Update on signaling pathways regulating polarized intercellular communication in Arabidopsis reproduction

Cell polarity refers to the asymmetric distribution of cellular components, structure, and function within a cell. Most instances of cell polarity in plants involve single cells or groups of cells that are attached to neighboring cells by the cell wall. Flowering plant reproduction is unique because it involves intercellular signaling between a highly polarized cell, the tip-growing pollen tube, that must navigate its way through various sporophytic tissues of the pistil to find the ovules and deliver its sperm cell cargo to the female gametes deep within the pistil where double fertilization occurs to form the next generation (Johnson et al. 2019). The pollen tube's journey involves a series of cell polarity changes in single cells occurring over short time periods in response to signals from another cell type. These interactions occur between cells that have different ploidy and potentially different genotypes if the pollen comes from a different plant. The haploid male gametophyte, or pollen grain, is formed in the anther and is the product of meiosis followed by 2 rounds of mitosis to form the vegetative cell with 2 embedded sperm cells (Fig. 1) (Hafidh and Honys 2021). In most angiosperms, the haploid female gametophyte, also known as the embryo sac, is formed in the ovule and is the product of meiosis followed by 3 rounds of mitosis to form a 7-celled structure that contains 2 gametes, the egg and central cell, and accessory cells known as the synergids and antipodal cells (Fig. 1) (Hater et al. 2020). The goal of pollination is to deliver the 2 sperm cells from the male gametophyte to the 2 gametes in the female gametophyte for double fertilization (Johnson et al. 2019). This goal requires interactions between the pollen grain and a diploid, sporophytic stigma papilla cell, followed by interactions between the tip-growing pollen tube and the sporophytic cells of the stigma, style, transmitting tract, septum, and funiculus (Fig. 1). Finally, the pollen tube interacts with the highly polarized haploid synergid cells of the female gametophyte and gets the signal to release the sperm cells so that polar interactions can occur with the egg and central cell. These interactions between the different cell types result in changes in polarity and growth direction of the tip-growing pollen tube in response to polar signals from the maternal sporophytic and gametophytic cells. (See Box 1 for an overview of tip growth in pollen tubes.) A recurrent theme in these intercellular communication events is the perception of peptide ligands by polarly localized receptor-like kinases (RLKs) leading to rapid cellular events. At several stages of plant reproduction, the CrRLK1L family of malectin-like RLKs respond to RAPID ALKALINIZATION FACTOR (RALF) or other peptide ligands secreted from a different cell type (paracrine signaling) or from the same cell (autocrine signaling) to trigger signal transduction cascades that regulate pollen tube growth direction and integrity (Zhu et al. 2021). In this Update, we will discuss the latest advances in understanding the regulation of polarized pollen tube growth and intercellular signaling during plant reproduction, with an emphasis on research from the model plant Arabidopsis (Arabidopsis thaliana). Pollen tubes are highly polarized tip-growing cells. The pollen tube is a highly polarized tip-growing cell that needs to navigate the pistil to deliver the 2 sperm cells to the embryo sac. Pollen tube elongation requires a delicate balance of speed as pollen tubes that grow too quickly can burst from loss of cell wall integrity and could miss guidance cues secreted from the pistil, but pollen tubes that grow too slowly fail to fertilize ovules (Johnson et al. 2019). The features of pollen tube tip growth have been extensively reviewed (Mollet et al. 2013; Ge et al. 2019a; Johnson et al. 2019; Adhikari et al. 2020; Hayashi and Palmgren 2021; Ou and Yi 2022). Pollen tube elongation occurs at the apical region where exocytosis of vesicles drives plasma membrane expansion (Adhikari et al. 2020). This region is differentiated from the shank of the pollen tube by cell wall modifications including demethylesterification of pectin in the shank that strengthens the cell wall (Mollet et al. 2013; Adhikari et al. 2020). ROP1 functions as a major regulator of polarized exocytosis in pollen tubes and is activated by the RLKs PRK2/6 and ANX1/2 BUPS1/2 that bind to synergid secreted LUREs and pollen tube-secreted RALFs, respectively (Ou and Yi 2022). ROP effectors regulate polarized growth through regulation of the actin cytoskeleton, tip-focused Ca2+ oscillations, and ROS generation (Johnson et al. 2019; Ou and Yi 2022). Pollen tube Ca2+ channel mutants exhibit a range of phenotypes including altered Ca2+ oscillations, impaired pollen tube growth, and pollen tube discharge defects (Ge et al. 2019b; Johnson et al. 2019). ROS homeostasis is also important for maintaining pollen tube integrity during growth through the transmitting tract and may also contribute to pollen tube bursting in the embryo sac (Ge et al. 2019b). In addition, H+ oscillations at the pollen tube apex play a role in regulating cell wall plasticity and the actin cytoskeleton (Hayashi and Palmgren 2021). The regulation of pollen tube tip growth is crucial for reproduction as these signaling mechanisms allow for directional changes as the pollen tube perceives attractants, pauses in growth during pollen tube reception, and the eventual loss of integrity as the pollen tube bursts. The process of reproduction in Arabidopsis. The male gametophyte is pollen, which contains a vegetative cell and 2 sperm cells. The female gametophyte is a seven-celled structure called the embryo sac containing an egg cell, central cell, 2 synergids, and 3 antipodal cells. A pollen grain lands on a stigma and germinates a pollen tube carrying 2 sperm cells. The pollen tube grows into the stigma through the style and transmitting tract be