Xerophyta is a genus of monocot plants in the family Vellociazeae, consisting of around 50 species found across Africa, Madagascar, and the Arabian Peninsula. They are closely related to the Vellozia genus of plants found in Southern America, primarily Brazil, and likely diverged after the break-up of the Gondwana supercontinent. Like their New World counterparts, all Xerophyta species are well-adapted to xeric environments and all tested species display vegetative desiccation tolerance (VDT). As many important crops are also monocots, investigating the mechanisms behind VDT in monocot resurrection plants could be an important step in transferring this knowledge to model plants.
Two Xerophyta species are the focus of the RESIST project. Xerophyta humilis (or “reënmeterjie” – “rain gauge” in English) is a poikilochlorophyllous resurrection plant found in northern South Africa and Namibia. It is a small, low-growing perennial found in mats in arid, shallow-soiled grasslands. It has slender, waxy leaves which fold down the middle when dry. Plants will often bloom immediately after rain, producing small, star-shaped purple-white flowers. Poikilochlorophylly is assumed to be a necessary adaption to prevent photooxidative damage in this plant (and other Xerophyta species) since they will often spend long periods in the desiccated state, uncovered and in high-irradiance areas. X. humilis has been the center of several previous studies, including microarray and RNA-Seq. Most recently, transcriptomics analysis was used to compare gene expression in maturing seeds and desiccating leaves of X. humilis plants (Lyall et al., 2020). This is the first study to analyse gene expression during seed maturation in a resurrection plant and showed that while the protective mechanisms induced during desiccation in leaves are similar to those in seeds, the regulatory mechanisms appear to be different.
Picture 1: Flowering Xerophyta humilis in its wild habitat.
The second species, Xerophyta elegans, is one of the few homoichlorophyllous Xerophyta species and is endemic to South Africa. X. elegans is larger than X. humilis, with wider, longer leaves and larger white, star-shaped flowers. Unlike X. humilis, it grows in shaded, wet areas, usually on rock faces and often near water - habitats superficially similar to those of H. rhodopensis. Poikilochlorophylly would be unnecessary in such an environment, though whether X. elegans retains an ancestral homoichlorophyllous trait or lost the poikilochlorophyllous trait is unknown. X. elegans has not been the focus of any omics-based studies to date, but its unique features compared to other Xerophyta species mean it will make an excellent comparative species to both H. rhodopensis and X. humilis.
Picture 2: Flowering Xerophyta elegans in its natural habitat.
- Lyall R., Schlebusch S., Proctor J., … Illing N. (2020) Vegetative desiccation tolerance in the resurrection plant Xerophyta humilis has not evolved through reactivation of the seed canonical LAFL regulatory network. The Plant Journal. 101(6): 1349–1367