guidance, fertilization or early embryo development. Unexpectedly, nearly half of the mutants isolated in this study were found to be primarily defective in post-fertilization processes dependent on the maternal allele, suggesting that genes expressed from the female gametophyte or the maternal genome play a major role in the early development of plant embryos. Sequence identification of the genes disrupted in the mutants revealed genes involved in protein degradation, cell death, signal transduction and transcriptional regulation required for embryo sac development, fertilization and early embryogenesis. These results provide a first comprehensive overview of the genes and gene products involved in female gametophyte development and function within a flowering plant.

Sperm delivery for double fertilization of flowering plants relies on interactions between the pollen tube (PT) and two synergids, leading to programmed cell death (PCD) of the PT and one synergid. The mechanisms underlying the communication among these cells during PT reception is unknown. We discovered that the synergids control this process by coordinating their distinct calcium signatures in response to the calcium dynamics and growth behavior of the PT. Induced and spontaneous aberrant calcium responses in the synergids abolish the two coordinated PCD events. Components of the FERONIA (FER) signaling pathway are required for initiating and modulating these calcium responses and for coupling the PCD events. Intriguingly, the calcium signatures are interchangeable between the two synergids, implying that their fates of death and survival are determined by reversible interactions with the PT. Thus, complex intercellular interactions involving a receptor kinase pathway and calcium-mediated signaling control sperm delivery in plants.

Parasitic plants in the Orobanchaceae develop haustoria in response to contact with host roots or chemical haustoriainducing factors. Experiments in this manuscript test the hypothesis that quinolic-inducing factors activate haustorium development via a signal mechanism initiated by redox cycling between quinone and hydroquinone states. Two cDNAs were previously isolated from roots of the parasitic plant Triphysaria versicolor that encode distinct quinone oxidoreductases. QR1 encodes a single-electron reducing NADPH quinone oxidoreductase similar to z-crystallin. The QR2 enzyme catalyzes two electron reductions typical of xenobiotic detoxification. QR1 and QR2 transcripts are upregulated in a primary response to chemical-inducing factors, but only QR1 was upregulated in response to host roots. RNA interference technology was used to reduce QR1 and QR2 transcripts in Triphysaria roots that were evaluated for their ability to form haustoria. There was a significant decrease in haustorium development in roots silenced for QR1 but not in roots silenced for QR2. The infrequent QR1 transgenic roots that did develop haustoria had levels of QR1 similar to those of nontransgenic roots. These experiments implicate QR1 as one of the earliest genes on the haustorium signal transduction pathway, encoding a quinone oxidoreductase necessary for the redox bioactivation of haustorial inducing factors.

The directional growth of the pollen tube from the stigma to the embryo sac in the ovules is regulated by pollen-pistil interactions based on intercellular communication. Although pollen tube growth is regulated by the cytoplasmic Ca2+ concentration ([Ca2+]cyt), it is not known whether [Ca2+]cyt is involved in pollen tube guidance and reception. Using Arabidopsis expressing the GFP-based Ca2+-sensor yellow cameleon 3.60 (YC3.60) in pollen tubes and synergid cells, we monitored Ca2+ dynamics in these cells during pollen tube guidance and reception under semi-in vivo fertilization conditions. In the pollen tube growing towards the micropyle, pollen tubes initiated turning within 150 μm of the micropylar opening; the [Ca2+]cyt in these pollen tube tips was higher than in those not growing towards an ovule in assays with myb98 mutant ovules, in which pollen tube guidance is disrupted. These results suggest that attractants secreted from the ovules affect Ca2+ dynamics in the pollen tube. [Ca2+]cyt in synergid cells did not change when the pollen tube grew towards the micropyle or entered the ovule. Upon pollen tube arrival at the synergid cell, however, [Ca2+]cyt oscillation began at the micropylar pole of the synergid, spreading towards the chalazal pole. Finally, [Ca2+]cyt in the synergid cell reached a maximum at pollen tube rupture. These results suggest that signals from the pollen tube induce Ca2+ oscillations in synergid cells, and that this Ca2+ oscillation is involved in the interaction between the pollen tube and synergid cell.