Combining Genetic and Transcriptomic Approaches to Identify Transporter-Coding Genes as Likely Responsible for a Repeatable Salt Tolerance QTL in Citrus

The excessive accumulation of chloride (Cl-) in leaves due to salinity is frequently related to decreased yield in citrus. Two salt tolerance experiments to detect quantitative trait loci (QTLs) for leaf concentrations of Cl-, Na+, and other traits using the same reference progeny derived from the salt-tolerant Cleopatra mandarin (Citrus reshni) and the disease-resistant donor Poncirus trifoliata were performed with the aim to identify repeatable QTLs that regulate leaf Cl- (and/or Na+) exclusion across independent experiments in citrus, as well as potential candidate genes involved. A repeatable QTL controlling leaf Cl- was detected in chromosome 6 (LCl-6), where 23 potential candidate genes coding for transporters were identified using the C. clementina genome as reference. Transcriptomic analysis revealed two important candidate genes coding for a member of the nitrate transporter 1/peptide transporter family (NPF5.9) and a major facilitator superfamily (MFS) protein. Cell wall biosynthesis- and secondary metabolism-related processes appeared to play a significant role in differential gene expression in LCl-6. Six likely gene candidates were mapped in LCl-6, showing conserved synteny in C. reshni. In conclusion, markers to select beneficial Cleopatra mandarin alleles of likely candidate genes in LCl-6 to improve salt tolerance in citrus rootstock breeding programs are provided.

Norovirus detection in environmental samples in norovirus outbreaks in closed and semi-closed settings

BACKGROUND

Environmental surfaces are a potential vehicle for the transmission of norovirus outbreaks in closed and semi-closed settings. Testing of environmental samples may help control outbreaks.

AIM

To assess the level of environmental contamination by norovirus in acute gastroenteritis outbreaks in closed or semi-closed settings (nursing homes, schools, kindergartens, youth accommodations, hospitals and social health centres) in the Barcelona region between January 2017 and March 2019.

METHODS

A prospective surveillance study was carried out. Environmental samples (529) were collected in 46 of the 50 outbreaks of acute norovirus gastroenteritis from environmental surfaces of common areas, bathrooms and kitchens in closed and semi-closed settings when the outbreak was notified and 10 days later. Instructions for taking environmental samples were distributed to public health inspectors. Norovirus was detected by reverse transcription polymerase chain reaction.

FINDINGS

Environmental samples were positive for norovirus in 31 (67.4%) outbreaks. Norovirus was most frequently detected on elevator buttons (4/17, 24%), toilet handles (16/66, 24%) and handrail bars (7/34, 21%). Positive samples from the first sampling were mainly found in bathrooms and greater viral persistence in the second sampling was found on elevator buttons and TV remote controls. Nursing homes were the setting with the most types of environmental surfaces contaminated (82% in first samples and 55% in second samples).

CONCLUSION

The probability of virus detection is independent of the time between notification of the outbreak or symptom onset and sample collection. Our results suggest possible defects in cleaning protocols and disinfection in closed and semi-closed settings.

Genetic dissection of tomato rootstock effects on scion traits under moderate salinity

Rootstock HKT1 genotype affected fruit [Na(+)] and non-commercial fruit yield; QTL analysis of rootstock-mediated scion nutrition is a powerful forward genetic approach to identify wild genes for rootstock breeding. The present study approaches the QTL dissection of rootstock effects on a commercial hybrid variety grafted on a population of RILs derived from Solanum pimpinellifolium, genotyped for 4370 segregating SNPs from the SolCAP tomato panel and grown under moderate salinity. Results are compared to those previously obtained under high salinity. The most likely functional candidate genes controlling the scion [Na(+)] were rootstock HKT1;1 and HKT1;2 as it was previously reported for non-grafted genotypes. The higher fruit [Na(+)] found when rootstock genotype was homozygote for SpHKT1 supports the thesis that scion HKT1 is loading Na(+) into the phloem sap in leaves and unloading it in sink organs. A significant increment of small, mostly seedless, fruits was found associated with SlHKT1 homozygous rootstocks. Just grafting increased the incidence of blossom end rot and delayed fruit maturation but there were rootstock RILs that increased commercial fruit yield under moderate salinity. The heritability and number of QTLs involved were lower and different than those found under high salinity. Four large contributing (>17%) rootstock QTLs, controlling the leaf concentrations of B, K, Mg and Mo were detected whose 2 Mbp physical intervals contained B, K, Mg and Mo transporter-coding genes, respectively. Since a minimum of 3 QTLs (two of them coincident with leaf K and Ca QTLs) were also found governing rootstock-mediated soluble-solids content of the fruit under moderate salinity, grafting desirable crop varieties on stress-tolerant rootstocks tenders an opportunity to increase both salt tolerance and quality.

