Dynamics and control of phloem loading of indole-3-acetic acid in seedling cotyledons of Ricinus communis

Synthesis and characterization of an artificial glucosinolate bearing a chlorthalonil-based aglycon as a potent inhibitor of glucosinolate transporters

Glucosinolates (GSLs) are specialized metabolites in plants of the order Brassicales. GSL transporters (GTRs) are essential for the redistribution of GSLs and also play a role in controlling the GSL content of seeds. However, specific inhibitors of these transporters have not been reported. In the current study, we described the design and synthesis of 2,3,4,6-tetrachloro-5-cyanophenyl GSL (TCPG), an artificial GSL bearing a chlorothalonil moiety as a potent inhibitor of GTRs, and evaluated its inhibitory effect on the substrate uptake mediated through GTR1 and GTR2. Molecular docking showed that the position of the β-D-glucose group of TCPG was significantly different from that of the natural substrate in GTRs and the chlorothalonil moiety forms halogen bonds with GTRs. Functional assays and kinetic analysis of the transport activity revealed that TCPG could significantly inhibit the transport activity of GTR1 and GTR2 (IC₅₀ values (mean ± SD) being 79 ± 16 μM and 192 ± 14 μM, respectively). Similarly, TCPG could inhibit the uptake and phloem transport of exogenous sinigrin by Arabidopsis thaliana (L.) Heynh leaf tissues, while not affecting that of esculin (a fluorescent surrogate for sucrose). TCPG could also reduce the content of endogenous GSLs in phloem exudates. Together, TCPG was discovered as an undescribed inhibitor of the uptake and phloem transport of GSLs, which brings novel insights into the ligand recognition of GTRs and provides a new strategy to control the GSL level. Further tests on the ecotoxicological and environmental safety of TCPG are needed before using it as an agricultural or horticultural chemical in the future.

Long-distance hormone transport via the phloem

Several key plant hormones are synthesised in the shoot and are advected within the phloem to the root tip. In the root tip, these hormones regulate growth and developmental processes, and responses to environmental cues. However, we lack understanding of how environmental factors and biological parameters affect the delivery of hormones to the root tip. In this study, we build on existing models of phloem flow to develop a mathematical model of sugar transport alongside the transport of a generic hormone. We derive the equations for osmotically driven flow in a long, thin pipe with spatially varying membrane properties to capture the phloem loading and unloading zones. Motivated by experimental findings, we formulate solute membrane transport in terms of passive and active components, and incorporate solute unloading via bulk flow (i.e. advection with the water efflux) by including the Staverman reflection coefficient. We use the model to investigate the coupling between the sugar and hormone dynamics. The model predicts that environmental cues that lead to an increase in active sugar loading, an increase in bulk flow sugar unloading or a decrease in the relative root sugar concentration result in an increase in phloem transport velocity. Furthermore, the model reveals that such increases in phloem transport velocity result in an increase in hormone delivery to the root tip for passively loaded hormones.

Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

Maintaining phloem transport under water stress is expected to be crucial to whole-plant drought tolerance, but the traits that benefit phloem function under drought are poorly understood. Nearly half of surveyed angiosperm species, including important crops, use sucrose transporter proteins to actively load sugar into the phloem. Plants can alter transporter abundance in response to stress, providing a potential mechanism for active-loading species to closely regulate phloem loading rates to avoid drought-induced reductions or failures in phloem transport. We developed an integrated xylem-phloem-stomatal model to test this hypothesis by quantifying the joint impacts of transporter kinetics, phloem anatomy, and plant water status on sucrose export to sinks. We parameterized the model with phloem hydraulic resistances and sucrose transporter kinetic parameters compiled from the literature, and simulated loading regulation by allowing loading rates to decline exponentially with phloem pressure to prevent excessive sucrose concentrations from inducing viscosity limitations. In the absence of loading regulation, where loading rates were independent of phloem pressure, most resistance values produced unrealistic phloem pressures owing to viscosity effects, even under well-watered conditions. Conversely, pressure-regulated loading helped to control viscosity buildup and improved export to sinks for both lower and higher resistant phloem pathways, while maintaining realistic phloem pressures. Regulation also allowed for rapid loading and export in wet conditions while maintaining export and viable phloem pressures during drought. Therefore, we expect feedbacks between phloem pressure and loading to be critical to carbon transport in active-loading species, especially under drought, and for transporter kinetics to be strongly coordinated with phloem architecture and plant water status. This work provides an important and underexplored physiological framework to understand the ecophysiology of phloem transport under drought and to enhance the genetic engineering of crop plants.

Use of D-glucose-fenpiclonil conjugate as a potent and specific inhibitor of sucrose carriers

Until now, specific inhibitors of sucrose carriers were not available. This led us to study the properties of the recently synthesized D-glucose-fenpiclonil conjugate (D-GFC). This large amphiphilic glucoside exhibited an extremely low phloem systemicity in contrast to L-amino acid-fenpiclonil conjugates. Using Ricinus seedlings, the effect of D-GFC on 0.5 mM [14C]sucrose (Suc), 3-O-[3H]methylglucose, and [3H]glutamine uptake by cotyledon tissues was compared with that of p-chloromercuribenzenesulfonic acid (PCMBS). D-GFC dramatically inhibited H+-Suc symport at the same concentrations as PCMBS (0.5 and 1 mM), but in contrast to the thiol reagent, it did not affect 3-O-methylglucose and glutamine transport, nor the acidification of the incubation medium by cotyledon tissues. Similarly, 0.5 mM D-GFC inhibited active Suc uptake by Vicia faba leaf tissues and by Saccharomyces cerevisiae cells transformed with AtSUC2, a gene involved in Suc phloem loading in Arabidopsis, by approximately 80%. The data indicated that D-GFC was a potent inhibitor of Suc uptake from the endosperm and of Suc phloem loading. It is the first chemical known to exhibit such specificity, at least in Ricinus, and this property permitted the quantification of the two routes involved in phloem loading of endogenous sugars after endosperm removal.