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Patchy patterns of protective products

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The evolutionary response of many land plants to UV-exposure has been the development of ability to synthesize UV-screening compounds. Further, many plants accumulate higher concentrations of these compounds in response to higher doses of UV radiation. Phenolic compounds present in leaf epidermis form a large and vital part of this UV-protective chemical ‘armour’; however, we know very little about how the distribution of these compounds may change during the life of the leaf. To remedy this deficiency, Meyer et al. (Orsay and Paris, pp. 621–633) have developed imaging techniques to monitor epidermal UV-absorbance during the development of Quercus petraea (oak) leaves. Young leaves exhibited high UV-absorbance in the upper epidermis. This declined slightly during leaf development. UV-absorbance was initially much lower in the lower epidermis but rose during development to reach the same level as the upper epidermis. However, these general patterns hide a more complex distribution at the cellular level. In young leaves, the distribution of epidermal UV-absorbance was patchy on both sides of the leaf. This patchiness was ascribed to asynchrony in the development of epidermal cells and in particular in development of their ability to accumulate flavonoids in their vacuoles. On the upper leaf surface it is clear that this ability spreads out from the epidermal cells located above the veins, suggesting a source–sink relationship at cellular level. Whatever the cause, this patchiness implies that some cells in young leaves may be more vulnerable to UV. Later in development, UV-absorbance was uniform across the leaf as the slower-developing cells caught up with the leaders. Later still, the leaves' spectral characteristics implied that flavanoids were supplemented with hydroxycinnamic acids which, among other functions, may provide an additional UV-screen at the leaf surface. Finally, the authors point out that these data can be used ‘in reverse’ by providing parameters for monitoring leaf development non-invasively in the field.

Ethylene puts up SUTs in rubber tap

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The sight of people wearing ‘Wellington’ boots in the streets of Cambridge on a recent rainy day reminded me of just how commonplace and familiar a substance is rubber. This familiarity may make us forget that it is also very unusual: a complex isoprenoid compound made in the cytoplasm of specific cells called lactifers. Further, this cytoplasm is extruded if the bark of the rubber tree is wounded or deliberately tapped; the cytoplasmic contents must then be replenished. Sucrose is very important for latex synthesis, as discussed by Dusotoit-Coucaud et al., a joint French–Thai group (pp. 635–647). Not only is it a key precursor for the two latex biosynthetic pathways but it also contributes to the high turgor pressure of the lactifers. Latex production is stimulated by ethylene and the authors have asked whether ethylene affects sucrose uptake into the lactifers. Here we focus on a selection of their results. A latex-specific cDNA library was the source from which they cloned seven cDNAs encoding sucrose transporters. Real-time RT–PCR showed that two of these, HbSUT1A and HbSUT1B, were expressed strongly in latex and bark of virgin (untapped) trees. The others were expressed at a very low level. Trees were treated with ethylene by applying Ethrel© to the bark, and were then tapped. Sixteen hours after treatment, expression of glutamine synthase, already known to be stimulated by ethylene, was seen in latex. A marked increase in the expression in latex of HbSUT1A and HbSUT2A was seen following the ethylene treatment, while expression of HbSUT1B, strongly expressed in untreated trees, was decreased. These effects did not occur in other tissues and did not happen if trees were treated with other hormones. Thus, ethylene stimulation of latex synthesis is clearly associated with regulation of genes encoding specific sucrose transporters.

Sequence studies reveal old oak crosses

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Hybridization followed by back-crossing to one of the parental species is a technique widely used by plant breeders to achieve introgression of desired traits into crop species. This can also happen in nature. Further, evidence for ancient introgressive gene flow can be obtained from analysis of chloroplast DNA (because of its uniparental inheritance and lack of recombination) while nuclear DNA can provide information on more recent events. These sources of information have been utilized by Lumaret and Jabbour-Zahab at Montpellier (pp. 725–736), working with Quercus suber and Q. ilex. These two oak species have different but overlapping ranges in the Mediterranean region and occasionally hybridize. The authors analysed chloroplast DNA sequences and the sequences of ten nuclear microsatellites in populations of both species. For chloroplast DNA, there is more interpopulation variation in Q. suber than in Q. ilex; however, in two regions, Catalonia and SW Morocco, many Q. suber individuals possess Q. ilex chloroplast DNA, indicating an ancient hybridization followed by back-crossing into the Q. suber population. Palynological analysis, providing evidence for ancient distributions, shows the feasibility of this idea and further suggests that the hybridization and back-crossing events may have occurred during the succession of glacial periods during the quaternary era. For nuclear DNA microsatellites it was Q. ilex that showed by far the greater variation. Analysis of individuals across the geographical range suggested that in areas where species co-exist, 1 % of individuals are F1 hybrids while approx. 4 % of individuals of both species showed evidence of bidirectional gene flow. These recent hybridizations have occurred predominantly in mixed stands and in all areas in which the species' ranges overlap. The differences between ancient and modern gene flow patterns lead the authors to suggest that introgression of Q. ilex chloroplast DNA into Q. suber occurred under selective pressure imposed by the glacial periods and enabled Q. suber to survive the colder periods.

ITS still a mystery down under

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The two ‘fairy rings’ that have appeared on my lawn are a frequent reminder of the teeming fungal life beneath the soil surface and of the fact that much of that life remains unknown. Indeed, even though it is estimated that over 92 % of land plants require mycorrhizal associations, there is a lot that we do not know about these relationships. Amongst angiosperm families, the Orchidaceae have a particularly strong reliance on mycorrhizal symbionts, as discussed by Huynh et al., a group based in the State of Victoria, Australia (pp. 757–765). They have worked with an endangered orchid species, Caladenia formosa, a member of a genus in which the plants do not form roots: mycorrhizal symbionts colonize the stem collar – the base of the plant's single leaf. For ex situ recovery of this species, it is important to understand the specific roles of symbionts at different stages of plant life. Stem collars were collected at different seasons and mycorrhizal fungi were isolated. The data patterns were complex, with variations between seasons and years of collection; however, it was very clear that individual stem collars yielded multiple mycorrhizal isolates. Different isolates from an individual collar varied considerably in their ability to promote germination and to support subsequent plant growth; further, the efficacy of an isolate to promote germination was not a guarantee of efficacy in aiding further growth and survival. Restriction fragment and sequence analysis of the internal transcribed spacer (ITS) of the rRNA genes gave a tight phylogenetic clustering with sequence similarities of 79–89 % with Sebacina vermiforma; thus, although closely related to the latter, the isolates cannot be regarded as conspecific with it. Finally, closeness of ITS sequence between two isolates was not an indicator of similar efficacy in promoting germination or growth. Thus, as with the fairy rings in my lawn, much remains to be discovered.

ContentSelect Extra: Public participation proves petal point

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There are areas of modern science in which amateurs have not only made a contribution but in some instances, exemplified by the discovery of comets, beaten the professionals to it. There are also research projects in which people can participate without owning expensive equipment (e.g. powerful telescopes). One such is reported by John Warren, working at Aberystwyth (pp. 785–788). In a widely disseminated appeal he recruited members of the public to count the petals on Ranunculus repens in meadows of known age. The data showed that the presence of one plant per hundred with extra petals equates to a meadow age of 7 years – an interesting and useful result that could have only been obtained with the help of many amateur botanists.


Articles from Annals of Botany are provided here courtesy of Oxford University Press