Heterologous expression of the yeast HAL5 gene in tomato enhances salt tolerance by reducing shoot Na+ accumulation in the long term

For salt tolerance to be achieved in the long-term plants must regulate Na(+)/K(+) homeostasis over time. In this study, we show that the salt tolerance induced by overexpression of the yeast HAL5 gene in tomato (Solanum lycopersicum) was related to a lower leaf Na(+) accumulation in the long term, by reducing Na(+) transport from root to shoot over time regardless of the severity of salt stress. Furthermore, maintaining Na(+)/K(+) homeostasis over time was associated with changes in the transcript levels of the Na(+) and K(+) transporters such as SlHKT1;2 and SlHAK5. The expression of SlHKT1;2 was upregulated in response to salinity in roots of transgenic plants but downregulated in the roots of wild-type (WT) plants, which seems to be related to the lower Na(+) transport rate from root to shoot in transgenic plants. The expression of the SlHAK5 increased in roots and leaves of both WT and transgenic plants under salinity. However, this increase was much higher in the leaves of transgenic plants than in those of WT plants, which may be associated with the ability of transgenic leaves to maintain Na(+)/K(+) homeostasis over time. Taken together, the results show that the salt tolerance mechanism induced by HAL5 overexpression in tomato is related to the appropriate regulation of ion transport from root to shoot and maintenance of the leaf Na(+)/K(+) homeostasis through modulation of SlHKT1 and SlHAK5 over time.

Genetic analysis of Na(+) and K (+) concentrations in leaf and stem as physiological components of salt tolerance in Tomato

The sodium and potassium concentrations in leaf and stem have been genetically studied as physiological components of the vegetative and reproductive development in two populations of F(8) lines, derived from a salt sensitive genotype of Solanum lycopersicum cv. Cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (142 lines), and S. cheesmaniae, the C population (116 lines). Genetic parameters of ten traits under salinity and five of them under control conditions were studied by ANOVA, correlation, principal component and QTL analysis to understand the global response of the plant. Two linkage maps including some tomato flowering time and salt tolerance candidate genes encoding for SlSOS1, SlSOS2, SlSOS3, LeNHX1, LeNHX3, were used for the QTL detection. Thirteen and 20 QTLs were detected under salinity in the P and C populations, respectively, and four under control conditions. Highly significant and contributing QTLs (over 40%) for the concentrations of Na(+) and K(+) in stems and leaves have been detected on chromosome 7 in both the populations. This is the only genomic position where the concentration QTLs for both the cations locate together. The proportion of QTLs significantly affected by salinity was larger in the P population (64.3%, including all QTLs detected under control) than in the C population (21.4%), where the estimated genetic component of variance was larger for most traits. A highly significant association between the leaf area and fruit yield under salinity was found only in the C population, which is supported by the location of QTLs for these traits in a common region of chromososome C1. As far as breeding for salt tolerance is concerned, only two sodium QTLs (lnc1.1 and lnc8.1) map in genomic regions of C1 and C8 where fruit yield QTLs are also located but in both the cases the profitable allele corresponds to the salt sensitive, cultivated species. One of those QTLs, lnc1.1 might involve LeNHX3.

