Drought restricted sucrose transport from outer cottonseed coat to fiber and further inhibited cellulose synthesis during cotton fiber thickening

The formation of cotton fiber strength largely relies on continuous and steady sucrose supply to cellulose synthesis and is greatly impaired by drought. However, the effects of drought on sucrose import into fiber and its involvement in cellulose biosynthesis within fiber remain unclear. To end this, moisture deficiency experiments were conducted using two Gossypium hirsutum cultivars of Dexiamian 1 (drought-tolerant) and Yuzaomian 9110 (drought-sensitive). Fiber strength was significantly decreased under drought. The results of 13C isotope labeling indicated that drought notably reduced sucrose efflux from cottonseed coat to fiber, and this was caused by down-regulation of sucrose transporter genes (GhSWEET10 and GhSWEET15) in the outer cottonseed coat, finally leading to decreased sucrose accumulation in fiber. Further, under drought, the balance of sucrose allocation within fiber was disrupted by increasing the flow of sucrose into β-1,3-glucan synthesis and lignin synthesis but hindering that into cellulose synthesis in both cultivars. Additionally, glycolysis and starch synthesis were specifically enhanced by drought in Yuzaomian 9110, which further reduced the flow of sucrose into cellulose synthesis. Under drought, the cellulose deposition was decreased due to promoted cellulose degrading process in Dexiamian 1 and stunted cellulose synthesis in Yuzaomian 9110. Consequently, reduced cellulose content was measured in drought-stressed fibers for both cultivars. In summary, the inhibited cellulose accumulation caused by drought was mainly due to reduced sucrose translocation from the outer cottonseed coat to fiber, and less sucrose partitioned to cellulose synthesis pathway under the condition of intensified competition for sucrose by different metabolic pathways within fiber, finally degrading the fiber strength.

Sucrose-dependence of sugar uptake, quorum sensing and virulence of the rice blight pathogen Xanthomonas oryzae pv. oryzae

Virulence of Xanthomonas oryzae pv. oryzae (Xoo), which causes bacterial leaf blight of rice, depends on induction of host SWEET sucrose efflux transporters. It remained unknown whether secreted sucrose serves bacterial nutrition or host defense. Here we identified the sux sucrose uptake/utilization locus of Xoo and demonstrate that it is necessary and sufficient for sucrose acquisition. Induction of sux genes during infection closely tracked induction of rice SWEET11a. sux mutants were defective in swimming, swarming, extracellular polysaccharide (EPS) production and biofilm formation. EPS synthesis in mutants was restored by the quorum-sensing factor DSF. Notably, transcripts for rate limiting steps in DSF production were unaffected by sucrose, transcripts of the DSF receptor were sucrose-inducible and increased during infection, indicating sensitization to DSF in response to sucrose supply. Sucrose induced the sigma factors transcripts for RpoN1 and RpoN2 that regulate swimming, EPS and virulence. Furthermore, in contrast to Xanthomonas axonopodis pv. manihotis, virulence of Xoo depended critically on sux gene function. Together, pathogen-induced sucrose efflux from host cells likely induces bacterial sigma factors and sensitizes quorum signaling necessary for biofilm formation and colonization of the xylem, serves as energy source for swimming against the xylem stream, and as nutrient for growth.

The RNA-binding protein RGGA is a sugar-responsive cofactor of the 5'-3' exonuclease XRN4 that post-transcriptionally meditates plant growth

