Background
After a pollen grain lands on the stigmatic surface of the pistil (female structure of the flower), it forms a pollen tube — a long polar process that transports all of the cellular contents, including the sperm. Pollen tubes invade the pistil and migrate past several different cell types, growing between the walls of the stigma cells, traveling through the extracellular matrix of the transmitting tissue, and finally arriving at the ovary, where they migrate up the funiculus (a stalk that supports the ovule), and enter the micropyle to deliver the two sperm cells–one fertilizes an egg and other the central cell (Figure 1). Typically, only one pollen tube enters the ovule through the micropyle opening, terminates its journey within a synergid cell, and bursts to release sperm cells to form a seed. The ovary develops into a fruit to support the developing seeds. Therefore, a pollen tube’s journey (Illustration 1) to an egg cell within the pistil involves a series of cell-cell interactions such as attraction, repulsion, and adhesion.
What do we do
- One question that we are addressing in the lab: “How are pollen tubes precisely guided to their target”? We identify and characterize the guidance signals generated by the Arabidopsis thaliana (Arabidopsis) pistils to guide pollen tubes to their final target in the embryo sac
(Figure 1). - “Is the pollen tube growth vulnerable to heat stress”? is another focus of the lab. We are using tomatoes as a model to investigate how to mitigate the adverse effects of heat stress on fruit and seed yield.
Why do what we do
Primary Research benefits: Characterization of pollen tube guidance in Arabidopsis is crucial as it focuses on a process that is (i) very unique to plants, (ii) poorly understood at the molecular level, (iii) amenable to genetic, cell biological, and biochemical techniques, and (iv) a rapid way to identify novel plant signals that allow communication between cells possible despite their thick extracellular walls
Applied benefits: If pollen tube growth and guidance are defective, seeds are not produced. 80% of the world’s staple food is derived from the seeds of crop plants. So, identifying genes involved in this process will help us i). regulate inter-species hybridizations, and thereby generate novel plant hybrids, and ii) control pollen spreading from genetically modified crops–a possibility that will reassure concerned public and regulatory agencies.
Additionally, studying this process in an agriculturally essential crop such as tomato and how it is negatively affected by heat stress will help us identify heat-tolerant varieties and combat climate change.
Research
The variety and inaccessibility of pistil cells and tissues have made it challenging to characterize the dynamics of pollen tube migration and identify the guidance signals. To overcome these shortcomings, we developed an in vitro assay in the model plant Arabidopsis to study pollen tube guidance to ovules (Palanivelu and Preuss (2006); BMC Plant Biology, 6:7). We are currently using an in vitro assay (Figure 2) to isolate and characterize the signals that control each of the following three signaling events by employing a variety of traditional approaches (genetics, cell biology, biochemistry) in combination with global approaches (proteomics, microarray and metabolomics). Please refer to the indicated papers from our lab for additional details:
- Pollen tube germination and growth are modified upon growth on the female tissues (Qin et al 2009; Qin et al 2011; Johnson et al 2019),
- Unfertilized Arabidopsis ovules emit diffusible, developmentally regulated, species-specific attractants (Palanivelu and Preuss, 2006; Yetisen et al 2011; Johnson et al 2019)
- Pollen tube arrest growth in the synergid cell before releasing the two sperm cells (Nikalova et al 2007; Tsukamoto et al 2010; Liu et al 2016)
- Effect of heat stress on pollen (Sze et al 2021) and (Sze et al 2024).
Research in the Palanivelu lab is supported by funding from National Science Foundation (NSF):
IOS 0723421 – 2007-2010), (IOS 1045314 – 2010-2012), (IOS 1146090 – 2012-2017), and (IOS 1939255 – 2020-2025)