Molecular Control of Cytokinin Degradation and Shoot Growth

In contrast to animals, plants have astonishing developmental plasticity and regeneration capacity. Their development is continuous and new plant organs are formed throughout their life by groups of undifferentiated cells called meristems. The shoot apical meristem (SAM) gives rise to organs like the leaves and flowers, and its activity thus not only determines the plant architecture but also agriculturally relevant traits, such as the plant biomass and yield. How meristems are maintained is one of the key questions in plant biology. Plant hormones are endogenous chemical signals playing a vital role in regulating activity of meristems. Cytokinin is a major plant hormone with a promotive effect on SAM activity and its concentration is strictly regulated by various molecular processes. It has been shown that the metabolic breakdown of this hormone through cytokinin oxidase/dehydrogenase (CKX) proteins is decisive for cytokinin homeostasis in the SAM. In our search for novel molecular components of the cytokinin regulatory system, we have recently identified molecular processes largely determining the activity of CKX proteins and thereby the SAM function and shoot development. These processes involve the protein quality control in the endoplasmic reticulum, which has a central role in protein biosynthesis and maturation. Based on recent data suggesting the functional significance of the endoplasmic reticulum in regulating the subcellular compartmentalization of the cytokinin, the major objective of this research project is to provide mechanistic insight into the crosstalk between protein quality control and cytokinin activity in shaping plant development. To accomplishthis, molecular and biochemical approacheswill be combined with comprehensive genetic analyses in the model plant Arabidopsis thaliana. This work will promote our understanding of the functional principles of plant meristems and yield valuable information for new biotechnological strategies to improve crop plant performance.

Plants are sessile organisms exhibiting a post-embryonic mode of development and displaying a striking ability to modulate their development in response to a wide range of environmental stimuli and stresses. This phenotypic plasticity enables plants to constantly adapt to their local environment and optimize their growth. The developmental flexibility arises from meristem tissues that continuously produce new organs throughout the plant life cycle. The shoot apical meristem (SAM) gives rise to organs like the leaves and flowers, and its activity thus not only determines the entire plant architecture but also agriculturally relevant traits, such as the plant biomass and yield. The plant hormone cytokinin exerts a pivotal function in controlling SAM activity, and its concentrations must be precisely regulated by various metabolic processes. It has been shown that the metabolic breakdown of this hormone through CYTOKININ OXIDASE/DEHYDROGENASE (CKX) proteins is crucial for cytokinin homeostasis in the SAM. This project aimed at understanding the control mechanisms of cytokinin degradation during shoot growth. We have revealed molecular processes largely determining the activity of CKX proteins, which involve the protein quality control in the endoplasmic reticulum (ER) and the functionally connected ER-associated degradation pathway (ERAD). This project identified specific molecular factors, such as, for example, the distinct group of HEAVY METAL-ASSOCIATED ISOPRENYLATED PLANT PROTEINS (HIPPs), mediating the ERAD of CKX proteins. We demonstrated that the isolated HIPP proteins determine the abundance of the active CKX protein species in the ER and thereby regulate cytokinin responses and plant development. The proposed model suggests a function of HIPP proteins in modulating the retrotranslocation step of plant ERAD. The project provided mechanistic insight into the crosstalk between protein quality control and cytokinin activity in plants. The research also contributed significantly to the understanding of the molecular regulation of plant meristems and provided knowledge that can potentially be employed in biotechnological approaches to tailor crop performance.