Organising Tissue Cell Polarity and Growth in Plants

Unlike most things that we manufacture, like cars and phones, the biological structures we see around us are generated through growth: a tree grows from a seed and a human child grows from a microscopic fertilised egg. This growth is not guided by an external hand but is self-organised, coordinated by internal rules. Complex structures can arise from this process because growth and deformation occur at varying rates over tissues and preferentially in particular directions. A major challenge is to understand how this process of self-construction through growth operates. Answering this question may allow us to modulate growing structures in a predictive way and thus enable us to exploit, manage and interact with living systems more effectively.

A major block to achieving this goal is that we currently understand little about how orientations are specified and coordinated within growing tissues. Our best working hypothesis is that this process depends on each cell having a polarity, like an internal compass, but how this polarity is propagated and coordinated across a tissue remains unclear. This project aims to address this problem by studying polarity coordinated in plants and how it may lead to tissue growing out in particular orientations. Plant cells are surrounded by walls which impede direct communication from one cell to another. Polarity coordination is therefore thought to operate indirectly, mediated by small molecules, such as the plant hormone auxin, that can diffuse through the cell wall. We have recently developed a general hypothesis for how this process may lead to coordinated polarity across tissues and a major aim of this project is to test this and other hypotheses through a combination of experimental and computational methods.

To understand the mechanism of polarity coordination, we aim to switch on genes that have been implicated in polarity control in patches of cells within growing leaf tissue. By activating these genes in small patches of tissue we hope to see how the polarity of the surrounding tissue is affected and how this affects the growth of the cells, which we can follow in detail with bioimaging. These experiments will be conducted in normal plants and also plants in which certain polarity components have been removed through mutation. We also intend to develop computational modelling systems that will allow us to determine whether particular hypotheses can account for the observed patterns of polarity and growth change. This will involve developing new software to deal with both the cellular nature of tissues and the way they grow and deform. Taken together these experimental and computational approaches should enable us to arrive at a new and deeper understanding of how polarity and growth are coordinated and thus advance our ability to manage and exploit biological systems.

Although many genes contributing to tissue cell polarity have been defined, it remains unclear how these components interact and organise polarity over extended domains. We propose to address this problem in plants through a combination of experimental and modelling approaches that build on recent experimental and theoretical findings. Hypotheses for polarity coordination will be tested by generating sectors with a heat-shock inducible Cre-lox system in which candidate organiser genes and polarity components are expressed in ectopic patches in growing leaves. These genes include CUC, LAX and auxin biosynthesis genes which have been implicated in polarity coordination and are expressed in predicted organiser regions. They also include PIN and PID genes that are likely involved in polarity determination and propagation. Ectopic patches of gene activity will be marked by fusions to fluorescent proteins and induced by heat shocking entire seedlings or individual cells. The effects of induced ectopic patches of these genes on growth and local polarity will then be determined in wild-type and mutant backgrounds that carry polarity markers. Detailed dynamics will be extracted by tracking polarity, expression and morphogenetic changes at the cellular and tissue level over time as the tissue grows. In parallel, computational methods will be developed to allow cell polarity mechanisms to be implemented within growing and deforming tissues, applicable to 2D sheets and 3D volumes. Hypotheses will then be evaluated by creating models and determining whether they account for the observed interactions, growth, expression and polarity dynamics. Models will be further tested by combining sectors within the same plant in different mutant backgrounds. The results should provide major new insights into the mechanism of polarity coordination in plants and also provide modelling and theoretical frameworks that could be extended to tissue polarity in other systems.

This project will benefit non-academic beneficiaries, in the following ways:

1. Breeders will benefit from knowledge that will facilitate the selection of candidate genes for improving crop growth and yield by conventional or transgenic approaches. For example, leaf angle is a key determinant of yield in cereals and depends on formation of an outgrowth (the ligule). By identifying the mechanism and genetic control of outgrowths, it may be possible to tailor leaf angle to increase yield. The expected time frame for this beneficial impact will be 10 years after the start of the project.

2. Designers will benefit by being exposed to new ways to manipulate structures - through changes in local rules of growth and deformation rather than through design of the final product. The expected time frame for this beneficial impact is 10-20 years.

3. Biotech industries will benefit from our work in the long term, through the greater fundamental understanding of processes that underlie tissue properties in plants and animals. This may open up new avenues to exploit and manipulate growing tissues. They will also benefit from the tools developed in the project such as integrating computational methods with bioimaging approaches that may become applicable to their research and development programmes. The time frame for this type of impact is expected to be 10-20 years.

4. The general public and school children will benefit directly from this project through the proposed hands-on training events and through dissemination of latest research findings in an accessible way via media routes like YouTube videos, press articles and the Inner Worlds website. They will also benefit in the longer term because of the contribution that this project will make to maintaining and developing forward-looking scientific research that provides the foundations of a modern healthy and growing economy.