FLOE genes

Most plant seeds reach a quiescent dry state that allows them to remain viable under harsh conditions for decades to millennia. They do so by accumulating protective molecules and changing the biophysical properties of their cytoplasm from a fluid to a glassy state, stabilizing its components and halting metabolism. Upon water uptake, seeds regain their fluid state and resume metabolism to kickstart germination. Yet, seeds should only germinate when enough water is around. Germinating too early, and the seedling may die; too late and it may be outcom-peted by its peers. This presents an exceptional challenge to the dormant embryo: it needs to predict the future availability of water and act accordingly—germinate when plenty of water is around, stay dormant when conditions are limiting. Despite the ecological and agronomical importance of seed germination, it remained largely elusive how seeds pull this off.

Given their exquisite sensitivity the chemical environment, we hypothesized that disordered proteins could play a role in the sensing of the transition from the dry to wet states. We identified a previously uncharacterized disordered protein that was expressed in a seed-specific manner in. This protein, which we named FLOE1, could undergo a surprising hydration-dependent phase transition: In the desiccated embryo, FLOE1 is diffusely localized to the cytoplasm, but it spontaneously forms condensates upon hydration of the embryo (Dorone and Boeynaems et al., 2021). Importantly, this process is fully reversible as seeds that are re-dried or incubated in high salt solutions after imbibition do not exhibit any condensates unless they are exposed to water once again. Hence, these FLOE1 condensates are exquisitely sensitive to the water potential.

While the FLOE1 protein seems to sense the water potential, it was still unclear if this function would affect germination. Testing germination of FLOE1 knock-out and overexpression lines, showed that the protein acts as a dose-dependent inhibitor on germination under water stress. In other words, seeds lacking FLOE1 will germinate prematurely at water potentials that may not suffice to support the seedling, while overexpression lines require higher water potentials for germination. Nonetheless, the question remained whether this functional effect relates to its phase separating properties. By generating a mutant form of FLOE1 that lost its hydration-dependency (i.e., was always in a condensed state), we found that these lines had aberrant germination responses. These findings convincingly demonstrated that the reversibility of FLOE1 phase separation was absolutely critical for its function.

Importantly, we found intragenic, intraspecific, and interspecific natural variation in FLOE1 ex-pression and phase separation and showed that intragenic variation is associated with adaptive germination strategies in natural populations. Moreover, the FLOE gene family is conserved across the green plant lineage, highlighting the wealth of FLOE variation plants can make use of with potential applications for crop design and agriculture in times of climate change.

Fun Fact: FLOE1 was named after the second movement Floe in the Glassworks composition by Philip Glass. A “floe” or ice sheet is a phase separated body of water, and plant seeds undergo vitrification to form biological glasses.

Publications

Dorone Y*, Boeynaems S*, Jin B, Bossi F, Flores E, Lazarus E, Michiels E, De Decker M, Baatsen P, Holehouse AS, Sukenik S, Gitler AD, Rhee SY. A prion-like protein regulator of seed germination undergoes hydration-dependent phase separation. Cell 2021, vol. 184(16), p. 4284-4298. co-first
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