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Making T provides breathing space
Discussion of aerenchyma formation usually focuses on two mechanisms. In the lysigenous mechanism, air-conducting spaces are formed by selective cell death, whereas in the schizogenous mechanism, cell layers split apart. However, a third mechanism, based on cell expansion, is also known. The rather cumbersome term expansigenous aerenchyma has been coined and although it was first discovered over 100 years ago, relatively little is known about it. One species that exhibits this type of aerenchyma formation is the sponge gourd Luffa cylindrica. This is a flood-tolerant member of the Cucurbitaceae and subject of a study by Shimamura et al. (Ibakari and Fukuoka, Japan, pp. 1431–1439). The authors transferred young plants to ‘continuously waterlogged conditions’ with the water surface 10—20 mm above the soil. Within 4 days, new adventitious roots began to appear and as these roots developed they became very porous. There was also an increase in the porosity of the hypocotyl. Slits developed in the outer layers of the hypocotyl and of adventitious roots, possibly allowing direct access of O2 to internal tissues. Detailed observations over a period of 16 days showed that aerenchyma was formed by outward radial expansion of cortical cells. In the hypocotyl this was a ‘normal’ expansion that, to a limited extent, pushed apart some of the cells in layers external to the cortex to form aerenchyma. However, in the adventitious roots the pattern of radial expansion was very different. Expansion was confined to a particular zone on the outermost cell surfaces, resulting in the outgrowth of a protuberance that pushes against the next cell layer. The cortical cells thus became T-shaped and air spaces were formed between the protuberances of adjacent cells to give an extensive aerenchyma. It will be very interesting to know how this specific and fascinating pattern of cortical cell growth is regulated but, as the authors clearly state, we currently know nothing about its physiological control.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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