When is a land plant not a land plant?

When it’s an eelgrass.

OK, this needs some explanation. A popular notion is that the kingdom Plantae is essentially synonymous with the term land plants. And that is understandable; after all, this group of organisms is arguably all about the conquest of the land and adaptations to a terrestrial existence by entities that had previously lived an aquatic life. That is in accord with the consensus view of plant evolution.

Whilst it is usually explained, after due consideration of the myriad adaptations to terrestrial life, that some land plants either never fully escaped their watery legacy (e.g. the bryophytes, the “amphibians of the plant world“, and others have – secondarily – gone back to an aquatic life – e.g. plants of freshwater rivers and lakes, and those of marine environments, such as eelgrasses), that is all too often forgotten and leaves the impression that land plants are plants of the land.* Even though the more enlightened view is that this grouping should be broadened to also include the green algae –which contain the putative ancestors of true plants, and which are anything but terrestrial – and the ‘land plants’ themselves, that still retains that potentially misleading land adjective for all but the clearly aquatic algae.

In order to disabuse those who might be of the view that land plant necessarily means just land-dwelling, this ever so slightly mischievous piece showcases the sequencing of the genome of Zostera marina (common eelgrass), a marine angiosperm, by Jeanine Olsen et al.

Taking almost a decade to complete because of technical obstacles, the study extends considerably the earlier work by Lothar Wissler et al. [but which publication is curiously not cited in Olsen et al.], and reveals a lot about the steps involved in de-adapting from a terrestrial environment. For example, Zostera has lost the genes involved in production of stomata (which structures would be redundant in a plant that has no problems of water loss and which can absorb gases directly from the seawater into its photosynthetic epidermis**), genes that provide UV protection, and those involved in far-red light sensing (both wavelengths that don’t penetrate to the seabed-rooted, submarine plants). In adapting to its new home – which, surrounded by saline water, is about as far removed from a dry, terrestrial habitat as it is possible to get – the eelgrass has been drastically redesigned, e.g. with development of functions that enable them to cope with a fully saline habitat. All of which is great testament to the adaptability of plants, and a reminder that not all ‘land plants’ are necessarily land-based.

* And let’s not forget those aerial ‘land’ plants that are epiphytes upon the outer surfaces of trees – a lifestyle that’s not exactly land-based either.

** Yes, I do realise what I’ve said in that rather casual throw-away remark(!). The biology of Zostera in particular, and seagrasses more widely, is an amazing aspect of botany and deserving of more attention than it generally gets in broad plant biology courses. So, for more, why not try http://plantsinaction.science.uq.edu.au/edition1/?q=content/18-2-2-ecophysiology-seagrasses?