Tannic cell walls form a continuous apoplastic barrier sustaining Arabidopsis seed coat biophysical properties

Seeds are a late land plant evolution innovation that promoted the striking spread and diversity of angiosperms. The seed coat is a specialized dead tissue protecting the plant embryo from mechanical damage. In many species, including Arabidopsis thaliana, the seed coat also achieves a remarkable balancing act: it limits oxygen uptake, avoiding premature embryo oxidative damage, but not entirely so as to enable seed dormancy release. The seed coat biophysical features implementing the striking physiological properties of the seed remain poorly understood. Tannins, a type of flavonoids, are antioxidants known to accumulate in the Arabidopsis seed coat and transparent testa (tt) mutant seeds, deficient in flavonoid synthesis, exhibit low dormancy and viability. However, their precise contribution to seed coat architecture and biophysics remains evasive. A seed coat cuticle, covering the endosperm outer surface was, intriguingly, previously shown to be more permeable in tt mutants deficient not in cuticular component synthesis, but rather in flavonoid synthesis. Investigating the role of flavonoids in cuticle permeability led us to identify cell walls, originating from the seed coat inner integument 1 cells, impregnated with tannins. We found that tannic cell walls are tightly associated with the cuticle, forming two fused layers that regulate endosperm permeability. In addition, we show that tannic cell walls are prominent building blocks of the seed coat, constituting a continuous barrier around the seed living tissues. Altogether our findings reveal the existence of tannic cell walls as a previously unrecognized biological barrier sustaining the seeds key physiological properties. One sentence summaryThe seed coat is largely composed of plant cell walls impregnated with tannins, forming a thick and continuous protective barrier surrounding the embryo promoting seed viability and dormancy.