A project undertaken at The University of Melbourne, and supervised by Dr Edwin Lampugnani.
Drought is one of the most severe abiotic stresses that affects the health, livelihood, and food security of the world (1). With climate change, it is expected that the world will experience more intense drought conditions than ever before. The hotter and drier conditions will challenge the agricultural industry and food security if the crop plants cannot adapt to the rapidly changing weather conditions.
Thankfully, plants already possess many tools to withstand drought stress. Drought tolerance can be conferred by a combination of different chemical and physiological responses, including cell wall modification.
Plant cell walls are made up of many different types of sugars called polysaccharides and changing the composition of these sugars can also change its physical properties. For example, a type of polysaccharide called arabinan has been shown to confer increased cell wall flexibility (2), presumed to be a consequence of reduced ionic bonds between other cell wall polysaccharides.
In fact, increased arabinan in the cell wall is thought to be one of the mechanisms in which some resurrection plants can avoid cell wall breakage and withstand extreme desiccation (3). Although arabinan may be playing an important role in drought tolerance, the proteins involved in its molecular biosynthetic pathway is yet to be confirmed through biochemical activity characterisation.
An early diverging land plant group called liverworts have several species that are desiccation tolerant. Liverworts possess a cell wall that contain the same polysaccharides as flowering plants, but their genome is much simpler in comparison. The liverwort model organism Marchantia polymorpha possess only two candidate genes that may be involved in arabinan biosynthesis, making it an ideal system to study the genetic mechanism behind arabinan biosynthesis.
Using a reverse genetics approach such as the CRISPR technology and other molecular biology tools, Australia and Pacific Science Foundation funding has allowed us to show that the two genes of interest, ARADL1 and ARADL2 in Marchantia are involved in the biosynthesis of arabinose containing cell wall polysaccharides. ARADL2 in particular, plays an important role in osmotic stress tolerance, which opens up new research avenues to enhance drought stress tolerance. More over we have gathered evidence to suggest that desiccation tolerant liverworts, and in particular Reboulia queenslandica and Plagiochasma rupestre, increase cell wall arabinan as a response to desiccation, consistent with observations of other drought species. Arabinan therefore likely plays an important role in desiccation tolerance, possible by maintaining cell wall elasticity while cells undergo drastic changes in size and structure related to osmotic stress.
References
(1) Drought in numbers – Restoration for readiness and resilience (2022) UNCCD Report
(2) Moore et al., (2013) doi 10.1007/s00425-012-1785-9
(3) Moore et al., (2006) doi: 10.1104/pp.106.077701