ULTRASTRUCTURE OF THE ROOT HAIR TIP AND OVULE IN THE ARABIDOPSIS THALIANA FER-4 MUTANT

Abstract

The FERONIA (FER) receptor kinase is crucial for multiple aspects of plant growth and survival, extending beyond its initially identified role in female fertility in Arabidopsis thaliana. FER is a transmembrane receptor that acts with co-receptors, peptides and ligands in various cell types, forming signaling networks that influence multiple plant growth regulatory pathways. For example, FER collaborates with the LORELEI and NORTIA proteins to facilitate the communication between male and female gametophytes in the ovule during fertilization. FER also supports polarized cell growth in root hair development by partnering with RALF peptides and auxin to stimulate root hair elongation. Because FER regulates cell growth and cell wall integrity, we hypothesized that ultrastructure of the root hair tip and ovule are altered in a loss-of-function mutant, fer-4. I imaged the root hairs tips of both the mutant and wild type under different growth conditions (with and without sucrose) by scanning electron microscopy (SEM). We hypothesized that that the presence of sucrose in the growth media could cause variation in the phenotypes. I found that root hairs tips of fer-4 mutants were bulbous and exhibited various abnormalities, including bursting at the tips, stunted growth, and collapse upon emergence. In both WT and the fer-4 mutant, sucrose caused a distorted appearance of the root hairs. To test for cell wall differences in the ovule, I used transmission electron microscopy (TEM) to examine all surrounding tissues and cells in high-pressure frozen/freeze substituted FER mutant ovules and compared them to wildtype ovules. I found that cells of the fer-4 mutant embryo sac and surrounding tissues had defective cell walls compared to the wild type. Specifically, cell walls between the synergids and projections of the filiform apparatus lacked uniformity, and the surrounding integument walls were convoluted. Additionally, walls of the embryo sac and those surrounding the antipodals were thinner in fer-4 mutants. This research highlights the essential function of FER in maintaining cell wall integrity and its broader implications for plant development, providing useful insights for future studies on improving plant adaptability and growth.