How does cell division influence decision-making in plants?

Plant development involves cell division and cell elongation. Since the 19th century, it has been debated whether growth is organised at the cellular scale (cell-theory) or organ scale (organismal-theory). Mutants in which cell division is disrupted but organ growth is relatively normal have given support to the organismal theory of plant development.

We wish to challenge this idea, by investigating the consequences of cell division on cell-to-cell signalling. Rather than having a central nervous system, in plants each cell must integrate information from multiple sources to make decisions, making cell-to-cell communication very important. Information can pass between cells in the form of mobile molecules, for example, hormones, transcription factors or signalling peptides.

How are these signalling processes impacted by the growth and division of the cells within the tissues? The consequences of having a heterogeneous cellular environment on cell-to-cell communication have not been investigated. As a proof of principle of our approach, we will examine the effects of altering cell division on the plant circadian clock, which is a key timing system that relies on cell-cell communication.

We propose to use lines that alter cell division frequency in an inducible manner in consort with clock reporter lines that allow examination of the clock at the sub-tissue or single-cell level. The clock genes are not synchronised in all cells and high-resolution imaging has demonstrated spatial waves of clock gene expression including in aerial organs.

The circadian clock has a large influence controlling approximately 40% of expressed genes. The waves involve local cell-cell coupling, so changing the number of cells should modulate coupling strength. We will determine if altering cell division can modulate signal propagation and ultimately plant decision making.

Aims: Aim 1: Characterise clock spatial dynamics across aerial organs in tissues with altered cell density.

1.1 Cell division will be altered to generate large numbers of cell divisions in aerial organs including the hypocotyl which does not normally divide. This line will be crossed to luciferase reporters for the circadian clock.

1.2 Images will be analysed using the pipeline previously developed in the Locke lab to quantify the phase, period and amplitude of the clock and compare the induced and uninduced cell division lines. We will test whether the wave speed is modulated by increasing cell number, as well as if higher cell density causes more coherent rhythms due to increased coupling.

Aim2: Determine how local heterogeneity in cell division alters signal movement

2.1 We will induce cell divisions in a fluorescent protein clock reporter line and use time-lapse confocal microscopy to examine the dynamics of the cell-to-cell coupling at single-cell resolution. We will compare the movement between cells when the cells are large as in the wild-type hypocotyl or small as in the induced state. As the induction is not always perfect there are often some cells that do not divide when the tissue is induced.

We will take advantage of this to determine if the propagation of the wave is disrupted by tissue heterogeneity.

2.2 We will test our understanding of the effects of cell division and cell number on plant clock coordination by incorporating our findings into an existing mathematical model of plant clock coordination. Conclusion

Here we present a test case to investigate how cell density affects plant decision-making in terms of timing. If successful we will expand our study to investigate using cell division as a tool to manipulate cell-to-cell communication speed and noise in other systems such as pathogen response and coordination of development.