Divergent metabolome and proteome suggest functional independence of dual phloem transport systems in cucurbits

• Published in: Proceedings of the National Academy of Sciences - PNAS 2010-07, Vol.107 (30), p.13532-13537
• Main Authors: Zhang, Baichen, Tolstikov, Vladimir, Turnbull, Colin, Hicks, Leslie M., Fiehn, Oliver, Zambryski, Patricia C.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Biological Sciences; Biological Transport; Carbohydrates - analysis; Chromatography, Liquid; Citrullus - anatomy & histology; Citrullus - metabolism; Cucumis sativus - anatomy & histology; Cucumis sativus - metabolism; Cucurbita - anatomy & histology; Cucurbita - metabolism; Cucurbitaceae - classification; Cucurbitaceae - genetics; Cucurbitaceae - metabolism; Exudation; Flowers & plants; Gels; Mass Spectrometry; Metabolites; Metabolome; Microscopy, Fluorescence - methods; Models, Anatomic; Models, Biological; Molecular Sequence Data; Petioles; Phloem; Phloem - anatomy & histology; Phloem - metabolism; Physiology; Plant Exudates - metabolism; Plant Proteins - analysis; Plant Proteins - genetics; Plant Proteins - metabolism; Plant Stems - anatomy & histology; Plant Stems - metabolism; Plants; Proteins; Proteome - analysis; Proteome - genetics; Proteome - metabolism; Proteomes; Proteomics; Ribonucleic acid; RNA; Sequence Homology, Amino Acid; Sieve elements; Sieve tubes; Species Specificity; Sugars; Tissues; Vascular bundles
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0910558107

Cucurbitaceous plants (cucurbits) have long been preferred models for studying phloem physiology. However, these species are unusual in that they possess two different phloem systems, one within the main vascular bundles [fascicular phloem (FP)] and another peripheral to the vascular bundles and scattered through stem and petiole cortex tissues [extrafascicular phloem (EFP)]. We have revisited the assumption that the sap released after shoot incision originates from the FP, and also investigated the long-standing question of why the sugar content of this sap is ∼30-fold less than predicted for requirements of photosynthate delivery. Video microscopy and phloem labeling experiments unexpectedly reveal that FP very quickly becomes blocked upon cutting, whereas the extrafascicular phloem bleeds for extended periods. Thus, all cucurbit phloem sap studies to date have reported metabolite, protein, and RNA composition and transport in the relatively minor extrafascicular sieve tubes. Using tissue dissection and direct sampling of sieve tube contents, we show that FP in fact does contain up to 1 M sugars, in contrast to low-millimolar levels in the EFP. Moreover, major phloem proteins in sieve tubes of FP differ from those that predominate in the extrafascicular sap, and include several previously uncharacterized proteins with little or no homology to databases. The overall compositional differences of the two phloem systems strongly indicate functional isolation. On this basis, we propose that the fascicular phloem is largely responsible for sugar transport, whereas the extrafascicular phloem may function in signaling, defense, and transport of other metabolites.