19.04.2017

It is not by accident, how large a petal or a seed becomes, because organ growth in plants is a finely tuned process. At the beginning of organ development cells divide. But then division halts and cells transition into a phase of expansion and differentiation. The distinctive ratio between cell division, cell expansion and cell differentiation causes a leaf or a petal to adopt its characteristic shape and size.

The independent research group Protein Recognition and Degradation headed by Nico Dissmeyer together with collaborators from Britain, China and Belgium have analyzed the molecular mechanisms that control organ growth. In their study recently published in Genes & Development, they elucidate how cells transition from division to expansion (Dong et al. Ubiquitylation activates a peptidase that promotes cleavage and destabilization of its activating E3 ligases and diverse growth regulatory proteins to limit cell proliferation in Arabidopsis Gen. Dev. 31, 197-208, 2017, doi: 10.1101/gad.292235.116). As in many cellular processes, in this case, too, proteostasis plays a major role. Proteostasis describes the process of regulating the amount of active protein in a cell or cell compartment. For proteostatic purposes, proteins often become linked with a marking protein called ubiquitin - a process termed ubiquitination. Special enzymes, so-called ubiquitin ligases transfer one or more ubiquitins to their specific target protein. Often several ubiquitins decorate a target protein in form of chains. Depending on the length, number, position or linkage type of the ubiquitin chain(s), the target protein will be subsequently degraded, activated, deactivated or translocated. Thus, ubiquitination resembles a readable mark for the protein's cellular fate.

It has been previously known that cell division during organ development is inhibited by three different enzymes: a peptidase and two ubiquitin ligases (Figure 1). The IPB researchers were now able to shed light on their interplay. Different proteins control the initial cell division. Then, cell division is stopped as those proteins are cleaved and thereby inactivated by the peptidase. As a result, cells begin to increase their volume and grow. For this scenario to be triggered, the peptidase has to be temporarily activated first. Dissmeyer and colleagues found that the ubiquitin ligases activate the peptidase through ubiquitination. A negative feedback loop ensures that this activation happens only temporarily. In fact, the peptidase not only deactivates the factors important for cell division, it also deactivates its own activators, the ubiquitin ligases. As a protein-cleaving enzyme, the peptidase cleaves off a short sequence at the ubiquitin ligase's N-terminus. This modification exposes a previously internal tyrosine residue at the N-terminus.

Here, the Dissmeyer lab's expertise on N-end-rule proteostasis comes into play. According to the N-end rule that describes destabilizing effects of N-terminal amino acids, tyrosine at the N-terminus shortens the lifespan of a protein, marking it for degradation. The researchers demonstrated that the ubiquitin ligase with an exposed N-terminal tyrosine is recognized by the cellular N-end-rule machinery and is being degraded. While several N-end rule substrates have been found in other organisms, the above-mentioned ubiquitin ligase is the first plant protein shown to be subject to degradation due to recognition of its processed N-terminus.

This multilayered interplay of peptidase and ubiquitin ligases allows a switch from cell division to cell expansion and differentiation and thereby contributes to determine organ size. This study using the model plant Arabidopsis provides insight into organ growth on the molecular level. Ultimately, the findings can be of agronomic relevance with regard to producing larger fruit, seeds or biomass in crop plants.