Phloem Unloading in Developing Rice Caryopses and its Contribution to Non-Structural Carbohydrate Translocation from Stems and Grain Yield Formation

Phloem unloading plays an important role in photoassimilate partitioning and grain yield improvements in cereal crops. The phloem unloading strategy and its effects on photoassimilate translocation and yield formation remain unclear in rice. In this study, plasmodesmata were observed at the interface between the sieve elements (SEs) and companion cells (CCs), and between the SE-CC complex and surrounding parenchyma cells (PCs) in phloem of the dorsal vascular bundle in developing caryopses. Carboxyfluorescein (CF) signal was detected in the phloem of caryopses, which showed that CF was unloaded into caryopses. These results indicated that the SE-CC complex was symplasmically connected with adjacent PCs by plasmodesmata. Gene expression for sucrose transporter (SUT) and cell wall invertase (CWI), and OsSUT1 and OsCIN1 proteins were detected in developing caryopses, indicating that rice plants might actively unload sucrose into caryopses by the apoplasmic pathway. Among three rice recombinant inbred lines, R201 exhibited lower plasmodesmal densities at the boundaries between cell types (SE-CC, SE-PC and CC-PC) in developing caryopses than R91 and R156. R201 also had lower expression of SUT and CWI genes and lower protein levels of OsSUT1 and OsCIN1, as well as CWI activity, than R91 and R156. These data agreed with stem non-structural carbohydrate (NSC) translocation and grain yields for the three lines. The nitrogen application rate had no significant effect on plasmodesmal densities at the interfaces between different cells types, and did not affect CF unloading in the phloem of developing caryopses. Low nitrogen treatment enhanced expression levels of OsSUT and OsCIN genes in the three lines. These results suggested that nitrogen application had no substantial effect on symplasmic unloading but affected apoplasmic unloading. Therefore, we concluded that poor symplasmic and apoplasmic unloading in developing caryopses might result in low stem NSC translocation and poor grain yield formation of R201.

Different evolutionary pathways to generate plant fructan exohydrolases

This article comments on: Oku S, Ueno K, Sawazaki Y, Maeda T, Jitsuyama Y, Suzuki T, Onodera S, Fujino K, Shimura H. 2022. Functional characterization and vacuolar localization of fructan exohydrolase derived from onion (Allium cepa). Journal of Experimental Botany 73,4908–4922.

CWIN-sugar transporter nexus is a key component for reproductive success

Reproductive development is critical for completion of plant life cycle and realization of crop yield potential. Reproductive organs comprise multiple distinctive or even transgenerational tissues, which are symplasmically disconnected from each other for protection and better control of nutrition and development. Cell wall invertases (CWINs) and sugar transporters are often specifically or abundantly expressed in these apoplasmic interfaces to provide carbon nutrients and sugar signals to developing pollens, endosperm and embryo. Emerging evidence shows that some of those genes were indeed targeted for selection during crop domestication. In this Opinion paper, I discuss the functional significance of the localized expression of CWINs and sugar transporters in reproductive organs followed by an analysis on how their spatial patterning may be regulated at the molecular levels and how the localized CWIN activity may be exploited for improvement of reproductive output.

Amylopectin Chain Length Dynamics and Activity Signatures of Key Carbon Metabolic Enzymes Highlight Early Maturation as Culprit for Yield Reduction of Barley Endosperm Starch after Heat Stress

Abiotic environmental stresses have a negative impact on the yield and quality of crops. Understanding these stresses is an essential enabler for mitigating breeding strategies and it becomes more important as the frequency of extreme weather conditions increases due to climate change. This study analyses the response of barley (Hordeum vulgare L.) to a heat wave during grain filling in three distinct stages: the heat wave itself, the return to a normal temperature regime, and the process of maturation and desiccation. The properties and structure of the starch produced were followed throughout the maturational stages. Furthermore, the key enzymes involved in the carbohydrate supply to the grain were monitored. We observed differences in starch structure with well-separated effects because of heat stress and during senescence. Heat stress produced marked effects on sucrolytic enzymes in source and sink tissues. Early cessation of plant development as an indirect consequence of the heat wave was identified as the major contributor to final yield loss from the stress, highlighting the importance for functional stay-green traits for the development of heat-resistant cereals.

