Scientists discover key regulators of seed development in a parental stalemate

The inside of two developing Arabidopsis thaliana seeds at 4 days after pollination. While the wild-type seed has not yet initiated endosperm cellularization, the seed with increased cARF activity is already completely cellularized. (Nicolas Butel photo)

(May 29, 2024) - Researchers at the Max Planck Institute of Molecular Plant Physiology have made a groundbreaking discovery about the molecular mechanisms that control seed development in flowering plants. 

This research, published in the prestigious journal Nature Plants, reveals how maternally expressed genes regulate the cellularization of the endosperm, a crucial tissue that supports the developing plant embryo and encompasses most of the worlds agricultural crop yield.

Endosperm: The Embryo's Lifeline

The endosperm is a vital tissue in seeds that provides essential nutrients to the growing plant embryo in the seed. 

During the early stages of seed development, the endosperm frequently grows without forming cell walls, creating one large cell filled with many nuclei. 

Subsequently, cell walls form around these nuclei in a process called the cellularization. This step is critical as it ensures the embryo can thrive on the endosperm's resources. If this process fails, the embryo dies.

Parental Conflict in Seed Development

Seed development is influenced by parental conflict over resource allocation to the seed. 

Female flowers are not particularly faithful and can be pollinated with pollen from many partners. Thus, the developing seeds, each holding paternal genes from different fathers but maternal genes from the same mother, compete for resources from the same mother plant. 

Paternal genes aim to maximize resource allocation towards the individual seed they inhabit, while maternal genes strive to distribute resources equally among all seeds. This is because the maternal parent is equally related to all her progeny, thus ensuring the overall survival of her offspring. 

Since delayed endosperm cellularization goes hand in hand with increased resource allocation, paternal genes delay cellularization while maternal genes promote it. The differential expression of paternal and maternal genes creates a balance that is crucial for determining seed size and viability.

A maternally expressed set of genes controls cellularization

The research team, led by Prof. Dr. Claudia Köhler at the Department Plant Reproductive Biology and Epigenetics, identified a family of genes called clustered auxin response factors (cARFs) controlling seed development. 

Lead author of this study Dr. Nicolas Butel found that these cARFs are expressed only from the maternal genome and play a key role in triggering endosperm cellularization. By comparing seeds that had undergone cellularization with those that had not, the team pinpointed these cARFs as the critical factors initiating the process.

The researchers demonstrated that these cARFs are active just before cellularization, promoting this essential process. Initial data indicate that this set of genes may counter a paternally controlled hormone pathway in the seed, which promotes endosperm proliferation.

Controlling cellularization will help to control seed size and crop yield

Most of the agricultural yield worldwide consists of seeds. Understanding how and when endosperm cellularization happens has significant implications for agriculture, as the timing of this process affects seed size and viability. 

If cellularization happens too early, seeds may be too small; if it happens too late, seeds might be larger but less viable. By manipulating cARF expression, breeders could potentially enhance seed size and improve crop yields, offering exciting new possibilities for plant breeding.

This new finding represents a major advance in plant science, shedding light on the complex genetic interactions that govern seed development. It opens up new avenues for improving crop productivity by targeting these molecular mechanisms and holds promise for the future of plant breeding, with the potential to significantly impact global food security.