Flowering Plant Reproduction
The evolutionary success of the >350,000 flowering plant species is due to several key innovations such as evolution of the pollen grain. Pollen grains are tricellular, desiccation resistant structures that allow for sexual reproduction by transporting two sperm cells to the female gametes within the pistil of the flower. They do this by growing long pollen tubes that invasively grow through the flower ovary where they are guided to the female gametophyte housed within the ovules. For this to occur, the pollen tube must autonomously control the integrity of its cell wall through cell wall sensing proteins of the Catharanthus roseus RLK1-like (CrRLK1L) signalling pathway. Male and female-expressed CrRLK1L modules regulate growth of the pollen tube at all stages of its development, including pollen grain recognition, polytuby block, and pollen tube reception (see below).
Gametophyte interactions
The final step of the pollen tubes (male gametophyte) journey is to signal with the female gametophyte housed within the ovule before rupturing and releasing its two sperm cells to fertilize the egg cell (EC) and central cell (CC). Here, pollen tubes first arrive at the filiform apparatus of the synergid cells (SC) that mediate pollen tube rupture in a process called pollen tube reception. This process is orchestrated by paracrine signalling between CrRLK1L receptor complexes expressed on the synergid and pollen tube surfaces that regulate a myriad of intracellular responses such as cytosolic calcium signalling.
Synergid cells
Synergid cells are great for studying basic cell biology questions as they are haploid, have strong synergid-specific promoters, are polarized in their development, undergo extensive cell-cell signaling, and exhibit programmed cell death. Furthermore, the are relevant for applied research since they are gatekeepers to species-specific pollen tube guidance and sperm delivery.
Synergids as models for cell biology
Much research over the past two decades has uncovered the role of key proteins localizing all around the cell that play important functions in carrying out pollen tube reception. I am interested in using synergid cells as models for cell biology to understand how extracellular signals (from the pollen) can be recognized and transduced to elicit pollen tube rupture through the various proteins below.
Live imaging
While the process of pollen tube reception is hidden deep within the ovary, it can be imaged using the semi-in vitro fertilization assay. This involves excising ovules or the septum and placing them onto media alongside excised, pollinated stigmas, to be targeted for fertilization by the pollen tube. The optical transparency of the ovules and availability of strong synergid promoters makes this assay a powerful tool for imaging cell-cell interactions.
Future Research
Much research points to cytological and molecular parallels between the process of fungal invasion and pollen tube (PT) invasion. Recently, I demonstrated that just before sperm delivery, the pollen tube breaks through the synergid cell wall and is surrounded by a synergid-derived membrane invagination, the peritubular membrane. At a cytological scale this process is reminiscent of the the periarbuscular membrane formation during root cell invasion by arbuscular mycorrhizae. Future research will be aimed at elucidating cellular and molecular changes in the synergid during peritubular membrane formation by drawing upon the wealth of knowledge on such changes during fungal invasion.