Comparative genome based cis-elements analysis in the 5' upstream and 3' downstream region of cell wall invertase and Phenylalanine ammonia lyase in Nicotiana benthamiana

Plant secondary metabolites are widely used in human disease treatment; though primary metabolism provides precursors for secondary metabolism, not much has been studied to unravel the link connecting both the processes. Most common form of gene regulation interconnecting diverse metabolism occurs at the transcriptional and/or posttranscriptional level mediated by regulatory cis-elements. The present study aims at understanding the common cis-elements network connecting the major primary metabolic enzyme, cell wall invertase (CWIN) and secondary metabolism genes in Nicotiana benthamiana (N. benthamiana). The CWIN and thirty one other gene sequences were extracted from N. benthamiana genome, followed by cis-element analysis of their 5' upstream and 3' downstream region using different programs (Genomatix software suite; PLACE and PlantCARe). Comparative cis-element analysis of CWIN (N. benthamiana and other plant species) and other primary, secondary metabolism and transcription factor genes (N. benthamiana) revealed the occurrence of common stress associated cis-elements. Predominantly, AHBP, L1BX, MYBL, MADS, MYBS, GTBX, DOFF and CCAF were found in the 5' upstream region of all genes, whereas AHBP, MYBL, L1BX, HEAT, CCAF and KAN1 were largely occurring in the 3' downstream region of all genes; indicating common function of these elements in transcriptional and posttranscriptional gene regulation. Further, genomic analysis using FGENESH, GenScan and homology based methods (BlastX and BlastN) was performed on the N. benthamiana contigs harboring CWIN and PAL, in an attempt to identify genomic neighborhood genes. The 5' upstream and 3' downstream region of genes in the genomic neighborhood of CWIN and PAL were also subjected to similar cis-element analysis, and the results indicated cis-elements profile similar to CWIN, PAL and other primary, secondary metabolism and transcription factor genes. The results of evolutionary studies confirmed that the 5' upstream region of NbCWINs significantly showed more proximity to secondary metabolism genes 4CL and the redox gene SOD, followed by the phenylpropanoid pathway gene CHI. The 3' downstream regions of NbCWINs were more closely related to other plant CWINs, followed by the redox gene, SOD and primary metabolism gene FBA. Thus, the commonly found stress responsive cis-elements in our study can play a vital role in modulating key pathways of both primary and secondary metabolism; thereby postulating their role in regulating plant growth and metabolisms under unfavourable growth conditions.

Transcriptomic and metabolomics responses to elevated cell wall invertase activity during tomato fruit set

Fruit set is a developmental transition from ovaries to fruitlets that determines yield potential. Cell wall invertase (CWIN) is essential for fruit and seed set, but the underlying molecular basis remains elusive. We addressed this issue by using CWIN-elevated transgenic tomato, focusing on ovaries and fruitlets at 2 d before and after anthesis, respectively. RNAseq analyses revealed that ovaries and fruitlets exhibited remarkable differences in their transcriptomic responses to elevated CWIN activity. Ovaries 2 d before anthesis were far more responsive to elevated CWIN activity compared with the fruitlets. We identified several previously unknown pathways that were up-regulated by elevated CWIN activity during fruit set. The most notable of these were expression of genes for defence, ethylene synthesis and the cell cycle along with a large number of cell wall-related genes. By contrast, expression of photosynthetic, protein degradation and some receptor-like kinase genes were generally decreased as compared with the wild type ovaries. GC-MS analyses revealed that 22 out of 24 amino acids exhibited reduced levels in the RNAi ovaries as compared with that in the wild type, probably owing to a down-regulated expression of protein degradation genes. Overall, the data indicate that (i) ovaries are much more sensitive to metabolic intervention than fruitlets; (ii) high CWIN activity could promote fruit set by improving resistance against pathogens and altering cell cycle and cell wall synthesis.

Grape hexokinases are involved in the expression regulation of sucrose synthase- and cell wall invertase-encoding genes by glucose and ABA

