Origin of anisotropic growth in plants: symmetry breaking

Plants know how to grow in one specific direction. Just look at tree trunks or vines that spread in your garden. Plants create such long structures by elongating their cells in specific directions. But plant cells are like balloons under extremely high pressure of 5 bar and higher. For them not to explode, they are contained in a structure called cell wall, made from cellulose, that restricts their growth and glues cells together. Although scientists know that it is this rigid cell wall that shapes growth, it is still unclear how cells control in which direction they grow. This project investigates this fundamental question using single cells from plants. Such cells lost their cell wall but are rebuilding it from scratch, making it possible to observe the wall formation and the emergence of a growth axis in real-time. Early findings revealed that cells do not have a clear direction in their cell wall structure, which would allow them to grow in one direction only. Instead, the new wall forms as a random network of cellulose fibers. The randomness of the network causes some regions to be mechanically weaker than others, and once the cell builds up internal pressure, the network breaks precisely at these weak spots. Thus, the cell elongates in one direction and creates an elongation axisan unexpected mechanism that challenges existing growth models. In the project we aim to uncover how such mechanical weak points arise and how cellulose fibers realign later so that the cell can elongate in one direction only. For this, we will use high-resolution time-lapse microscopy to track individual fibers as the cell transitions from a round to an elongated shape. At the same time, imaging the cells interior will reveal the cellular processes underlying the cells growth. In parallel, with a computational model we will simulate how pressure and fiber arrangement interact to trigger onset of elongation and later directional growth. This will allow us to create a novel biophysical model of how the cells organize themselves to undergo coordinated cell expansion. Since the discovery of cellulose as the basic component of plant cell walls over 100 years ago, it is known that cellulose alignment is crucial for establishing and maintaining growth. However, little was known about how plant cells organize themselves to achieve aligned cellulose and hence directional growth. The initial findings already show that this project provides insights into a so-far unknown mechanism of how plant cells establish mechanical asymmetries and hence directional growth, giving answers to century-long questions. Understanding directional growth is key not only for basic plant biology but also for future applications in agriculture, such as developing more resilient crops or improving biomass production. The project therefore contributes to a deeper understanding of how plants build and remodel their bodies at the cellular level.

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