Glycosylphosphatidylinositol anchor lipid remodeling directs proteins to the plasma membrane and governs cell wall mechanics.

Glycosylphosphatidylinositol (GPI) anchoring is a common protein modification that targets proteins to the plasma membrane (PM). Knowledge about the GPI lipid tail, which guides the secretion of GPI-anchored proteins (GPI-APs), is limited in plants. Here, we report that rice (Oryza sativa) BRITTLE CULM16 (BC16), a membrane-bound O-acyltransferase (MBOAT) remodels GPI lipid tails and governs cell wall biomechanics. The bc16 mutant exhibits fragile internodes, resulting from reduced cell wall thickness and cellulose content. BC16 is the only MBOAT in rice and is located in the endoplasmic reticulum and Golgi apparatus. Yeast gup1Δ mutant restoring assay and GPI lipid composition analysis demonstrated BC16 as a GPI lipid remodelase. Loss of BC16 alters GPI lipid structure and disturbs the targeting of BC1, a GPI-AP for cellulose biosynthesis, to the PM lipid nanodomains. Atomic force microscopy revealed compromised deposition of cellulosic nanofibers in bc16, leading to an increased Young's modulus and abnormal mechanical properties. Therefore, BC16-mediated lipid remodeling directs the GPI-APs, such as BC1, to the cell surface to fulfill multiple functions, including cellulose organization. Our work unravels a mechanism by which GPI lipids are remodeled in plants and provides insights into the control of cell wall biomechanics, offering a tool for breeding elite crops with improved support strength.

Xylan-based nanocompartments orchestrate plant vessel wall patterning.

Nanoclustering of biomacromolecules allows cells to efficiently orchestrate biological processes. The plant cell wall is a highly organized polysaccharide network but is heterogeneous in chemistry and structure. However, polysaccharide-based nanocompartments remain ill-defined. Here, we identify a xylan-rich nanodomain at pit borders of xylem vessels. We show that these nanocompartments maintain distinct wall patterns by anchoring cellulosic nanofibrils at the pit borders, critically supporting vessel robustness, water transport and leaf transpiration. The nanocompartments are produced by the activity of IRREGULAR XYLEM (IRX)10 and its homologues, which we show are de novo xylan synthases. Our study hence outlines a mechanism of how xylans are synthesized, how they assemble into nanocompartments and how the nanocompartments sustain cell wall pit patterning to support efficient water transport throughout the plant body.

MYB61 is regulated by GRF4 and promotes nitrogen utilization and biomass production in rice.

Nitrogen (N) is a macronutrient that boosts carbon (C) metabolism and plant growth leading to biomass accumulation. The molecular connection between nitrogen utilization efficiency (NUE) and biomass production remains unclear. Here, via quantitative trait loci analysis and map-based cloning, we reveal that natural variation at the MYB61 locus leads to differences in N use and cellulose biogenesis between indica and japonica subspecies of rice. MYB61, a transcriptional factor that regulates cellulose synthesis, is directly regulated by a known NUE regulator GROWTH-REGULATING FACTOR4 (GRF4), which coordinates cellulosic biomass production and N utilization. The variation at MYB61 has been selected during indica and japonica domestication. The indica allele of MYB61 displays robust transcription resulting in higher NUE and increased grain yield at reduced N supply than that of japonica. Our study hence unravels how C metabolism is linked to N uptake and may provide an opportunity to reduce N use for sustainable agriculture.

Rice Homeobox Protein KNAT7 Integrates the Pathways Regulating Cell Expansion and Wall Stiffness.

During growth, plant cells must coordinate cell expansion and cell wall reinforcement by integrating distinct regulatory pathways in concert with intrinsic and external cues. However, the mechanism underpinning this integration is unclear, as few of the regulators that orchestrate cell expansion and wall strengthening have been identified. Here, we report a rice (Oryza sativa) Class II KNOX-like homeobox protein, KNOTTED ARABIDOPSIS THALIANA7 (KNAT7), that interacts with different partners to govern cell expansion and wall thickening. A loss-of-function mutation in KNAT7 enhanced wall mechanical strength and cell expansion, resulting in improved lodging resistance and grain size. Overexpression of KNAT7 gave rise to the opposite phenotypes, with plants having weaker cell walls and smaller grains. Biochemical and gene expression analyses revealed that rice KNAT7 interacts with a secondary wall key regulator, NAC31, and a cell growth master regulator, Growth-Regulating Factor 4 (GRF4). The KNAT7-NAC31 and KNAT7-GRF4 modules suppressed regulatory pathways of cell expansion and wall reinforcement, as we show in internode and panicle development. These modules function in sclerenchyma fiber cells and modulate fiber cell length and wall thickness. Hence, our study uncovers a mechanism for the combined control of cell size and wall strengthening, providing a tool to improve lodging resistance and yield in rice production.