Biology subjects

Hennacy, J. H.

Publications and source records attributed to Hennacy, J. H..

4 recordsLinked to original sources

bioRxiv2026

SBE1 drives the circumferential growth of the starch sheath around the Chlamydomonas reinhardtii pyrenoid

Pyrenoids are CO2-fixing organelles responsible for approximately one-third of global CO2 fixation. The pyrenoids of many algae are surrounded by a starch sheath proposed to perform the critical function of slowing leakage of concentrated CO2 out of the pyrenoid. How the cell shapes starch granules into curved starch plates that encase the pyrenoid to enable efficient CO2 fixation is currently unknown. Here, we elucidate how starch transitions from granules into a fully formed pyrenoid starch sheath using confocal microscopy of the model green alga Chlamydomonas reinhardtii. We observe that after initiation, starch granules grow circumferentially along the surface of the pyrenoid matrix to encapsulate it. We show that the starch branching enzyme SBE1 localizes to the pyrenoid and is essential for this circumferential starch granule growth. Our data suggest that SBE1 promotes the circumferential growth of pyrenoid-associated starch granules by branching starch at the matrix-granule-stroma interface. Our findings advance the understanding of pyrenoid starch sheath assembly and, more broadly, starch-shaping mechanisms. Significance StatementThe shape of the starch granules that surround the pyrenoid is critical for efficient carbon fixation, but how these granules are shaped into curved plates is currently not understood. We find that starch granules spread along the pyrenoid surface rather than growing equally in all directions, and that this bias requires SBE1, a branching enzyme localized to the surface of the spherical pyrenoid matrix condensate. The findings support a model in which localizing a metabolic enzyme directs polymer growth to create curvature. This mechanism connects branching enzyme placement to starch granule geometry and contributes to the understanding of how cells convert local biosynthetic activity into large-scale organelle architecture.

molecular biology↗

bioRxiv2025

Kinase KEY1 controls pyrenoid condensate size throughout the cell cycle by disrupting phase separation interactions

Biomolecular condensates spatially organize cellular functions, but the regulation of their size, number, dissolution, and re-condensation is poorly understood. The pyrenoid, an algal biomolecular condensate that mediates one-third of global CO2 fixation, typically exists as one large condensate per chloroplast, but during cell division it transiently dissolves and reconfigures into multiple smaller condensates. Here, we identify a kinase, KEY1, in the model alga Chlamydomonas reinhardtii that regulates pyrenoid condensate size and number dynamics throughout the cell cycle and is necessary for normal pyrenoid function and growth. Unlike wild type, key1 mutant cells have multiple smaller condensates throughout the cell cycle that fail to dissolve during cell division. We show that KEY1 localizes to the condensates and promotes their dissolution by disrupting interactions between their core constituents, the CO2-fixing enzyme Rubisco and its linker protein EPYC1, through EPYC1 phosphorylation. We develop a biophysical model that recapitulates KEY1-mediated condensate size and number regulation and suggests a mechanism for controlling condensate position. These data provide a foundation for the mechanistic understanding of the regulation of size, number, position, and dissolution in pyrenoids and other biomolecular condensates.

cell biology↗

bioRxiv2024

Biogenesis, engineering and function of membranes in the CO2-fixing pyrenoid

Summary ParagraphApproximately one-third of global CO2 assimilation is performed by the pyrenoid1, a liquid-like organelle found in most algae and some plants2. Specialized membranes are hypothesized to drive CO2 assimilation in the pyrenoid by delivering concentrated CO23,4, but their biogenesis and function have not been experimentally characterized. Here, we show that homologous proteins SAGA1 and MITH1 mediate the biogenesis of the pyrenoid membrane tubules in the model alga Chlamydomonas reinhardtii and are sufficient to reconstitute pyrenoid-traversing membranes in a heterologous system, the plant Arabidopsis thaliana. SAGA1 localizes to the regions where thylakoid membranes transition into tubules and is necessary to initiate tubule formation. MITH1 localizes to the tubules and is necessary for their extension through the pyrenoid. Tubule-deficient mutants exhibit growth defects under CO2-limiting conditions, providing evidence for the function of membrane tubules in CO2 delivery to the pyrenoid. Furthermore, these mutants form multiple aberrant condensates of pyrenoid matrix, indicating that a normal tubule network promotes the coalescence of a single pyrenoid. The reconstitution of pyrenoid-traversing membranes in a plant represents a key milestone toward engineering a functional pyrenoid into crops for improving crop yields. More broadly, our study demonstrates the functional importance of pyrenoid membranes, identifies key biogenesis factors, and paves the way for the molecular characterization of pyrenoid membranes across the tree of life.

molecular biology↗

bioRxiv2017

Minimum requirements for reprogramming and maintaining cell fate in the Arabidopsis root

Changes in gene regulation during differentiation are governed by networks of transcription factors. To identify the minimal network for endodermal differentiation in the Arabidopsis root, we asked what transcription factors are sufficient to program a non-native cell-type into endodermis. Our results show the transcription factors SHORTROOT and MYB36 have limited ability to reprogram a non-native cell-type (the epidermis) and that this reprogramming is reversible in the absence of additional cues. The stele-derived signaling peptide CIF2 stabilizes SHORTROOT-induced reprogramming. The outcome is a partially impermeable barrier deposited in the sub-epidermal cell layer that has a transcriptional signature similar to endodermis. The trans-differentiation mechanism depends on the expression of genes downstream in the gene regulatory network but is independent of SHORTROOT movement. These results highlight a non cell-autonomous induction mechanism for endodermis that resembles differentiation in many animal systems.\n\nOne Sentence SummarySHORTROOT and CIF2 combined can induce and stabilize an endodermis in sub-epidermal cells and do so in a non cell-autonomous manner.

developmental biology↗