Review: Nutrient loading of developing seeds

Abstract

Abstract. Interest in nutrient loading of seeds is fuelled by its central importance to plant reproductive success and human nutrition. Rates of nutrient loading, imported through the phloem, are regulated by transport and transfer processes located in sources (leaves, stems, reproductive structures), phloem pathway and seed sinks. During the early phases of seed development, most control is likely to be imposed by a low conductive pathway of differentiating phloem cells serving developing seeds. Following the onset of storage product accumulation by seeds, and, depending on nutrient species, dominance of path control gives way to regulation by processes located in sources (nitrogen, sulfur, minor minerals), phloem path (transition elements) or seed sinks (sugars and major mineral elements, such as potassium). Nutrients and accompanying water are imported into maternal seed tissues and unloaded from the conducting sieve elements into an extensive post-phloem symplasmic domain. Nutrients are released from this symplasmic domain into the seed apoplasm by poorly understood membrane transport mechanisms. As seed development progresses, increasing volumes of imported phloem water are recycled back to the parent plant by process(es) yet to be discovered. However, aquaporins concentrated in vascular and surrounding parenchyma cells of legume seed coats could provide a gated pathway of water movement in these tissues. Filial cells, abutting the maternal tissues, take up nutrients from the seed apoplasm by membrane proteins that include sucrose and amino acid/H + symporters functioning in parallel with non-selective cation channels. Filial demand for nutrients, that comprise the major osmotic species, is integrated with their release and phloem import by a turgorhomeostat mechanism located in maternal seed tissues. It is speculated that turgors of maternal unloading cells are sensed by the cytoskeleton and transduced by calcium signalling cascades.

Moving micronutrients from the soil to the seeds: Genes and physiological processes from a biofortification perspective

“…Even if minerals have arrived at the seed covering tissues, physical barriers between the maternal plant and developing seed and lack of transpirational tension will necessitate phloem transport to the seed tissues [126,127]. Experimental results support the idea that micronutrients pass through pods [18] or glumes [128] prior to transport to seeds.…”

“…The unloading of these nutrients at the seed decreases the pressure and maintains a gradient. Micronutrients will not be present in high enough concentrations to create a pressure gradient on their own, and thus will move with the major nutrients-that is, K + , Cl − , and sugars [126,157] in the phloem. The alkaline phloem sap, at pH 7.2-8.2 [157], necessitates chelation of Fe, which decreases in solubility as pH rises [158].…”

“…tissues to the sink (seed) tissues [126]. The pressure gradient is generated by loading nutrients such as carbohydrates and potassium into phloem at source tissues to provide osmotic potential that draws in water.…”

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…Importantly, sucrose creates the driving force for long-distance translocation of all other compounds in the phloem. Sucrose is imported into the developing embryo by plasma membrane SUT sucrose/proton cotransporters (Patrick and Offler, 1995;Baud et al, 2005;Zhang et al, 2007).…”

Gradual Accumulation of Heavy Metals in an Industrial Wheat Crop from Uranium Mine Soil and the Potential Use of the Herbage

“…Flag leaves with internodal sheaths and glumes were the sink tissues with the highest mineral concentrations (Tables 3 and 4; Figures 2 and 6). Sucrose and potassium are the major osmotic species [93] to move the phloem sap via differences in the hydrostatic pressure from the donor tissues to the grains [94].…”