Protonophore‐ and pH‐insensitive glucose and sucrose accumulation detected by FRET nanosensors in Arabidopsis root tips

Abstract

SummaryAlthough soil contains only traces of soluble carbohydrates, plant roots take up glucose and sucrose efficiently when supplied in artificial media. Soluble carbohydrates and other small metabolites found in soil are in part products from exudation from plant roots. The molecular nature of the transporters for uptake and exudation is unknown. Here, fluorescence resonance energy transfer (FRET) glucose and sucrose sensors were used to characterize accumulation and elimination of glucose and sucrose in Arabidopsis roots tips. Using an improved image acquisition set-up, FRET responses to perfusion with carbohydrates were detectable in roots within less than 10 sec and over a wide concentration range. Accumulation was fully reversible within 10-180 sec after glucose or sucrose had been withdrawn; elimination may be caused by metabolism and/or efflux. The rate of elimination was unaffected by pre-incubation with high concentrations of glucose, suggesting that elimination is not due to accumulation in a short-term buffer such as the vacuole. Glucose and sucrose accumulation was insensitive to protonophores, was comparable in media differing in potassium levels, and was similar at pH 5.8, 6.8 and 7.8, suggesting that both influx and efflux may be mediated by protonindependent transport systems. High-resolution expression mapping in root tips showed that only a few proton-dependent transport of the STP (Sugar Transport Protein) and SUT/SUC (Sucrose Transporter/Carrier) families are expressed in the external cell layers of root tips. The root expression maps may help to pinpoint candidate genes for uptake and release of carbohydrates from roots.

Ethylene Suppression of Sugar-Induced Anthocyanin Pigmentation in Arabidopsis

“…Indeed, SUC1-independent Suc uptake pathways in the root have been proposed previously (i.e. one mediated by endocytosis and another by pHindependent sugar transporters; Chaudhuri et al, 2008). However, we failed to inhibit anthocyanin induction using the endocytic inhibitor wortmannin (data not shown, Emans et al, 2002;Etxeberria et al, 2005) or the uncouplers CCCP and nigericin ( Fig.…”

“…7B). In Col0 and the mutants investigated in this study, Glc is the most abundant form of soluble sugar, representing 70% to 90% of total sugar, probably due to rapid Suc hydrolysis before or after entry into the cell (Chaudhuri et al, 2008).…”

“…If we assume that SUC1 is also regulated at the transcriptional level as for other SUCs (Zhou et al, 2009), the increase in SUC1 transcript levels induced by light and sugar would result in enhanced SUC1 activity. Thus, it is most likely that SUC1 plays a role in transporting exogenous Suc taken by proton-independent sugar transporters in the root tips (Chaudhuri et al, 2008) to the shoot. In accordance with this viewpoint, the sugar content in seedlings after root Step 1, Light and sugar signals generated from photosynthetic electron transport (PET) activates the MBW regulatory complex.…”

“…However, we failed to inhibit anthocyanin induction using the endocytic inhibitor wortmannin (data not shown, Emans et al, 2002;Etxeberria et al, 2005) or the uncouplers CCCP and nigericin ( Fig. 5; Chaudhuri et al, 2008). Several pH-independent sugar transporters are expressed in roots, including the sugar transporter family ERD6-like homologs (At1g08920 and At1g08930), two plastidic Glc transporters (At1g79820 and At5g16150), and four members of the aquaporin gene family (PIP1:2, PIP1:3, PIP2:8, and SIP1:1; Lalonde et al, 2004;Kaldenhoff et al, 2007;Chaudhuri et al, 2008).…”

Feedback Inhibition of Ammonium Uptake by a Phospho-Dependent Allosteric Mechanism inArabidopsis

“…For axenic culture experiments, Arabidopsis seeds were surface sterilized and germinated on full nutrient (FN) medium (2 mM NH 4 NO 3 , 1 mM KH 2 PO 4 , 1 mM MgSO 4 , 250 mM K 2 SO 4 , 250 mM CaCl 2 , 100 mM NaFe-EDTA, 50 mM KCl, 50 mM H 3 BO 3 , 5 mM MnSO 4 , 1 mM ZnSO 4 , 1 mM CuSO 4 , 1 mM NaMoO 4 , 1 mM MES, 1% [w/v] (Chaudhuri et al, 2008). After 9 d, seedlings were transferred to the medium lacking nitrogen.…”