Effects of Boron on Proton Transport and Membrane Properties of Sunflower (Helianthus annuus L.) Cell Microsomes

Boron deficiency and toxicity inhibit ATP-dependent H+ pumping and vanadate-sensitive ATPase activity in sunflower roots and cell suspensions. The effects of boron on H+ pumping and on passive H+ conductance, as well as on fluorescence anisotropy in KI-washed microsomes isolated from sunflower (Helianthus annuus L. cv Enano) cell suspensions, have been investigated. Boron deficiency reduced the total and vanadate-sensitive ATPase activities as well as the vanadate-sensitive ATP-dependent H+ pumping without affecting the amount of antigenic ATPase protein as measured by immunoblotting with an Arabidopsis thaliana plasma membrane anti-H+-ATPase polyclonal antibody. Kinetic studies revealed that boron deficiency reduced Vmax of vanadate-sensitive ATPase activity with little change in the apparent Km for Mg2+-ATP. Proton leakage was greater in microsomal vesicles isolated from cells grown without boron and incubated in reaction medium without added boron, and this effect was reversed by addition of boron to the reaction medium. Fluorescence anisotropy indicated that diphenyl hexatriene and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene probes were immobilized to a greater extent in microsomes from cells grown without boron than in those from cells grown with 100 [mu]M H3BO3. The apparent decrease of membrane fluidity in microsomes from cells grown without boron was reversed by the addition of boron to the reaction medium. Taken together these data suggest that inhibition of H+ gradient formation in microsomes from sunflower cells grown in the absence of boron could be due to the combined effects of reduced H+-ATPase activity and increased passive conductance across the membrane, possibly resulting from increased membrane rigidity.

Characterization of proton extrusion in sunflower cell cultures

Cell suspensions derived from callus root tips of sunflower (Helianthus annuus L., cv. enano) were obtained in order to assess the effects of different chemical and physical agents on cell H+ extrusion. Cell H+ efflux was sensitive to temperature, pH, inhibitors of plasmalemma H(+)-ATPase and Ca2+ and K+ concentrations in the assay medium, as well as to the light intensity at which cells were cultivated. Thus, in the darkness and at 60 mumol/m2/s of illumination, a strong inhibition of H+ extrusion was detected as compared to cells grown at 30 mumol/m2/s. H+ extrusion by cells grown at 30 mumol/m2/s was unaffected by the presence of calcium in the assay medium, while at 60 mumol/m2/s such an activity increased when calcium was removed. These results provide the basis for the use of cell suspensions as an appropriate model to investigate the involvement of membrane-associated processes in plant tolerance mechanisms to different environmental stresses.

Effect of temperature conditioning on chilling injury of cucumber cotyledons: possible role of abscisic Acid and heat shock proteins

Endogenous abscisic acid levels and induced heat shock proteins were measured in tissue exposed for 6 hours to temperatures that reduced their subsequent chilling sensitivity. One-centimeter discs excised from fully expanded cotyledons of 11-day-old seedlings of cucumber (Cucumis sativus L., cv Poinsett 76) were exposed to 12.5 or 37 degrees C for 6 hours followed by 4 days at 2.5 or 12.5 degrees C. Ion leakage, a qualitative indicator of chilling injury, increased after 2 to 3 day exposure to 2.5 degrees C, but not to 12.5 degrees C, a nonchilling temperature. Exposure to 37 degrees C before chilling significantly reduced the rate of ion leakage by about 60% compared to tissue exposed to 12.5 degrees C before chilling, but slightly increased leakage compared to tissue exposed to 12.5 or 37 degrees C and held at the nonchilling temperature of 12.5 degrees C. There was no relationship between abscisic acid content following exposure to 12.5 or 37 degrees C and chilling tolerance. Five heat shock proteins, with apparent molecular mass of 25, 38, 50, 70, and 80 kilodaltons, were induced by exposure to 37 or 42 degrees C for 6 hours, and their appearance coincided with increased chilling resistance. Heat shock treatments reduced the synthesis of three proteins with apparent molecular mass of 14, 17, and 43 kilodaltons. Induction of heat shock proteins could be a possible cause of reduced chilling injury in tissue exposed to 37 or 42 degrees C.