Hexokinase (HXK, EC 2.7.1.1) is a multifunctional protein that both is involved in catalyzing the first step of glycolysis and plays an important role in sugar signaling. However, the supporting genetic evidence on hexokinases (CsHXKs) from grape (Vitis vinifera L. cv. Cabernet Sauvignon) berries has been lacking. Here, to investigate the role of CsHXK isoforms as glucose (Glc) and abscisic acid (ABA) sensors, we cloned two hexokinase isozymes, CsHXK1 and CsHXK2 with highly conserved genomic structure of nine exons and eight introns. We also found adenosine phosphate binding, substrate recognition and connection sites in their putative proteins. During grape berry development, the expression profiles of two CsHXK isoforms, sucrose synthases (SuSys) and cell wall invertase (CWINV) genes increased concomitantly with high levels of endogenous Glc and ABA. Furthermore, we showed that in wild type grape berry calli (WT), glucose repressed the expression levels of sucrose synthase (SuSy) and cell wall invertase (CWINV) genes, while ABA increased their expression levels. ABA could not only effectively improve the expression levels of SuSy and CWINV, but also block the repression induced by glucose on the expression of both genes. However, after silencing CsHXK1 or CsHXK2 in grape calli, SuSy and CWINV expression were enhanced, and the expressions of the two genes are insensitive in response to Glc treatment. Interestingly, exogenous ABA alone could not or less increase SuSy and CWINV expression in silencing CsHXK1 or CsHXK2 grape calli compared to WT. Meantime, ABA could not block the repression induced by glucose on the expression of SuSy and CWINV in CsHXK1 or CsHXK2 mutants. Therefore, Glc signal transduction depends on the regulation of CsHXK1 or CsHXK2. ABA signal was also disturbed by CsHXK1 or CsHXK2 silencing. The present results provide new insights into the regulatory role of Glc and ABA on the enzymes related to sugar metabolism in grape berry.

Sucrose phloem unloading follows an apoplastic pathway with high sucrose synthase in Actinidia fruit

Phloem unloading plays a pivotal role in photoassimilate partitioning and the accumulation of sugars in sink organs, e.g. fruit. Here, we investigated the pathway of sucrose unloading in kiwifruit (Actinidia deliciasa cv. Qinmei) using a combination of electron microscopy, transport of the phloem-mobile symplastic tracer carboxyfluorescein and enzyme activity and gene expression assays. Our structural investigation revealed that the sieve element-companion cell complex of bundles feeding the fruit flesh was symplastically isolated from its surrounding parenchyma cells throughout fruit development, whereas numerous plasmodesmata were present between the phloem parenchyma cells. Consistent with this, carboxyfluorescein unloading showed that the dye remained confined in the phloem strands during fruit development. The activities and expression of cell wall acid invertase in fruit flesh were lower than those of other enzymes that catalyze sucrose dissociation. However, sucrose synthase showed higher enzyme activities and mRNA expression in fruit flesh compared with other detected enzymes. These results imply that, in kiwifruit flesh, phloem unloading of sucrose is predominantly an apoplastic pathway during fruit development, and that sucrose synthase is a key enzyme for sucrose post-unloading pathways.

Suppression of extracellular invertase inhibitor gene expression improves seed weight in soybean (Glycine max)

Cell wall invertase (CWI) and vacuolar invertase (VI) play multiple functions in plant growth. As well as depending on transcriptional and post-transcriptional regulation, there is growing evidence that CWI and VI are also subject to post-translational control by small inhibitory proteins. Despite the significance of this, genes encoding inhibitors, their molecular and biochemical properties, and their potential roles in regulating seed production have not been well documented in soybean (Glycine max). In this study, two invertase inhibitor isoforms, GmCIF1 and GmC/VIF2, were characterized to possess inhibitory activities in vitro via heterologous expression. Transcript analyses showed that they were predominantly expressed in developing seeds and in response to ABA. In accordance with this, surveys of primary targets showed subcellular localizations to the apoplast in tobacco epidermis after expressing YFP-fusion constructs. Investigations using RNAi transgenic plants demonstrated marked elevations of CWI activities and improvements in seed weight in conjunction with higher accumulations of hexoses, starch, and protein in mature seeds. Further co-expression analyses of GmCIF1 with several putative CWI genes corroborated the notion that GmCIF1 modulation of CWI that affects seed weight is mainly contingent on post-translational mechanisms. Overall, the results suggest that post-translational elevation of CWI by silencing of GmCIF1 expression orchestrates the process of seed maturation through fine-tuning sucrose metabolism and sink strength.

Functional characterization and expression analysis of cucumber (Cucumis sativus L.) hexose transporters, involving carbohydrate partitioning and phloem unloading in sink tissues

Many hexose transporters (HTs) have been reported to play roles in sucrose-transporting plants. However, little information about roles of HTs in RFOs (raffinose family oligosaccharides)-transporting plants has been reported. Here, three hexose transporters (CsHT2, CsHT3, and CsHT4) were cloned from Cucumis sativus L. Heterologous expression in yeast demonstrated that CsHT3 transported glucose, galactose and mannose, with a K(m) of 131.9 μM for glucose, and CsHT4 only transported galactose, while CsHT2 was non-functional. Both CsHT3 and CsHT4 were targeted to the plasma membrane of cucumber protoplasts. Spatio-temporal expression indicated that transcript level of CsHT3 was much higher than that of CsHT2 and CsHT4 in most tissues, especially in peduncles and fruit tissues containing vascular bundles. GUS staining of CsHT3-promoter-β-glucuronidase (GUS) transgenic Arabidopsis plants revealed CsHT3 expression in tissues with high metabolic turnover, suggesting that CsHT3 is involved in sugar competition among different sink organs during plant development. The transcript levels of CsHT3 and cell wall invertase genes increased in peduncles and fruit tissues along with cucumber fruit enlargement, and CsHT3 localized to phloem tissues by immunohistochemical localization; These results suggest that CsHT3 probably plays an important role in apoplastic phloem unloading of cucumber fruit.

