CSB Seminar Series – Ravishankar Palanivelu (University of Arizona) – Pollen tube growth: critical link in stress tolerance and evolution of angiosperm reproduction

Prof Ravishankar Palanivelu
School of Plant Sciences
University of Arizona

Pollen tube growth: critical link in stress tolerance and evolution of angiosperm reproduction

Sexual reproduction in flowering plants relies on cell-cell interactions between the pollen grain and the pistil, the female reproductive organ. After pollination, the pollen grain develops a pollen tube to transport the two immotile sperm cells through the pistil tissues to fertilize the female gametes contained within a female gametophye in an ovule. The first direct contact between the male gametophyte and the female gametophyte occurs during pollen tube reception, which involves pollen tube growth arrest and release of sperm cells to affect fertilization and seed formation (Johnson et al., 2019). My lab identified and characterized the role of LORELEI (LRE), a glycosylphosphatidylinositol (GPI)-anchored membrane protein, in Arabidopsis pollen tube reception and how it functions with its co-receptor FERONIA(FER) inpermitting the release of sperm cells for fertilization (Tsukamoto et al., 2010 and Liu et al., 2016). In addition to reproduction, variants of this signaling complex mediate diverse processes ranging from root hair growth, hypocotyl elongation, and plant pathogen-host interactions. These studies have shown that the core signaling complex in A. thaliana is composed of a GPI-anchored membrane protein (GAP), a receptorlike kinase (RLK), and a small cysteine-rich peptide (CRP). In A. thaliana, each protein in the tripartite signaling complex are represented by 4, 17, and 33 gene family members, respectively, yielding potentially 2,244 unique combinations of the complex. The small LRE gene family presented an opportunity to explore the conservations of molecular functions within the family, the evolutionary history of the family in land plants, and the mechanisms that explain the functional diversification of this signaling complex.
In this seminar, I will present the evidence which showed that expression divergence of LLGs (regulatory subfunctionalization), rather than functional divergence, explains the retention of paralogs in Brassicales. All but one extant monocot and eudicot species examined maintained an LLG copy with preferential expression in male reproductive tissues, with the other duplicate copies showed highest levels of expression in female or vegetative tissues. Interestingly, the single LLG copy in Amborella (sister to all other angiosperms) is expressed vastly higher in male compared to female reproductive or vegetative tissues. Reconstruction of expression evolution showed that the highest inferred expression levels for the single copy ancestral angiosperm LLG was in male reproductive tissues. Because Arabidopsis LLG2 and LLG3 are critical for pollen tube growth through the transmitting tract, we propose that expression of LLGs in pollen and pollen tubes underlie the key evolutionary innovation of rapid pollen tube growth in the closed carpel of angiosperms.
Pollen tube growth is also critical for heat stress tolerance, as at elevated temperatures, pollen tubes fail to grow through the pistil and deliver sperm cells for fertilization and seed formation. Our central hypothesis is that pollen tubes are uniquely sensitive to heat stress, but that thermotolerant variants have been selected that facilitate reproductive success at high temperature. An update will be provided on our progress in using genomics to define these molecular adaptations and determine whether they are sufficient to enhance fruit production of thermosensitive cultivars at high temperature.

HOST: Daphne Goring

Zoom ID and passcode:

https://utoronto.zoom.us/j/87447422547

Meeting ID: 874 4742 2547

Passcode: 050579