Isotopic evidence for nitrogen exchange between autotrophic and heterotrophic tissues in variegated leaves
Cyril Abadie A , Marlène Lamothe-Sibold B , Françoise Gilard B and Guillaume Tcherkez A CA Research School of Biology, ANU College of Medicine, Biology and Environment, Australian National University, Canberra, ACT 2601, Australia.
B Université Paris-Sud, Institute of Plant Sciences Paris-Saclay IPS2 (Bâtiment 630), UMR CNRS-INRA 9213, Université Paris-Saclay, 91405 Orsay, France.
C Corresponding author. Email: guillaume.tcherkez@anu.edu.au
Functional Plant Biology 43(3) 298-306 https://doi.org/10.1071/FP15187
Submitted: 7 July 2015 Accepted: 12 November 2015 Published: 12 January 2016
Abstract
Many plant species or cultivars form variegated leaves in which blades are made of green and white sectors. On the one hand, there is little photosynthetic CO2 assimilation in white tissue simply because of the lack of functional chloroplasts and thus, leaf white tissue is heterotrophic and fed by photosynthates exported by leaf green tissue. On the other hand, it has been previously shown that the white tissue is enriched in nitrogenous compounds such as amino acids and polyamines, which can, in turn, be remobilised upon nitrogen deficiency. However, the origin of organic nitrogen in leaf white tissue, including the possible requirement for N-reduction in leaf green tissue before export to white tissue, has not been examined. Here, we took advantage of isotopic methods to investigate the source of nitrogen in the white tissue. A survey of natural isotope abundance (δ15N) and elemental composition (%N) in various variegated species shows no visible difference between white and green tissues, suggesting a common N source. However, there is a tendency for N-rich white tissue to be naturally 15N-enriched whereas in the model species Pelargonium × hortorum, white sectors are naturally 15N-depleted, indicating that changes in metabolic composition and/or N-partitioning may occur. Isotopic labelling with 15N-nitrate on illuminated leaf discs clearly shows that the white tissue assimilates little nitrogen and thus relies on nitrate reduction and metabolism in the green tissue. The N-sink represented by the white tissue is considerable, accounting for nearly 50% of total assimilated nitrate.
Additional keywords: isotopes, labelling, metabolism, N-assimilation, Pelargonium, variegation.
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