Carbohydrate partitioning is the process of carbon assimilation and distribution from source tissues, such as leaves, to sink tissues, such as stems, roots and seeds. Sucrose, the primary carbohydrate transported long distance in many plant species, is loaded into the phloem and unloaded into distal sink tissues. However, many factors, both genetic and environmental, influence sucrose metabolism and transport. Therefore, understanding the function and regulation of sugar transporters and sucrose metabolic enzymes is key to improving agriculture. In this review, we highlight recent findings that (i) address the path of phloem loading of sucrose in rice and maize leaves; (ii) discuss the phloem unloading pathways in stems and roots and the sugar transporters putatively involved; (iii) describe how heat and drought stress impact carbohydrate partitioning and phloem transport; (iv) shed light on how plant pathogens hijack sugar transporters to obtain carbohydrates for pathogen survival, and how the plant employs sugar transporters to defend against pathogens; and (v) discuss novel roles for sugar transporters in plant biology. These exciting discoveries and insights provide valuable knowledge that will ultimately help mitigate the impending societal challenges due to global climate change and a growing population by improving crop yield and enhancing renewable energy production.

During daylight, plants produce excess photosynthates, including sucrose, which is temporarily stored in the vacuole. At night, plants remobilize sucrose to sustain metabolism and growth. Based on homology to other sucrose transporter (SUT) proteins, we hypothesized the maize (Zea mays) SUCROSE TRANSPORTER2 (ZmSUT2) protein functions as a sucrose/H þ symporter on the vacuolar membrane to export transiently stored sucrose.To understand the biological role of ZmSut2, we examined its spatial and temporal gene expression, determined the protein subcellular localization, and characterized loss-offunction mutations. ZmSut2 mRNA was ubiquitously expressed and exhibited diurnal cycling in transcript abundance. Expressing a translational fusion of ZmSUT2 fused to a red fluorescent protein in maize mesophyll cell protoplasts revealed that the protein localized to the tonoplast. Under field conditions, zmsut2 mutant plants grew slower, possessed smaller tassels and ears, and produced fewer kernels when compared to wild-type siblings. zmsut2 mutants also accumulated two-fold more sucrose, glucose, and fructose as well as starch in source leaves compared to wild type. These findings suggest (i) ZmSUT2 functions to remobilize sucrose out of the vacuole for subsequent use in growing tissues; and (ii) its function provides an important contribution to maize development and agronomic yield.

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Sweet corn is one of the most important vegetables in the United States and Canada. Here, we present a de novo assembly of a sweet corn inbred line Ia453 with the mutated shrunken2-reference allele (Ia453-sh2). This mutation accumulates more sugar and is present in most commercial hybrids developed for the processing and fresh markets. The ten pseudochromosomes cover 92% of the total assembly and 99% of the estimated genome size, with a scaffold N50 of 222.2 Mb. This reference genome completely assembles the large structural variation that created the mutant sh2-R allele. Furthermore, comparative genomics analysis with six field corn genomes highlights differences in single-nucleotide polymorphisms, structural variations, and transposon composition. Phylogenetic analysis of 5,381 diverse maize and teosinte accessions reveals genetic relationships between sweet corn and other types of maize. Our results show evidence for a common origin in northern Mexico for modern sweet corn in the U.S. Finally, population genomic analysis identifies regions of the genome under selection and candidate genes associated with sweet corn traits, such as early flowering, endosperm composition, plant and tassel architecture, and kernel row number. Our study provides a high-quality reference-genome sequence to facilitate comparative genomics, functional studies, and genomic-assisted breeding for sweet corn.

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Agricultural crops are exposed to greater water deficits as production regions receive less rainfall and producers move into more arid landscapes to meet demand. Maintenance of root elongation is vital for seedling establishment in these types of climates. Some maize (Zea mays L.) lines can maintain primary root elongation under severe water stress (SS) conditions. Twelve maize inbred lines were examined at the seedling stage before leaf expansion to assess the phenotypic diversity in primary root elongation rate and root elongation zone abscisic acid (ABA) content at three water potentials, −0.03 MPa, −0.3 MPa, and −1.6 MPa. A phylogenetic tree was constructed using 93 simple sequence repeat (SSR) markers to examine genetic and phenotypic relationships among the lines in relation to stress response. Statistical analysis revealed phenotypic diversity in both primary root elongation rate and ABA content. No correlations were detected between the amount of ABA present in the root tip and the rate of root elongation in any of the treatments. Histograms of primary root elongation response to the varying water deficits suggest multiple mechanisms may be responsible for the response to water stress observed in different maize lines. Different phenotypic responses toward water stress were observed for lines with small genetic distances, which will aid in identification of specific alleles for maintenance of primary root growth under water deficits.

Insect endosymbionts influence many important metabolic and developmental processes of their host. It has been speculated that they may also help to manipulate and suppress plant defenses to the benefit of herbivores. Recently, endosymbionts of the root herbivore Diabrotica virgifera virgifera have been reported to suppress the induction of defensive transcripts in maize roots, which may explain the finding of another study that once attacked plants become more susceptible to subsequent D. v. virgifera attack. To test this hypothesis, we cured D. v. virgifera from its major endosymbiont Wolbachia and tested whether endosymbiont-free individuals elicit different defense responses in maize roots. The presence of Wolbachia did not alter the induction of defense marker genes and resistance in a susceptible maize hybrid and a resistant line. Furthermore, attacked maize plants emitted the same amount of (E)-β-caryophyllene, a volatile signal that serves as foraging cue for both entomopathogenic nematodes and D. v. virgifera. Finally, the effectiveness of the entomopathogenic nematode Heterorhabditis bacteriophora to infest D. v. virgifera was not changed by curing the larvae from their endosymbionts. These results show that the defense mechanisms of maize are not affected by Wolbachia. Consequently, D. v. virgifera does not seem to derive any plant-defense mediated benefits from its major endosymbiont.

An easy‐to‐perform protocol for isolating and quantifying soluble sugars (sucrose, glucose, and fructose) and starch from maize (Zea mays) leaf tissue is described. The method has been optimized to extract non‐structural carbohydrates (NSC) from frozen, finely ground tissue in a methanol:chloroform:water solution. Three rounds of tissue extraction for 30 min each at 50°C provide quantitative recovery of soluble sugars. The use of alternative extraction solvents is discussed, as well as the advantages and disadvantages of these solvents. Additionally, we provide two support protocols. The first quantifies the isolated NSC via commercially available enzymatic kits that couple the amount of each specific sugar to the production of NADPH, which is detected using equipment readily available to most laboratories. The second describes the preparation of a purification column to remove strongly charged or hydrophobic molecules from the extracted sugar solution, which is required prior to quantification with high‐pressure liquid chromatography or high‐performance anion‐exchange chromatography. The protocols are robust and easily adapted for use in measuring NSC extracted from other plant species or tissues, making them ideal for new users. © 2016 by John Wiley & Sons, Inc.