Genetic mapping shows intraspecific variation and transgressive segregation for caterpillar‐induced aphid resistance in maize

Plants in nature have inducible defences that sometimes lead to targeted resistance against particular herbivores, but susceptibility to others. The metabolic diversity and genetic resources available for maize (Zea mays) make this a suitable system for a mechanistic study of within‐species variation in such plant‐mediated interactions between herbivores. Beet armyworms (Spodoptera exigua) and corn leaf aphids (Rhopalosiphum maidis) are two naturally occurring maize herbivores with different feeding habits. Whereas chewing herbivore‐induced methylation of 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one glucoside (DIMBOA‐Glc) to form 2‐hydroxy‐4,7‐dimethoxy‐1,4‐benzoxazin‐3‐one glucoside (HDMBOA‐Glc) promotes caterpillar resistance, lower DIMBOA‐Glc levels favour aphid reproduction. Thus, caterpillar‐induced DIMBOA‐Glc methyltransferase activity in maize is predicted to promote aphid growth. To test this hypothesis, the impact of S. exigua feeding on R. maidis progeny production was assessed using seventeen genetically diverse maize inbred lines. Whereas aphid progeny production was increased by prior caterpillar feeding on lines B73, Ki11, Ki3 and Tx303, it decreased on lines Ky21, CML103, Mo18W and W22. Genetic mapping of this trait in a population of B73 × Ky21 recombinant inbred lines identified significant quantitative trait loci on maize chromosomes 1, 7 and 10. There is a transgressive segregation for aphid resistance, with the Ky21 alleles on chromosomes 1 and 7 and the B73 allele on chromosome 10 increasing aphid progeny production. The chromosome 1 QTL coincides with a cluster of three maize genes encoding benzoxazinoid O‐methyltransferases that convert DIMBOA‐Glc to HDMBOA‐Glc. Gene expression studies and benzoxazinoid measurements indicate that S. exigua ‐induced responses in this pathway differentially affect R. maidis resistance in B73 and Ky21.     

A direction‐selective local‐thresholding method,DSLT, in combination with a dye‐based method for automated three‐dimensional segmentation of cells and airspaces in developing leaves

Quantifying the anatomical data acquired from three‐dimensional (3D) images has become increasingly important in recent years. Visualization and image segmentation are essential for acquiring accurate and detailed anatomical data from images; however, plant tissues such as leaves are difficult to image by confocal or multi‐photon laser scanning microscopy because their airspaces generate optical aberrations. To overcome this problem, we established a staining method based on Nile Red in silicone‐oil solution. Our staining method enables color differentiation between lipid bilayer membranes and airspaces, while minimizing any damage to leaf development. By repeated applications of our staining method we performed time‐lapse imaging of a leaf over 5 days. To counteract the drastic decline in signal‐to‐noise ratio at greater tissue depths, we also developed a local thresholding method (direction‐selective local thresholding, DSLT) and an automated iterative segmentation algorithm. The segmentation algorithm uses the DSLT to extract the anatomical structures. Using the proposed methods, we accurately segmented 3D images of intact leaves to single‐cell resolution, and measured the airspace volumes in intact leaves.     

Comparative Metabolism of Free‐living Bodo saltans and Parasitic Trypanosomatids

Comparison of the genomes of free‐living Bodo saltans and those of parasitic trypanosomatids reveals that the transition from a free‐living to a parasitic life style has resulted in the loss of approximately 50% of protein‐coding genes. Despite this dramatic reduction in genome size, B. saltans and trypanosomatids still share a significant number of common metabolic traits: glycosomes; a unique set of the pyrimidine biosynthetic pathway genes; an ATP‐PFK which is homologous to the bacterial PP_i‐PFKs rather than to the canonical eukaryotic ATP‐PFKs; an alternative oxidase; three phosphoglycerate kinases and two GAPDH isoenzymes; a pyruvate kinase regulated by fructose‐2,6‐bisphosphate; trypanothione as a substitute for glutathione; synthesis of fatty acids via a unique set of elongase enzymes; and a mitochondrial acetate:succinate coenzyme A transferase. B. saltans has lost the capacity to synthesize ubiquinone. Among genes that are present in B. saltans and lost in all trypanosomatids are those involved in the degradation of mureine, tryptophan and lysine. Novel acquisitions of trypanosomatids are components of pentose sugar metabolism, pteridine reductase and bromodomain‐factor proteins. In addition, only the subfamily Leishmaniinae has acquired a gene for catalase and the capacity to convert diaminopimelic acid to lysine.     

