This column is always happy to promote the underdog, especially if that neglected item is a bit of plant anatomy. So, let’s give a bit of space to the hydathode. Hydathodes are openings within the epidermis of plants that release water during the process known as guttation. Although the opening/closing mechanism of stomata is stated as being absent from hydathodes, structurally and superficially hydathodes do look like stomata. In many respects, hydathodes can be considered the ‘pore relations’ of stomata, and are sometimes alternatively named ‘water stomata’. Hydathodes aren’t much talked about nowadays and live very much in the shadow of their more high-profile, gas-exchange-controlling neighbours. However, recent discoveries mean they hydathodes can no longer be ignored, at least in the context of plant infection by microbes.
Investigating hydathodes in Brassica oleracea var. botrytis cultivar Clovis (cauliflower and Arabidopsis thaliana (ecotype Col-0, ‘the plant equivalent of the laboratory rat’), Aude Cerutti et al. demonstrate the importance of these structures as potential infection routes to plants. In particular they show that the hydathodal pore did not close when challenged with flg22 flagellin peptide (a component of bacterial flagella that can trigger plant immunity responses to microbial infection). This behaviour contrasts with the stomata, which do close in response to flg22, and which thereby gain some protection against entry of potentially harmful microbes.
Apart from emphasising the ‘open door’ to microbial infection that hydathodes pose* why should we be surprised that the hydathodal pore doesn’t close (after all, the textbooks say they don’t possess this property)? Well, interestingly, Cerutti et al. (2017) also show that Arabidopsis hydathodes close in response to ABA (abscisic acid), and open further in response to light, both in a manner similar to stomata (a discovery which contradicts the textbooks!)** Hydathodes, not such an open-and-shut case after all?
* Although both stomata and hydathodes have long been recognised as potential entry routes for plant-infecting microbes, at least as far back as the late 19th Century (Russell 1898 cited in Cerutti et al. (2017)), this puts a 21st century molecular gloss on that work in considering hydathodal entry by Xanthomonas campestris pv. campestris (which causes black rot disease in crucifers, such as cauliflower and Arabidopsis).
** However, in contrast to stomata, these hydathodes were still open in darkness. Nevertheless, the apparent similarities in biology of these two epidermal structures leads one to wonder when hydathodes and stomata diverged in terms of function, and which developed first? And are the ‘stomata’ observed in the surfaces of plant fossils (and which have been around in the Plant Kingdom for >400 million years), actually stomata or hydathodes?