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Functional Plant Biology Functional Plant Biology Society
Plant function and evolutionary biology
REVIEW

Nutrient loading of developing seeds

Wen-Hao Zhang A , Yuchan Zhou B E , Katherine E. Dibley B E , Stephen D. Tyerman C , Robert T. Furbank D and John W. Patrick B F
+ Author Affiliations
- Author Affiliations

A Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China.

B School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2238, Australia.

C School of Agriculture, Food and Wine, Adelaide University, Waite Campus, PMB #1, Glen Osmond, SA 5064, Australia.

D CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.

E These authors contributed equally to this work.

F Corresponding author. Email: john.patrick@newcastle.edu.au

G This paper originates from an International Symposium in Memory of Vincent R. Franceschi, Washington State University, Pullman, Washington, USA, June 2006.

Functional Plant Biology 34(4) 314-331 https://doi.org/10.1071/FP06271
Submitted: 26 October 2006  Accepted: 30 January 2007   Published: 19 April 2007

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 turgor-homeostat 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.

Additional keywords: membrane transport, nutrients, phloem transport, remobilisation, seeds, symplasmic transport.


Acknowledgements

This review is dedicated to the memory of Vincent Franceschi, a great friend and colleague who contributed significantly to conceptual advances in understanding nutrient loading of seeds. In particular, he made seminal findings as to the role paraveinal mesophyll play in assimilate partitioning and compartmentation (1983–2000). By a generous sharing of his immense intellect and innovative use of imaging and microtechniques, Vince has provided a continuing legacy that inspires and underpins efforts to discover mechanisms regulating nutrient transport to and within developing seeds. Studies reported from the authors’ laboratories were supported by grants from the Australian Research Council, Natural Science Foundation of China (30570136) and Grain Research and Development Corporation.


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