Sugar-regulated gene expression is a ubiquitous mechanism for carbohydrate allocation and utilization by keeping a balance among source and sink organs in plants. Previous studies have shown that excess sugar represses the expression of the genes implicated in photosynthesis and sugar metabolism. However, the mechanism is still largely unknown. Here, we found that the mutant of Arabidopsis RGGA, RGG repeats RNA-binding protein A coding gene, grew faster than wild type (Col-0) in MS medium. In rgga, mRNA half-live of the genes related to sucrose transport and metabolism, chlorophyll synthesis, root development as well as certain transcription factors was obviously longer than those of Col-0. Further study revealed that AtRGGA could interact with 5'-3' exonuclease AtXRN4, and guide it to the target mRNAs for their degradation. When AtRGGA is absent or its interaction domain is deleted, AtXRN4 self can't recognize the target mRNAs, which leads to a dramatically increase in transcript levels of the above gene subsets, and thus promotes the growth of Arabidopsis with exogenous sucrose supply. And only 5-day sucrose supply could trigger the vigorous growth of rgga. These findings suggest that the regulation of mRNA stability mediated by RGGA plays a critical role in sugar suppression, and implicates a possibility to unlock the growth potential by modulating sugar utilization at post-transcriptional level in plants.

Suppressed ABA signal transduction in the spike promotes sucrose use in the stem and reduces grain number in wheat under water stress

Water stress is a primary trigger for reducing grain number per spike in wheat during the reproductive period. However, under stress conditions, the responses of plant organs and the interactions between them at the molecular and physiological levels remain unclear. In this study, when water stress occurred at the young microspore stage, RNA-seq data indicated that the spike had 970 differentially expressed genes, while the stem, comprising the two internodes below the spike (TIS), had 382. Abscisic acid (ABA) signal transduction genes were down-regulated by water stress in both these tissues, although to a greater extent in the TIS than in the spike. A reduction in sucrose was observed, and was accompanied by increases in cell wall invertase (CWIN) and sucrose:sucrose 1-fructosyl-transferase (1-SST) activities. Hexose and fructan were increased in the TIS but decreased in the spike. ABA was increased in the spike and TIS, and showed significant positive correlation with CWIN and 1-SST activities in the TIS. Overall, our results suggest that water stress induces the conversion of sucrose to hexose by CWIN, and to fructan by 1-SST, due to increased down-regulation of ABA signal transduction related-genes in the TIS; this leads to deficient sucrose supply to the spike and a decrease in grain number.

Exogenous Sucrose Supply Regulates the Non-Structural Carbohydrate, Endogenous Hormone and Mineral Element Levels before the Flower Blooming and Fruit Abscission Stages in the Biodiesel Tree Sapindus mukorossi Gaertn.

Background: Nutrient deficiency leads to a high fruit abscission rate in Sapindus mukorossi Gaertn. (Soapberry), which is one of the most widely cultivated biodiesel feedstock forests in China. Exogenous sucrose can provide a solution to nutrient deficiency and fruit abscission, which was applied to whole trees at two stages, 20 days before blooming (DBB stage) and fruit abscission (DBFA stage). Six sucrose concentrations, 0%, 1%, 1.5%, 3%, 5% and 7%, were sprayed three times at a completely randomized block design with 5 replications and 6 treatments. 13CO2 labelling experiments were carried out after the 3 sprayings. Results: (1) In the DBB stage, the 3% treatment significantly increased the inflorescence fructose and glucose contents 1-1.2 times, also resulted in the highest fruit GA3, leaf IAA, fruit IAA and fruit ZT contents, while it decreased the inflorescence ABA from 16 μg/g to 4 μg/g. (2) The 1.5% and 3% treatments significantly increased the carbohydrate content and decreased fruit ABA content to 30%-50% of the control level in the DBFA stage. (3) High-concentration sugar treatment (over 3%) increased the nitrogen, phosphorus, and potassium contents, which decreased the calcium and magnesium contents. (4) The 13C-dispatching ability of the inflorescence was 3 times greater than leaves under high-concentration sugar treatment in the DBB stage. Supplying 1.5% sucrose nearly doubled the allocation capacity in the DBFA stage. (5) The source-sink nutrient migration pathway showed that leaf and fruit sugars were directly correlated via phosphorus. Fruit fructose and glucose contents affected the leaf mineral element contents. Conclusions: It can conclude that exogenous application of 3% sucrose in the DBB stage and 1.5% sucrose in the DBFA stage can increase the sink nutrition allocation capacity, and phosphorus is the main mineral element correlated with leaf and fruit sugars.

