Plant biology: Stigmatic ROS decide whether pollen is accepted or rejected.

A new study reports that self-incompatibility in Brassica triggers the production of stigmatic ROS that are responsible for the rejection of incompatible pollen.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization.

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Plant fertilization: bursting pollen tubes!

Higher plants don't have motile sperm; they rely on pollen tubes to deliver them. Recent research has identified key components involved in pollen tube tip bursting that allow sperm release and fertilization.

Gametophytic self-incompatibility inhibits pollen tube growth using different mechanisms.

Self-incompatibility (SI) is one of the most important mechanisms used by plants to prevent self-pollination and consequently inbreeding. It is genetically controlled by the S-locus, which allows the recognition and rejection of 'self' (S-phenotypically identical) pollen. Gametophytically controlled SI (GSI) is the most widespread SI system. To date, only two forms have been elucidated in detail at the molecular level, revealing two different stigmatic S-genes. Here we summarize the evidence for the use of two different mechanisms to inhibit incompatible pollen tube growth. Because the limited data suggest the independent evolution of these two GSI systems, it would be interesting to explore other GSI systems to determine the extent of the mechanistic diversity.

Investigating mechanisms involved in the self-incompatibility response in Papaver rhoeas.

Sexual reproduction in flowering plants is controlled by recognition mechanisms involving the male gametophyte (the pollen) and the female sporophyte (the pistil). Self-incompatibility (SI) involves the recognition and rejection of self- or incompatible pollen by the pistil. In Papaver rhoeas, SI uses a Ca(2+)-based signalling cascade triggered by the S-protein, which is encoded by the stigmatic component of the S-locus. This results in the rapid inhibition of incompatible pollen tube growth. We have identified several targets of the SI signalling cascade, including protein kinases, the actin cytoskeleton and nuclear DNA. Here, we summarize progress made on currently funded projects in our laboratory investigating some of the components targeted by SI, comprising (i) the characterization of a pollen phosphoprotein (p26) that is rapidly phosphorylated upon an incompatible SI response; (ii) the identification and characterization of a pollen mitogen-activated protein kinase (p56), which exhibits enhanced activation during SI; (iii) characterizing components involved in the reorganization and depolymerization of the actin cytoskeleton during the SI response; and (iv) investigating whether the SI response involves a programmed cell death signalling cascade.