Hexokinase (HXK) is a sugar-phosphorylating enzyme involved in sugar-sensing. It has recently been shown that HXK in guard cells mediates stomatal closure and coordinates photosynthesis with transpiration in the annual species tomato and Arabidopsis. To examine the role of HXK in the control of the stomatal movement of perennial plants, we generated citrus plants that express Arabidopsis HXK1 (AtHXK1) under KST1, a guard cell-specific promoter. The expression of KST1 in the guard cells of citrus plants has been verified using GFP as a reporter gene. The expression of AtHXK1 in the guard cells of citrus reduced stomatal conductance and transpiration with no negative effect on the rate of photosynthesis, leading to increased water-use efficiency. The effects of light intensity and humidity on stomatal behavior were examined in rooted leaves of the citrus plants. The optimal intensity of photosynthetically active radiation and lower humidity enhanced stomatal closure of AtHXK1-expressing leaves, supporting the role of sugar in the regulation of citrus stomata. These results suggest that HXK coordinates photosynthesis and transpiration and stimulates stomatal closure not only in annual species, but also in perennial species.

SUMMARYThe loss of rooting capability following the transition from the juvenile to the mature phase is a known phenomenon in woody plant development. Eucalyptus grandis was used here as a model system to study the differences in gene expression between juvenile and mature cuttings. RNA was prepared from the base of the two types of cuttings before root induction and hybridized to a DNA microarray of E. grandis. In juvenile cuttings, 363 transcripts were specifically upregulated, enriched in enzymes of oxidation/reduction processes. In mature cuttings, 245 transcripts were specifically upregulated, enriched in transcription factors involved in the regulation of secondary metabolites. A gene encoding for nitrate reductase (NIA), which is involved in nitric oxide (NO) production, was among the genes that were upregulated in juvenile cuttings. Concomitantly, a transient burst of NO was observed upon excision, which was higher in juvenile cuttings than in mature ones. Treatment with an NO donor improved rooting of both juvenile and mature cuttings. A single NIA gene was found in the newly released E. grandis genome sequence, the cDNA of which was isolated, overexpressed in Arabidopsis plants and shown to increase NO production in intact plants. Therefore, higher levels of NIA in E. grandis juvenile cuttings might lead to increased ability to produce NO and to form adventitious roots. Arabidopsis transgenic plants constantly expressing EgNIA did not exhibit a significantly higher lateral or adventitious root formation, suggesting that spatial and temporal rather than a constitutive increase in NO is favorable for root differentiation.

A detailed phenotypic analysis of fruit-quality traits was conducted among 46 mandarin varieties within the Israeli Citrus breeding collection, belonging to genetically different natural subgroups, including common mandarin (C. reticulata Blanco), clementine (C. clementina Hort. ex. Tan), satsuma (C. unshiu Marcovitch), Mediterranean mandarin (C. deliciosa Tenore), King mandarin (C. nobilis Loureiro), and mandarin hybrids, such as tangor (C. reticulata × C. sinensis) and tangelo (C. reticulata × C. paradisi). Evaluated qualities included physical attributes (size, shape, color, peel thickness, and seed number); physiological properties (ripening period, peelability, and segmentation); nutritional and biochemical composition (vitamin C, phenol, flavonoid, and carotenoid contents and total antioxidant activity); and sensory attributes (total soluble solids and acid levels, flavor preference, sweetness, sourness, and fruitiness). The results indicated wide genetic variability in fruit-quality traits among mandarin varieties and natural subgroups, and statistical and hierarchical clustering analysis revealed multiple correlations among attributes. Such phenomic analysis is an obligatory requirement for identification of molecular markers for distinct fruit-quality traits and for selection of appropriate parents for future breeding programs.

During the last decade, there has been a continuous rise in consumption and global marketing of fresh, easy-to-peel mandarins, with current annual production of nearly 29 million tons. Nevertheless, most of the existing knowledge on quality traits of citrus fruit comes from research conducted on oranges and grapefruit, which are the main products for the citrus juice manufacturing industry; relatively little is yet known regarding the unique fruit quality traits of mandarins, nor about the great diversity in these traits among the various natural sub-groups and varieties of mandarins. In the present review we discuss the physiological, biochemical, and molecular factors governing key fruit quality attributes of mandarins, including fruit colour, size and shape, ease of peeling, seedlessness, flavour, and nutritional quality. Fruit colour, size, and shape contribute to external appearance; peelability and seedlessness to ease of consumption; and flavour and nutritional quality to internal quality.

BackgroundThe ability to form adventitious roots (AR) is an economically important trait that is lost during the juvenile-to-mature phase change in woody plants. Auxin treatment, which generally promotes rooting in juvenile cuttings, is often ineffective when applied to mature cuttings. The molecular basis for this phenomenon in Eucalyptus grandis was addressed here.ResultsA comprehensive microarray analysis was performed in order to compare gene-expression profiles in juvenile and mature cuttings of E. grandis, with or without auxin treatment on days, 0, 1, 3, 6, 9 and 12 post AR induction. Under these conditions AR primordia were formed only in auxin-treated juvenile cuttings. However, clustering the expression profiles revealed that the time after induction contributed more significantly to the differences in expression than the developmental phase of the cuttings or auxin treatment. Most detected differences which were related to the developmental phase and auxin treatment occurred on day 6, which correlated with the kinetics of AR-primordia formation. Among the functional groups of transcripts that differed between juvenile and mature cuttings was that of microtubules (MT). The expression of 42 transcripts annotated as coding for tubulin, MT-associated proteins and kinesin motor proteins was validated in the same RNA samples. The results suggest a coordinated developmental and auxin dependent regulation of several MT-related transcripts in these cuttings. To determine the relevance of MT remodeling to AR formation, MTs were subjected to subtle perturbations by trifluralin, a MT disrupting drug, applied during auxin induction. Juvenile cuttings were not affected by the treatment, but rooting of mature cuttings increased from 10 to more than 40 percent.ConclusionsThe data suggest that juvenile-specific MT remodeling is involved in AR formation in E. grandis.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-826) contains supplementary material, which is available to authorized users.