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Life is sweet – but short
In an era in which our science is apparently dominated by studies at the molecular level, it is easy to assume that investigations of plant structure can teach us relatively little. However, such an assumption would be wrong. The renaissance in microscopy and the interfaces between microscopy, ultra-microscopy, computing and modelling (and indeed, with molecular studies) are providing exciting new information about plant anatomy, morphology and development. Aspects of this are beautifully illustrated in the paper by Gaffal et al. (Erlangen and Freudenberg, Germany, pp. 593–607). They have investigated the relationship between the phloem and the floral nectaries in Digitalis purpurea. In this species, the nectaries are temporary organs and so not only is there the question of how they receive sucrose during their active phase but what happens to them when they cease to function. It is not possible in this short commentary to do justice to the authors’ elegant and very thorough work but the key findings were as follows. In the secreting nectary, the nectiferous layer is located below the epidermis. However, it is not in direct contact with the phloem sieve elements that deliver the sucrose. Instead, the latter are abutted by intercellular spaces or ducts, suggesting that transfer from phloem to nectiferous cells is via an apoplastic route, a suggestion supported by calculations on the required rates of sucrose delivery. However, the direction of the traffic changes as the nectary becomes exhausted. Even during the nectar-secretion phase, the nectary exhibits early signs of senescence, and after it is exhausted the cells show the typical progression of developmentally regulated programmed cell death, including increased vacuolation, loss of internal structural integrity, degradation of plastids, breakdown and eventual disappearance of chromatin, finally culminating in complete lysis of the cell contents. The stillintact phloem is thus able to take up the soluble lysis products and redistribute them within the plant.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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