Assessing Long-distance Transport from Photosynthetic Source Leaves to Heterotrophic Sink Organs with [14C]CO2

PUB30-mediated downregulation of the HB24-SWEET11 module is involved in root growth inhibition under salt stress by attenuating sucrose supply in Arabidopsis

One of the strategies that plants adopt to cope with an unfavorable environment is to sacrifice their growth for tolerance. Although moderate salt stress can induce root growth inhibition, the molecular mechanisms regulating this process have yet to be elucidated. Here, we found that overexpression of a zinc finger-homeodomain family transcription factor, HOMEOBOX PROTEIN 24 (HB24), led to longer primary roots than in the wild-type in the presence of 125 mM NaCl, whereas this phenotype was reversed for the hb24 loss-of-function mutant, indicating a negative impact of HB24 on salt-induced root growth inhibition. We then found that salt stress triggered the degradation of HB24 via the ubiquitin-proteasome pathway, as mediated by a plant U-box type E3 ubiquitin ligase 30 (PUB30) that directly targets HB24. We verified that HB24 is able to directly bind to the promoters of Sugars Will Eventually be Exported Transporter 11/12 (SWEET11/12) to regulate their expression in roots. Through genetic and biochemical assays, we further demonstrated that the HB24-SWEET11 module plays a negative role in salt-induced root growth inhibition. Therefore, we propose that under salt stress, PUB30 mediates HB24's degradation, thereby downregulating the expression of SWEET11, resulting in reduced sucrose supply and root growth inhibition.

Assessing Long-Distance Carbon Partitioning from Photosynthetic Source Leaves to Heterotrophic Sink Organs with Photoassimilated [14C]CO2

Phloem loading and long-distance transport of photoassimilate from source leaves to sink organs are essential physiological processes that contribute to plant growth and yield. At a minimum, three steps are involved: phloem loading in source organs, transport along the phloem path, and phloem unloading in sink organs. Each of these can have variable rates contingent on the physiological state of the plant, and thereby influence the overall transport rate. In addition to these phloem transport steps, rates of photosynthesis and photosynthate movement in the pre-phloem path, as well as photosynthate utilization in post phloem tissues of sink organs also contribute to phloem transport. The protocol described here estimates carbon allocation along the entire path from initial carbon fixation to delivery to sink organs after a labeling pulse: [¹⁴C]CO₂ is photoassimilated in source leaves and loading and transport of the ¹⁴C label to heterotrophic sink organs (roots) is quantified by scintillation counting. This method is flexible and can be adapted to quantify long-distance transport in many plant species.

Assessing Rates of Long-distance Carbon Transport in Arabidopsis by Collecting Phloem Exudations into EDTA Solutions after Photosynthetic Labeling with [14C]CO2

Phloem loading and transport of photoassimilate from photoautotrophic source leaves to heterotrophic sink organs are essential physiological processes that help the disparate organs of a plant function as a single, unified organism. We present three protocols we routinely use in combination with each other to assess (1) the relative rates of sucrose (Suc) loading into the phloem vascular system of mature leaves ( Yadav et al., 2017a ), (2) the relative rates of carbon loading and transport through the phloem (this protocol), and (3) the relative rates of carbon unloading into heterotrophic sink organs, specifically roots, after long-distance transport ( Yadav et al., 2017b ), We propose that conducting all three protocols on experimental and control plants provides a reliable comparison of whole-plant carbon partitioning, and minimizes ambiguities associated with a single protocol conducted in isolation ( Dasgupta et al., 2014 ; Khadilkar et al., 2016 ). In this protocol, [14C]CO2 is photoassimilated in source leaves and phloem loading and transport of photoassimilate is quantified by collecting phloem exudates into an EDTA solution followed by scintillation counting.

Quantifying the Capacity of Phloem Loading in Leaf Disks with [14C]Sucrose

Phloem loading and transport of photoassimilate from photoautotrophic source leaves to heterotrophic sink organs are essential physiological processes that help the disparate organs of a plant function as a single, unified organism. We present three protocols we routinely use in combination with each other to assess (1) the relative rates of sucrose (Suc) loading into the phloem vascular system of mature leaves (this protocol), (2) the relative rates of carbon loading and transport through the phloem ( Yadav et al., 2017a ), and (3) the relative rates of carbon unloading into heterotrophic sink organs, specifically roots, after long-distance transport ( Yadav et al., 2017b ). We propose that conducting all three protocols on experimental and control plants provides a reliable comparison of whole-plant carbon partitioning, and minimizes ambiguities associated with a single protocol conducted in isolation ( Dasgupta et al., 2014 ; Khadilkar et al., 2016 ). In this protocol, Arabidopsis leaf disks isolated from mature rosette leaves are infiltrated with a buffered solution containing [14C]Suc. Suc transporters (SUCs or SUTs) load Suc into the phloem and excess, unloaded Suc in the leaf disk is then washed away. Loading of labeled Suc into the veins is visualized by autoradiography of lyophilized leaf disks and quantified by scintillation counting. Results are expressed as disintegration per minute per unit of leaf disk fresh weight or area.