Breaking down walls: cell elimination at the origin of seeds

The evolution of the seed habit can be traced back to a change in the cell fate of the nucellus, the sporophytic tissue responsible for female meiosis. Seeds originated when plants began retaining their female spores within the maternal tissue, the nucellus, instead of releasing them into the environment. This evolutionary innovation required the elimination of part of the nucellus to make room for the growth of the female gametophyte and, in angiosperms, the endosperm, the storage tissue that nourishes the embryo. Unlike most programmed cell death processes, which preserve the cell wall, this elimination is accompanied by the apparent dismantling of the cell wall. Until now, this process had only been described at the morphological level, while its molecular basis remained unknown.

In the model plant Arabidopsis, nucellus elimination begins with a modification of the cell wall: the de-methylesterification of homogalacturonan, the major cell wall pectin. This is followed by pectin degradation by pectate lyases and by fragmentation of nuclear DNA, a hallmark of programmed cell death. Blocking pectin de-methylesterification is sufficient to halt nucellus elimination, demonstrating a causal role for pectin status in determining cell fate. The transcription factor TRANSPARENT TESTA 16 (TT16), which emerged with seed plants, controls this process and likely contributed to the evolution of the seed. These findings reveal a convergence between animals and plants in the structure and function of extracellular polysaccharides. They highlight the essential role of these polysaccharides in the development of multicellular organisms. This work contributes to the INRAE objective "Building the quality of dietary patterns" and advances our understanding of biological mechanisms relevant to crop improvement. 

By modulating the degree of pectin methylesterification, it may become possible to control the rate of nucellus elimination. Because nucellus development influences grain size, these findings open new avenues for improving the yield of cereals and other seed crops. The same strategy could also be used to modulate the relative proportions of the nucellus and the endosperm according to the species, thereby tailoring seed composition to achieve desired quality traits.


Research developed at the Institute Jean-Pierre Bourgin for Plant Sciences in collaboration.