Cell wall acyl-lipids, proteins and polysaccharides in mature and germinated olive pollen

Proteins, acyl-lipids and polysaccharides from cell walls of mature and germinated olive pollen were studied. In general, hemicelluloses are the most abundant polysaccharides, arabinose in mature and glucose in germinated pollen being the main components of these macromolecules. Protein content and its amino acid composition are very similar in walls from mature and germinated pollen, these compounds showing a weak acid character. Free-fatty acids are the most abundant lipid molecules in mature and germinated pollen walls and a decrease in acyl-lipids, especially in polar lipids, as well as a higher unsaturation of their fatty acid components are observed after germination.

Preparation of Corn Root Plasmalemma with Low Mg-ATPase Latency and High Electrogenic H Pumping Activity after Phase Partitioning

Crude plasma membranes of corn (Zea mays L.) roots were obtained according to MI De Michelis and RM Spanswick (1986 Plant Physiol 81: 542-547). This preparation, which contained tightly sealed vesicles displaying Mg-ATP dependent H(+)-transport, was purified by phase partitioning. The percentage of inside-out vesicles (10%) was determined from the Mg-ATPase latency, revealed with lysophosphatidylcholine. A Triton X-100 treatment described previously (JP Grouzis, R Gibrat, J Rigaud, C Grignon 1987 Biochim Biophys Acta 903: 449-464) was applied to phase-partitioned plasma membranes. The percentage of catalytic sites freely accessible to Mg-ATP increased to 50% after Triton X-100 treatment. Treated vesicles remained capable of electrogenic H(+)-pumping, as demonstrated by Mg:ATP-dependent quinacrine fluorescence quenching and oxonol absorbance shift. As expected from the large increase of the catalytic sites accessibility, increases of the dye responses were observed. Concanavalin A binding was estimated from microelectrophoretic measurements of individual vesicles. Statistical analysis of concanavalin A binding and Mg-ATPase latency suggest that treated membranes have lost their asymmetric structure.

Lipid biosynthesis and composition in oil bodies and microsomes of olive fruit

Both lipid synthesis and composition in oil bodies and microsomes of olive fruit at the first stage of development have been studied. The rate of fatty-acid synthesis in isolated oil bodies was saturated by 4.0 microM [2-14C]-malonyl-CoA. The fatty-acids synthesized of phospholipids and neutral lipids were saturated and monounsaturated. Neutral lipids, galactolipids and, above all, phospholipids were the major acyl-lipid components of microsomal fraction, oleic and palmitic being their principal fatty-acids. When the lipids of microsomes were labelled in vivo with [1-14C]-acetate, phospholipids and neutral lipids exhibited a higher biosynthesis rate relative to the galactolipids. The increase in saturated and monounsaturated fatty-acid synthesis in microsomes, was also accompanied by an important [1-14C]-acetate incorporation into polyunsaturated acids. The data presented here, in conjunction with our previous morphological results, suggest the possibility that olive fruit oil bodies could contain the necessary enzymes for the reserve lipid biosynthesis.

Lipid biosynthesis, oxidative enzyme activities and cellular changes in growing olive fruit

Fruit of Olea europea L. was examined by light and electron microscopy to determine whether commencement of lipid accumulation depended upon the fruit achieving structural maturity. Maturation of fruit develops progressively from the smallest changes towards the largest in cellular structures. Important metabolic and structural changes have been observed: oil body formation, changes in the structural and reserve lipid biosynthesis and in the fatty acid of total lipid content, as well as in G6PDH and LOX activities. The labelling of fruit lipids by previously incubating the leaves with (1-14C)-acetate and (1,5-14C)-citrate or by putting the labelled substrates directly on the fruit surface, shows a 14C assimilate derived from acetate greater than that from citrate; the incorporation of the latter is higher in the methanol-water fractions. At the beginning of fruit development the lipid biosynthesis with both substrates is greater in polar lipids; on the contrary, the incorporation of 14C into neutral lipids increases during fruit maturation. Additionally, a maximum of substrate export from leaves to fruit coincides with an increase in the lipoxygenase and, above all, in the glucose-6-phosphate dehydrogenase activities. The transported 14C from leaves begins its activity before the small oil bodies close to the tonoplast can be observed in the fruit, and well before the beginning of maturation. The results suggest that structural development and some other rate controlling metabolic steps can govern the initiation of lipid accumulation in olive fruit.