With global population growth and the threats of climate change, producing hardy, pest-resistant crops with larger yields is the holy grail of plant science. Historically, selectively breeding crops may have taken thousands of years and there was no knowledge of the genetic mechanisms underlying advantageous traits. In recent decades, researchers have linked phenotypic traits to specific genes and have found that many desirable traits are determined by a set of genes involved in regulating plant meristems.
Meristems are regions of the plant that maintain pools of undifferentiated stem cells and enable it to generate new structures, like roots, branches or flowers, throughout its entire lifetime. It is the regulation of these meristems that controls the plant’s overall architecture, allowing it to respond to environmental inputs and adapt its growth accordingly. By taking advantage of naturally-arising differences in meristem regulation, scientists have been able to modify crop architecture to increase yields, for example by selecting tomato plants with larger floral meristems that produce more and lager fruits, or dwarf rice plants that divert more resources to seed production rather than stem tissue.
Despite their agricultural significance, we still don’t fully understand the fundamental ways in which meristems are controlled. To investigate this, the CLAVHUB project used the model plant Arabidopsis thaliana, a member of the brassica family whose small size, short lifecycle and relatively simple genetics makes it particularly suitable for study and propagation in the lab. CLAVHUB concentrated on two Arabidopsis proteins involved in meristem regulation: ARABIDOPSIS CRINKLY 4 (ACR4) and CLAVATA 1 (CLV1).
ACR4 and CLV1 are both receptor kinases, a class of proteins straddling the cell membrane that can receive messages from outside the cell and relay them to effectors inside the cell. One interesting feature of ACR4 and CLV1 is that, as well as being distributed throughout the cell membrane, they also concentrate at specific points along the membrane. These points correspond to plasmodesmata, membrane-lined channels that enable nutrients and informational molecules to move from cell to cell through the cell wall. While small molecules can freely diffuse through plasmodesmata, the movement of larger molecules, such as RNAs or transcription factors, is selective. This is particularly important in plant development, which relies on regulatory molecules being able to move between specific regions to establish cell identity. However, how this selectivity is achieved it not well understood.
This brings us back to ACR4 and CLV1 because, in addition to clustering at plasmodesmata, the two proteins form complexes there. Furthermore, these complexes differ from the complexes they form elsewhere in the plasma membrane. This suggests that ACR4 and CLV1 have a plasmodesmata-specific function. The aim of CLAVHUB was to try to find out what this function was: a) whether plasmodesmata simply provide a platform on which to assemble signalling hubs, and/or b) if ACR4 and CLV1 are somehow influencing the function of plasmodesmata by modulating their permeability, and c) what other proteins are involved.