Sugar availability suppresses the auxin-induced strigolactone pathway to promote bud outgrowth

SUMMARYApical dominance occurs when the growing shoot tip inhibits the outgrowth of axillary buds. Apically-derived auxin in the nodal stem indirectly inhibits bud outgrowth via cytokinins and strigolactones. Recently, sugar deprivation was found to contribute to this phenomenon.Using rose and pea, we investigated whether sugar availability interacts with auxin in bud outgrowth control, and the role of cytokinins and strigolactones, in vitro and in planta.We show that sucrose antagonizes auxin’s effect on bud outgrowth, in a dose-dependent and coupled manner. Sucrose also suppresses strigolactone-inhibition of outgrowth, and rms3 strigolactone-perception mutant is less affected by reducing sucrose supply; however, sucrose does not interfere with the regulation of cytokinin levels by auxin, and stimulates outgrowth even with optimal cytokinin supply. These observations were assembled into a computational model where sucrose represses bud response to strigolactones, largely independently of cytokinin levels. It quantitatively captures our observed dose-dependent sucrose-hormones effects on bud outgrowth, and allows us to express outgrowth response to various combinations of auxin and sucrose levels as a simple quantitative law.This study places sugars in the bud outgrowth regulatory network, and paves the way for better understanding of branching plasticity in response to environmental and genotypic factors.

Foliar Zn Spraying Simultaneously Improved Concentrations and Bioavailability of Zn and Fe in Maize Grains Irrespective of Foliar Sucrose Supply

Zinc (Zn) deficiency is a global nutritional problem that is reduced through agronomic biofortification. In the current study, the effects of foliar spraying of exogenous ZnSO4·7H2O (0.2% in Quzhou and 0.3% in Licheng, w/v) and/or sucrose (10.0%, w/v) on maize (Zea mays L.) agronomic traits; concentrations of Zn, iron (Fe), calcium (Ca), total phosphorus (P), phytic acid (PA) P, carbon (C), and nitrogen (N); C/N ratios; and Zn and Fe bioavailability (as evaluated by molar ratios of PA/Zn, PA × Ca/Zn, PA/Fe and PA × Ca/Fe) in maize grains were studied under field conditions for two years at two experimental locations. The results confirmed that there were no significant differences in maize agronomic traits following the various foliar treatments. Compared with the control treatment of foliar spraying with deionized water, foliar applications of Zn alone or combined with sucrose significantly increased maize grain Zn concentrations by 29.2–58.3% in Quzhou (from 18.4–19.9 to 25.2–29.6 mg/kg) and by 39.8–47.8% in Licheng (from 24.9 to 34.8–36.8 mg/kg), as well as its bioavailability. No significant differences were found between the foliar spraying of deionized water and sucrose, and between Zn-only and “sucrose + Zn” at each N application rate and across different N application rates and experimental sites. Similar results were observed for maize grain Fe concentrations and bioavailability, but the Fe concentration increased to a smaller extent than Zn. Foliar Zn spraying alone or with sucrose increased maize grain Fe concentrations by 4.7–28.4% in Quzhou (from 13.4–17.1 to 15.2–18.5 mg/kg) and by 15.4–25.0% in Licheng (from 24.0 to 27.7–30.0 mg/kg). Iron concentrations were significantly and positively correlated with Zn at each N application rate and across different N application rates and experimental locations, indicating that foliar Zn spraying facilitated the transport of endogenous Fe to maize grains. Therefore, foliar Zn spraying increased the Zn concentration and bioavailability in maize grains irrespective of foliar sucrose supply while also improving Fe concentrations and bioavailability to some extent. This is a promising agricultural practice for simultaneous Zn and Fe biofortification in maize grains, i.e., “killing two birds with one stone”.