Geographically widespread honeybee‐gut symbiont subgroups show locally distinct antibiotic‐resistant patterns

How long‐term antibiotic treatment affects host bacterial associations is still largely unknown. The honeybee‐gut microbiota has a simple composition, so we used this gut community to investigate how long‐term antibiotic treatment affects host‐associated microbiota. We investigated the phylogenetic relatedness, genomic content (GC percentage, genome size, number of genes and CRISPR) and antibiotic‐resistant genes (ARG) for strains from two abundant members of the honeybee core gut microbiota (Gilliamella apicola and Snodgrassella alvi). Domesticated honeybees are subjected to geographically different management policies, so we used two research apiaries, representing different antibiotic treatment regimens in their apiculture: low antibiotic usage (Norway) and high antibiotic usage (Arizona, USA). We applied whole‐genome shotgun sequencing on 48 G. apicola and 22 S. alvi. We identified three predominating subgroups of G. apicola in honeybees from both Norway and Arizona. For G. apicola, genetic content substantially varied between subgroups and distance similarity calculations showed similarity discrepancy between subgroups. Functional differences between subgroups, such as pectin‐degrading enzymes (G. apicola), were also identified. In addition, we identified horizontal gene transfer (HGT) of transposon (Tn10)‐associated tetracycline resistance (Tet B) across the G. apicola subgroups in the Arizonan honeybees, using interspace polymorphisms in the Tet B determinant. Our results support that honeybee‐gut symbiont subgroups can resist long‐term antibiotic treatment and maintain functionality through acquisition of geographically distinct antibiotic‐resistant genes by HGT.     

Major and minor QTL and epistasis contribute to fatty acid compositions and oil concentration in high-oil maize

High-oil maize is a useful genetic resource for genomic investigation in plants. To determine the genetic basis of oil concentration and composition in maize grain, a recombinant inbred population derived from a cross between normal line B73 and high-oil line By804 was phenotyped using gas chromatography, and genotyped with 228 molecular markers. A total of 42 individual QTL, associated with fatty acid compositions and oil concentration, were detected in 21 genomic regions. Five major QTL were identified for measured traits, one each of which explained 42.0% of phenotypic variance for palmitic acid, 15.0% for stearic acid, 27.7% for oleic acid, 48.3% for linoleic acid, and 15.7% for oil concentration in the RIL population. Thirty-six loci were involved in 24 molecular marker pairs of epistatic interactions across all traits, which explained phenotypic variances ranging from 0.4 to 6.1%. Seven of 18 mapping candidate genes related to lipid metabolism were localized within or were close to identified individual QTL, explaining 0.7–13.2% of the population variance. These results demonstrated that a few major QTL with large additive effects could play an important role in attending fatty acid compositions and increasing oil concentration in used germplasm. A larger number of minor QTL and a certain number of epistatic QTL, both with additive effects, also contributed to fatty acid compositions and oil concentration.     

Arabidopsis lipid droplet‐associated protein (LDAP) – interacting protein (LDIP) influences lipid droplet size and neutral lipid homeostasis in both leaves and seeds

Cytoplasmic lipid droplets (LDs) are found in all types of plant cells; they are derived from the endoplasmic reticulum and function as a repository for neutral lipids, as well as serving in lipid remodelling and signalling. However, the mechanisms underlying the formation, steady‐state maintenance and turnover of plant LDs, particularly in non‐seed tissues, are relatively unknown. Previously, we showed that the LD‐associated proteins (LDAPs) are a family of plant‐specific, LD surface‐associated coat proteins that are required for proper biogenesis of LDs and neutral lipid homeostasis in vegetative tissues. Here, we screened a yeast two‐hybrid library using the Arabidopsis LDAP3 isoform as ‘bait’ in an effort to identify other novel LD protein constituents. One of the candidate LDAP3‐interacting proteins was Arabidopsis At5g16550, which is a plant‐specific protein of unknown function that we termed LDIP (LDAP‐interacting protein). Using a combination of biochemical and cellular approaches, we show that LDIP targets specifically to the LD surface, contains a discrete amphipathic α‐helical targeting sequence, and participates in both homotypic and heterotypic associations with itself and LDAP3, respectively. Analysis of LDIP T‐DNA knockdown and knockout mutants showed a decrease in LD abundance and an increase in variability of LD size in leaves, with concomitant increases in total neutral lipid content. Similar phenotypes were observed in plant seeds, which showed enlarged LDs and increases in total amounts of seed oil. Collectively, these data identify LDIP as a new player in LD biology that modulates both LD size and cellular neutral lipid homeostasis in both leaves and seeds.