A new regulatory link between cell walls and hormones controls differential growth in plants

When hearing about walls, one thinks of rigid structures like medieval town walls that served as protection against intruders. Plant cell walls fulfil similar defence functions, but they are not as inert and rigid as town walls. On the contrary, they are much more flexible and dynamic and play a crucial role during plant growth and development.

“The cell wall is a highly complex network of proteins and several long-chain carbohydrate molecules like cellulose, hemicellulose and pectin”, explains Rishikesh Bhalerao, professor at the Swedish University of Agricultural Sciences and group leader at Umeå Plant Science Centre. “These key components have been known for a long time, but how they interact with each other and how these interactions functionally contribute to the control of plant growth and development is still poorly understood.”

A defective RG-II pectin dimerization causes weaker cell walls

Rishikesh Bhalerao and his colleagues from United Kingdom, Belgium, Switzerland, China and Canada focused on the cell wall component RG-II pectin. This is a highly modified pectin whose role in the cell wall has remained obscure. RG-II pectin is usually present as a dimer, meaning that two RG-II pectin molecules are bound together. To understand its role in plant development, the researchers used a thale cress mutant in which this dimerization did not work, resulting in weaker cell walls.

“We discovered nearly ten years ago that deficiency of RG-II pectin causes a particular defect in development of the apical hook, a structure that is formed by germinating seedlings that bend down when emerging from the soil”, continues Rishikesh Bhalerao. “We had already established the link between certain other pectin modifications, and mechano-chemical control of the distribution of the plant hormone auxin in apical hook development. Now, we could show that there is also a link between RG-II pectin and auxin and another hormone called brassinosteroid.”

The plant hormone auxin promotes cell growth and elongation in plants. During apical hook formation auxin accumulates on one side of the seedling. This promotes a faster cell growth on this side compared to the opposite side, which ultimately leads to the bending of the seedling. The researchers showed that RG-II pectin dimerization also affects the regulation of auxin distribution, but it acts more centrally by affecting the activity of several genes of the auxin transport machinery.

Cell wall components and hormones interact in a dynamic manner

Unexpectedly, the researchers also found a connection between RG-II pectin dimerization and brassinosteroids – another group of plant hormones involved in regulating cell division, elongation and differentiation. The defective RG-II pectin dimerization suppressed the biosynthesis of brassinosteroids which in turn impacted RG-II pectin, thus demonstrating a hormonal feedback on RG-II pectin dimerization. 

“Cell wall components and hormones interact with each other in a dynamic manner and thus regulate growth and development”, says Rishikesh Bhalerao. “It has taken a long time and considerable effort especially from my former postdoc Pawan Kumar Jewaria, who is now a group leader at the National Institute of Plant Genome Research in India, to reach this point.”

Rishikesh Bhalerao and his colleagues are convinced that the new regulatory mechanisms they discovered are not only relevant for apical hook formation but also for other processes that involve differential growth such as the directed growth towards light. They hope that other researchers in the field of cell wall mechanics and development can use these results now and build further on them to understand how cell wall mechanics and its interplay with plant hormones can regulate plant architecture.