Nutrient status regulates MED19a phase separation for ORESARA1-dependent senescence.

The mediator complex is highly conserved in eukgaryotes and is integral for transcriptional responses. Mediator subunits associate with signal-responsive transcription factors (TF) to activate expression of specific signal-responsive genes. As the key TF of Arabidopsis thaliana senescence, ORESARA1 (ORE1) is required for nitrogen deficiency (-N) induced senescence; however, the mediator subunit that associates with ORE1 remains unknown. Here, we show that Arabidopsis MED19a associates with ORE1 to activate -N senescence-responsive genes. Disordered MED19a forms inducible nuclear condensates under -N that is regulated by decreasing MED19a lysine acetylation. MED19a carboxyl terminus (cMED19a) harbors a mixed-charged intrinsically disordered region (MC-IDR) required for ORE1 interaction and liquid-liquid phase separation (LLPS). Plant and human cMED19 are sufficient to form heterotypic condensates with ORE1. Human cMED19 MC-IDR, but not yeast cMED19 IDR, partially complements med19a suggesting functional conservation in evolutionarily distant eukaryotes. Phylogenetic analysis of eukaryotic cMED19 revealed that the MC-IDR could arise through convergent evolution. Our result of MED19 MC-IDR suggests that plant MED19 is regulated by phase separation during stress responses.

Systemic movement of long non-coding RNA ELENA1 attenuates leaf senescence under nitrogen deficiency.

Nitrogen is an essential macronutrient that is absorbed by roots and stored in leaves, mainly as ribulose-1,5-bisphosphate carboxylase/oxygenase1,2. During nitrogen deficiency (-N), plants activate leaf senescence for source-to-sink nitrogen remobilization for adaptative growth3-6. However, how -N signals perceived by roots are propagated to shoots remains underexplored. We found that ELF18-INDUCED LONG NONCODING RNA 1 (ELENA1) is -N inducible and attenuates -N-induced leaf senescence in Arabidopsis. Analysis of plants expressing the ELENA1 promoter ß-glucuronidase fusion gene showed that ELENA1 is transcribed specifically in roots under -N. Reciprocal grafting of the wild type and elena1 demonstrated that ELENA1 functions systemically. ELENA1 dissociates the MEDIATOR SUBUNIT 19a-ORESARA1 transcriptional complex, thereby calibrating senescence progression. Our observations establish the systemic regulation of leaf senescence by a root-derived long non-coding RNA under -N in Arabidopsis.

The phase separation underlying the pyrenoid-based microalgal Rubisco supercharger.

The slow and promiscuous properties of the CO2-fixing enzyme Rubisco constrain photosynthetic efficiency and have prompted the evolution of powerful CO2 concentrating mechanisms (CCMs). In eukaryotic microalgae a key strategy involves sequestration of the enzyme in the pyrenoid, a liquid non-membranous compartment of the chloroplast stroma. Here we show using pure components that two proteins, Rubisco and the linker protein Essential Pyrenoid Component 1 (EPYC1), are both necessary and sufficient to phase separate and form liquid droplets. The phase-separated Rubisco is functional. Droplet composition is dynamic and components rapidly exchange with the bulk solution. Heterologous and chimeric Rubiscos exhibit variability in their tendency to demix with EPYC1. The ability to dissect aspects of pyrenoid biochemistry in vitro will permit us to inform and guide synthetic biology ambitions aiming to engineer microalgal CCMs into crop plants.