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Metal Transport in the Developing Plant Seed

Healthy plant growth depends on a balanced metal homeostasis at the organ, tissue and sub-cellular levels, which is mediated principally by plasma and vacuolar membrane metal transporters. The genetic bases of metal acquisition in developing seeds has long remained poorly understood. Recent technical advances have helped circumvent the difficulties of conducting metal nutrient research on the extremely small seeds of Arabidopsis thaliana. The review presents recent advances in our understanding of seed metal homeostasis focussing on this model plant. Metals are loaded from phloem to the seed coat and must pass through the endosperm to reach the embryo. The embryo comprises several apoplastic and symplastic pathways that strictly depend on the changing physiology of the developing seed organs. Metals that reach the developing embryo fuel immediate cellular processes or accumulate in vacuoles to support forthcoming germination. In the mature embryo, metal distribution is homogeneous, with the exception of iron and manganese which localize to distinct cell layers. These metal localizations are strictly dependent on expression of specific tonoplast transporters, with putative functions that go beyond the storage of metals. Accumulating evidence indicates that they can control the timing of metal entry into the embryo.