The Mechanism of Boron Mobility in Wheat and Canola Phloem    

Abstract

Low-molecular-weight borate complexes were isolated from canola (Brassica napus) and wheat (Triticum aestivum) phloem exudates, as well as the cytoplasm of the fresh-water alga Chara corallina, and identified using matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry. Phloem exudate was collected from field-grown canola inflorescence stalks by shallow incision, while wheat phloem exudate was collected by aphid stylectomy. Chara cytoplasm was collected by careful manual separation of the cell wall, vacuole, and cytosolic compartments. Matrix-assisted laser desorption/ionization time-offlight mass spectrometry showed the presence of isotopic borate complexes, at mass-to-charge ratio of 690.22/691.22 in the canola and wheat phloem and at 300.11/301.11 in canola phloem and Chara cytoplasm. Using reference compounds, the borate complexes with mass-to-charge ratio 690.22/691.22 was identified as a bis-sucrose (Suc) borate complex in which the 4,6-hydroxyl pairs from the two a-glucopyranoside moieties formed an [L 2 B] 21 complex. Further investigation using liquid chromatography electrospray ionization triple quadrupole mass spectrometry analysis confirmed the presence of the bis-Suc borate complex in wheat phloem with a concentration up to 220 mM. The 300.11/301.11 complex was putatively identified as a bis-N-acetyl-serine borate complex but its concentration was below the detection limits of the liquid chromatography electrospray ionization triple quadrupole mass spectrometer so could not be quantified. The presence of borate complexes in the phloem provides a mechanistic explanation for the observed phloem boron mobility in canola and wheat and other species that transport Suc as their primary photoassimilate.Boron (B) is an essential element for plant growth (Warington, 1923) and it plays a structural role in the cell wall where it cross-links the pectic fractions of the dimeric polysaccharide complex known as RG-II (Matoh et al

Sucrose phosphate synthase plays a key role in boron‐promoted sucrose synthesis in tobacco leaves

“…A similar mechanism was also described for the B transporter BOR1 (Takano et al, 2005). It has also been reported that some B-binding membrane proteins are localized in microdomains (Wimmer et al, 2009), and B was identified as a bissucrose borate complex in the phloem of wheat and canola (Stangoulis et al, 2010). However, whether SUT1 and BOR1 are coregulated has not yet been determined.…”

“…SUT1 belongs to the first system and SUT4 to the second one (Weise et al, 2000). Interestingly, the long-distance transport of sucrose and B both occurred through the plant phloem, and Stangoulis et al (2010) revealed that B exists as bis-sucrose borate complex in phloem exudates of wheat. Wimmer et al (2009) had isolated and identified some boron-binding membrane-associated proteins in roots of Arabidopsis thaliana and Zea mays.…”

Acclimation of eggplant (Solanum melongena) to low boron supply

“…Despite that a direct mechanistic association has been established between translocation of polyols and B mobility, a certain degree of B remobilization may be observed in plant species that lack polyols in the phloem or vice-versa (Lehto et al, 2004). In addition, sucrose translocation could itself support some B mobility according to recent reports (Stangoulis et al, 2010). Reports on B mobility in solanaceaous species are limited.…”

“…Despite the fact that B remobilization is more obvious in species that translocate polyols in the phloem, species that do not belong to the above group (i.e., they predominantly translocate sucrose) also show an appreciable level of B mobility (Shelp, 1988;Liu et al, 1993;Marentes et al, 1997;Stangoulis et al, 2001;Lehto et al, 2004). Recently (Stangoulis et al, 2010), a putative bis-sucrose borate complex has been identified in the phloem sap of canola and wheat. This result extends the possibility of B phloem mobility in plant species that translocate sucrose.…”

Improved techniques for measurement of nanolitre volumes of phloem exudate from aphid stylectomy

“…Both techniques could be used in future work exploring the elemental, metabolite and protein composition of phloem exudate collected from single stylets. This will help expand on the current body of work in phloem proteomics [10,18], micronutrient transport [19] and phloem metabolomics [20]. …”