Ectopic overexpression of the cell wall invertase gene CIN1 leads to dehydration avoidance in tomato

Drought stress conditions modify source-sink relations, thereby influencing plant growth, adaptive responses, and consequently crop yield. Invertases are key metabolic enzymes regulating sink activity through the hydrolytic cleavage of sucrose into hexose monomers, thus playing a crucial role in plant growth and development. However, the physiological role of invertases during adaptation to abiotic stress conditions is not yet fully understood. Here it is shown that plant adaptation to drought stress can be markedly improved in tomato (Solanum lycopersicum L.) by overexpression of the cell wall invertase (cwInv) gene CIN1 from Chenopodium rubrum. CIN1 overexpression limited stomatal conductance under normal watering regimes, leading to reduced water consumption during the drought period, while photosynthetic activity was maintained. This caused a strong increase in water use efficiency (up to 50%), markedly improving water stress adaptation through an efficient physiological strategy of dehydration avoidance. Drought stress strongly reduced cwInv activity and induced its proteinaceous inhibitor in the leaves of the wild-type plants. However, the CIN1-overexpressing plants registered 3- to 6-fold higher cwInv activity in all analysed conditions. Surprisingly, the enhanced invertase activity did not result in increased hexose concentrations due to the activation of the metabolic carbohydrate fluxes, as reflected by the maintenance of the activity of key enzymes of primary metabolism and increased levels of sugar-phosphate intermediates under water deprivation. The induced sink metabolism in the leaves explained the maintenance of photosynthetic activity, delayed senescence, and increased source activity under drought stress. Moreover, CIN1 plants also presented a better control of production of reactive oxygen species and sustained membrane protection. Those metabolic changes conferred by CIN1 overexpression were accompanied by increases in the concentrations of the senescence-delaying hormone trans-zeatin and decreases in the senescence-inducing ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) in the leaves. Thus, cwInv critically functions at the integration point of metabolic, hormonal, and stress signals, providing a novel strategy to overcome drought-induced limitations to crop yield, without negatively affecting plant fitness under optimal growth conditions.

Hormonal and metabolic regulation of tomato fruit sink activity and yield under salinity

Salinization of water and soil has a negative impact on tomato (Solanum lycopersicum L.) productivity by reducing growth of sink organs and by inducing senescence in source leaves. It has been hypothesized that yield stability implies the maintenance or increase of sink activity in the reproductive structures, thus contributing to the transport of assimilates from the source leaves through changes in sucrolytic enzymes and their regulation by phytohormones. In this study, classical and functional physiological approaches have been integrated to study the influence of metabolic and hormonal factors on tomato fruit sink activity, growth, and yield: (i) exogenous hormones were applied to plants, and (ii) transgenic plants overexpressing the cell wall invertase (cwInv) gene CIN1 in the fruits and de novo cytokinin (CK) biosynthesis gene IPT in the roots were constructed. Although salinity reduces fruit growth, sink activity, and trans-zeatin (tZ) concentrations, it increases the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) during the actively growing period (25 days after anthesis). Indeed, exogenous application of the CK analogue kinetin to salinized actively growing fruits recovered sucrolytic activities (mainly cwInv and sucrose synthase), sink strength, and fruit weight, whereas the ethylene-releasing compound ethephon had a negative effect in equivalent non-stressed fruits. Fruit yield was increased by both the constitutive expression of CIN1 in the fruits (up to 4-fold) or IPT in the root (up to 30%), owing to an increase in the fruit number (lower flower abortion) and in fruit weight. This is possibly related to a recovery of sink activity in reproductive tissues due to both (i) increase in sucrolytic activities (cwInv, sucrose synthase, and vacuolar and cytoplasmic invertases) and tZ concentration, and (ii) a decrease in the ACC levels and the activity of the invertase inhibitor. This study provides new functional evidences about the role of metabolic and hormonal inter-regulation of local sink processes in controlling tomato fruit sink activity, growth, and yield under salinity.