Understanding the connections between plant hormones and cellulose synthesis in plants: still a great unknown!

Cellulose is the major component of plant cell walls, and is known to be the major load-bearing component of plant cell walls. Cellulose is found both in the primary cell walls that surround the majority of plant cells, and also in secondary cell walls that surround specialised plant tissues. How cellulose is made, deposited and arranged in plant cell walls is of high interest to those trying to understand how variable plant structures develop from an invariant embryo, and how this process is influenced by internal and external factors that plants may encounter. Translating the impacts of internal and external factors to cellulose synthesis requires an intermediate, for which plant hormones are strong candidates.

Plant hormones are well-known to regulate plant growth and development at the cellular and whole-plant level, processes that are dependent on controlled synthesis, deposition and arrangement of cellulose in the cell wall. However, there are a variety of hormones currently known in plants, and the responses they cause are complex and often wide-reaching. Understanding exactly how plant hormones may directly impact cellulose production and deposition is therefore not entirely straightforward. In a recent review article in Annals of Botany, Wang and colleagues summarise what is known about connections between cellulose synthesis and plant hormones.

Wang and colleagues discuss the links that are known between cellulose biosynthesis and action of the plant hormones auxin, brassinosteroids, abscisic acid, gibberellins, cytokinin, ethylene, jasmonates, salicylic acid and strigolactone. Sifting through the complexity of this due to the broad effects of these hormones and the sheer number of them, the authors highlight that there is so far little data directly linking plant hormones and cellulose biosynthesis. One exception to this is that hormone-sensitive transcription factors are known to influence the formation of the cell wall, and particularly cellulose biosynthesis in the case of secondary cell walls.

Connections between hormone-influenced transcription factors and primary cell wall cellulose biosynthesis remain unclear, and a screen of mutants in transcription factors closely expressed with proteins required for cellulose biosynthesis in primary cell walls did not detect any obvious abnormalities in plant growth. Wang and colleagues also point out that hormones influence the production of small RNAs that are known to influence expression levels and patterns of other genes, and suggest that these are strong candidates for hormone-mediated regulation of cellulose synthesis related genes. Time will tell if this is the case.

Wang and colleagues also discuss another likely way in which plant hormones can influence cellulose synthesis, post-translational modifications, which can influence protein activity and abundance. Cellulose synthesis related proteins are known to be the target of multiple post-translational modifications, and one site has been shown to be the target of brassinosteroid-sensitive modification. The authors suggest that the effects of hormones on other known cellulose synthesis protein modification sites should be investigated further. In summary, the review by Wang and colleagues highlights how, although there must be connections between cellulose biosynthesis and the wide variety of hormones produced by plants, examples of these remain scarce. However, in the age of highly- available transcriptomics, proteomics and chemical biology toolkits, our knowledge of the connections between plant hormones and cellulose biosynthesis looks set to increase.