Seed coat suberin forms a barrier against chromium (Cr3+) during early seed germination in Arabidopsis thaliana

The Arabidopsis seed coat contains various lipid-based (suberin and cutin), carbohydrate-based (mucilage) and phenolic-based (proanthocyanidin) polymers that may influence seed permeability to various solutes. We investigated the roles of these polymers in protection against a potential soil contaminant, chromium (Cr3+), during seed germination in Arabidopsis thaliana. Wild-type and mutants impaired in one or more of these seed coat polymers were exposed to different Cr3+ concentrations (0, 50, 100, 200, 400 and 800 mg L-1) during seed stratification and early imbibition. Mutants with significant reductions in seed coat suberin were highly sensitive to Cr3+. Suberin-altered mutants demonstrated reduced seed germination with increasing Cr3+ concentrations. At the highest level of Cr3+ (800 mg L-1), mutants deficient in suberin showed reduced embryo viability, indicating that chromium was directly damaging the embryo. This was correlated with increased seed coat permeability of the suberin-deficient mutants to tetrazolium salts. A cutin mutant that is reduced in seed coat C16-C18 alpha,omega-dicarboxylic fatty acids of the polyester was not affected by Cr3+. Mutants that were strongly reduced in rhamnose, a component of rhamnogalacturonan-I (RG-I), had moderate sensitivity to Cr3+ in terms of seed germination rates. Although many proanthocyanidin mutants displayed reduced seed germination in the presence of Cr3+, the viabilities of their embryos were only slightly reduced at high Cr3+ concentrations. We speculate that Cr3+ exposure induced germination arrest in proanthocyanidin mutants, which have a defective endosperm-associated cuticle. Our findings provide evidence that various polymer types in Arabidopsis seed coat provide protection against Cr3+ by different mechanisms and degrees. Seed coat suberin was identified as the main physical barrier against Cr3+ toxicity. The results also highlight the risks associated with Cr3+ toxicity on the persistence of seeds that have relatively low seed coat suberin in soils contaminated with high levels of chromium.