ORCID Profile
0000-0002-8733-0103
Current Organisation
Western Sydney University
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Publisher: Oxford University Press (OUP)
Date: 27-04-2023
DOI: 10.1093/AOB/MCAD057
Abstract: The mechanisms of sugar sensing in grasses remain elusive, especially those using C4 photosynthesis even though a large proportion of the world's agricultural crops utilize this pathway. We addressed this gap by comparing the expression of genes encoding components of sugar sensors in C3 and C4 grasses, with a focus on source tissues of C4 grasses. Given C4 plants evolved into a two-cell carbon fixation system, it was hypothesized this may have also changed how sugars were sensed. For six C3 and eight C4 grasses, putative sugar sensor genes were identified for target of rapamycin (TOR), SNF1-related kinase 1 (SnRK1), hexokinase (HXK) and those involved in the metabolism of the sugar sensing metabolite trehalose-6-phosphate (T6P) using publicly available RNA deep sequencing data. For several of these grasses, expression was compared in three ways: source (leaf) versus sink (seed), along the gradient of the leaf, and bundle sheath versus mesophyll cells. No positive selection of codons associated with the evolution of C4 photosynthesis was identified in sugar sensor proteins here. Expressions of genes encoding sugar sensors were relatively ubiquitous between source and sink tissues as well as along the leaf gradient of both C4 and C3 grasses. Across C4 grasses, SnRK1β1 and TPS1 were preferentially expressed in the mesophyll and bundle sheath cells, respectively. Species-specific differences of gene expression between the two cell types were also apparent. This comprehensive transcriptomic study provides an initial foundation for elucidating sugar-sensing genes within major C4 and C3 crops. This study provides some evidence that C4 and C3 grasses do not differ in how sugars are sensed. While sugar sensor gene expression has a degree of stability along the leaf, there are some contrasts between the mesophyll and bundle sheath cells.
Publisher: Wiley
Date: 12-12-2021
DOI: 10.1111/TPJ.15581
Abstract: Photosynthetic efficiency and sink demand are tightly correlated with rates of phloem loading, where maintaining low cytosolic sugar concentrations is paramount to prevent the downregulation of photosynthesis. Sugars Will Eventually be Exported Transporters (SWEETs) are thought to have a pivotal role in the apoplastic phloem loading of C 4 grasses. SWEETs have not been well studied in C 4 species, and their investigation is complicated by photosynthesis taking place across two cell types and, therefore, photoassimilate export can occur from either one. SWEET13 homologues in C 4 grasses have been proposed to facilitate apoplastic phloem loading. Here, we provide evidence for this hypothesis using the C 4 grass Setaria viridis . Expression analyses on the leaf gradient of C 4 species Setaria and Sorghum bicolor show abundant transcript levels for SWEET13 homologues. Carbohydrate profiling along the Setaria leaf shows total sugar content to be significantly higher in the mature leaf tip compared with the younger tissue at the base. We present the first known immunolocalization results for SvSWEET13a and SvSWEET13b using novel isoform‐specific antisera. These results show localization to the bundle sheath and phloem parenchyma cells of both minor and major veins. We further present the first transport kinetics study of C 4 monocot SWEETs by using a Xenopus laevis oocyte heterologous expression system. We demonstrate that SvSWEET13a and SvSWEET13b are high‐capacity transporters of glucose and sucrose, with a higher apparent V max for sucrose, compared with glucose, typical of clade III SWEETs. Collectively, these results provide evidence for an apoplastic phloem loading pathway in Setaria and possibly other C 4 species.
Publisher: Oxford University Press (OUP)
Date: 08-03-2023
DOI: 10.1093/JXB/ERAD076
Abstract: In the developing seeds of all higher plants, filial cells are symplastically isolated from the maternal tissue supplying photosynthate to the reproductive structure. Photoassimilates must be transported apoplastically, crossing several membrane barriers, a process facilitated by sugar transporters. Sugars Will Eventually be Exported Transporters (SWEETs) have been proposed to play a crucial role in apoplastic sugar transport during phloem unloading and the post-phloem pathway in sink tissues. Evidence for this is presented here for developing seeds of the C4 model grass Setaria viridis. Using immunolocalization, SvSWEET4 was detected in various maternal and filial tissues within the seed along the sugar transport pathway, in the vascular parenchyma of the pedicel, and in the xylem parenchyma of the stem. Expression of SvSWEET4a in Xenopus laevis oocytes indicated that it functions as a high-capacity glucose and sucrose transporter. Carbohydrate and transcriptional profiling of Setaria seed heads showed that there were some developmental shifts in hexose and sucrose content and consistent expression of SvSWEET4 homologues. Collectively, these results provide evidence for the involvement of SWEETs in the apoplastic transport pathway of sink tissues and allow a pathway for post-phloem sugar transport into the seed to be proposed.
Publisher: Frontiers Media SA
Date: 12-2016
Publisher: Cold Spring Harbor Laboratory
Date: 11-07-2023
DOI: 10.1101/2023.07.10.548403
Abstract: Hexokinases (HXK) were the first sugar signalling proteins identified in plants and are well known for their feedback regulation of photosynthetic gene expression. In some C 3 plants, HXKs have been found to regulate stomatal function. However, the role of HXK in C 4 photosynthesis, which is inherently more water use efficient than C 3 metabolism, remains poorly understood. Here, we report on the first tissue-specific modification of HXK in a C 4 plant. SvHXK6 was expressed in the model C 4 grass Setaria viridis under the control of the ZmPEPC promoter ( ZmPEPC pro ), which directs expression in the leaf mesophyll tissue. Three S. viridis transgenic lines with increased abundance of SvHXK6 transcripts in the leaf tissue showed significant reduction in stomatal conductance with minimal effects on leaf CO 2 assimilation rate. Consequently, the transgenic lines had higher leaf-level water use efficiency relative to the control (wild-type and null) plants. Overexpression of SvHXK6 had no effect on shoot biomass or seed yield of the S. viridis plants. Our study shows conserved function of HXK in regulating stomatal conductance in a C 4 grass, demonstrating possible widespread utility in improving water use efficiency in C 4 as well as C 3 species.
No related grants have been discovered for Lily Chen.