Saline-alkali soil is a major environmental constraint impairing plant growth and crop productivity. In this study, we identified a Ca²⁺ sensor/kinase/plasma membrane (PM) H⁺-ATPase module as a central component conferring alkali tolerance in Arabidopsis (Arabidopsis thaliana). We report that the SCaBP3 (SOS3-LIKE CALCIUM BINDING PROTEIN3)/CBL7 (CALCINEURIN B-LIKE7) loss-of-function plants exhibit enhanced stress tolerance associated with increased PM H⁺-ATPase activity and provide fundamental mechanistic insights into the regulation of PM H⁺-ATPase activity. Consistent with the genetic evidence, interaction analyses, in vivo reconstitution experiments, and determination of H⁺-ATPase activity indicate that interaction of the Ca²⁺ sensor SCaBP3 with the C-terminal Region I domain of the PM H⁺-ATPase AHA2 (Arabidopsis thaliana PLASMA MEMBRANE PROTON ATPASE2) facilitates the intramolecular interaction of the AHA2 C terminus with the Central loop region of the PM H⁺-ATPase to promote autoinhibition of H⁺-ATPase activity. Concurrently, direct interaction of SCaPB3 with the kinase PKS5 (PROTEIN KINASE SOS2-LIKE5) stabilizes the kinase-ATPase interaction and thereby fosters the inhibitory phosphorylation of AHA2 by PKS5. Consistently, yeast reconstitution experiments and genetic analysis indicate that SCaBP3 provides a bifurcated pathway for coordinating intramolecular and intermolecular inhibition of PM H⁺-ATPase. We propose that alkaline stress-triggered Ca²⁺ signals induce SCaBP3 dissociation from AHA2 to enhance PM H⁺-ATPase activity. This work illustrates a versatile signaling module that enables the stress-responsive adjustment of plasma membrane proton fluxes.

Carbon and nitrogen are the two main nutrients in maize (Zea mays L.) kernels, and kernel filling and metabolism determine seed formation and germination. However, the molecular mechanisms underlying the relationship between kernel filling and corresponding carbon and nitrogen metabolism remain largely unknown. Here, we found that HEAT SHOCK PROTEIN 90.6 (HSP90.6) is involved in both seed filling and the metabolism processes of carbon and nitrogen. A single-amino acid mutation within the HATPase_c domain of HSP90.6 led to small kernels. Transcriptome profiling showed that the expression of amino acid biosynthesis- and carbon metabolism-related genes was significantly downregulated in the hsp90.6 mutant. Further molecular evidence showed strong interactions between HSP90.6 and the 26S proteasome subunits REGULATORY PARTICLE NON-ATPASE6 (RPN6) and PROTEASOME BETA SUBUNITD2 (PBD2). The mutation of hsp90.6 significantly reduced the activity of the 26S proteasome, resulting in the accumulation of ubiquitinated proteins and defects in nitrogen recycling. Moreover, we verified that HSP90.6 is involved in carbon metabolism through interacting with the 14-3-3 protein GENERAL REGULATORY FACTOR14-4 (GF14-4). Collectively, our findings revealed that HSP90.6 is involved in seed filling and development by interacting with the components controlling carbon and nitrogen metabolism.

Summary: The Elongator complex was originally identified as an interactor of hyperphosphorylated RNA polymerase II (RNAPII) in yeast and has histone acetyltransferase (HAT) activity. However, the genome‐wide regulatory roles of Elongator on transcriptional elongation and histone acetylation remain unclear. We characterized a maize miniature seed mutant, mn7 and map‐based cloning revealed that Mn7 encodes one of the subunits of the Elongator complex, ZmELP1. ZmELP1 deficiency causes marked reductions in the kernel size and weight. Molecular analyses showed that ZmELP1 interacts with ZmELP3, which is required for H3K14 acetylation (H3K14ac), and Elongator complex subunits interact with RNA polymerase II (RNAPII) C‐terminal domain (CTD). Genome‐wide analyses indicated that loss of ZmELP1 leads to a significant decrease in the deposition of H3K14ac and the CTD of phosphorylated RNAPII on Ser2 (Ser2P). These chromatin changes positively correlate with global transcriptomic changes. ZmELP1 mutation alters the expression of genes involved in transcriptional regulation and kernel development. We also showed that the decrease of Ser2P depends on the deposition of Elongator complex‐mediated H3K14ac. Taken together, our results reveal an important role of ZmELP1 in the H3K14ac‐dependent transcriptional elongation, which is critical for kernel development.

Carbon and nitrogen are the two most abundant nutrients in all living things, and their metabolism maintains normal plant growth. However, the molecular mechanism underlying carbon and nitrogen metabolism remains largely unknown. Here, we found that HSP90.6 is involved in the metabolism of carbon and nitrogen. We performed gene cloning and functional characterization of a maize EMS mutant ehsp90.6 , whose kernels were small. HSP90.6 encodes heat shock protein 90.6, which has a single-amino acid mutation within its HATPase_c domain. Transcriptome profiling showed that the expression of amino acid biosynthesis- and carbon metabolism-related genes was significantly downregulated in hsp90.6 . HSP90.6 is involved in the 26S proteasome degradation pathway, which affects nitrogen recycling to regulate amino acid synthesis; this occurs by interactions between HSP90.6 and the 26S proteasome subunits RPN6 and PBD2 (PRC2). The loss of HSP90.6 significantly reduced the activity of the 26S proteasome, resulting in the accumulation of ubiquitinated proteins and defects in nitrogen recycling. Moreover, HSP90.6 interacted with the 14-3-3 protein GF14-6 to participate in carbon metabolism. Together, these findings revealed that HSP90.6 regulates nutrient metabolism in maize seeds by affecting 26S proteasome-mediated nitrogen recycling and GF14-6-mediated carbon metabolism. One sentence summary: HSP90.6 is involved in nutrient metabolism via 26S proteasome-mediated protein degradation to promote nitrogen recycling and GF14-6 protein-mediated carbon metabolism. The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors ( https://academic.oup.com/plcell/pages/General-Instructions ) is Weibin Song ( songwb@cau.edu.cn ). Highlights: HATPase_c is necessary for HSP90.6 to regulate maize kernel development. HSP90.6 is involved in nitrogen recycling via the 26S proteasome degradation pathway. HSP90.6 interacts with the 14-3-3 protein GF14-6 to affect carbon metabolism. IN A NUTSHELL: Background: Seeds are the main harvested organs of maize. Understanding the regulatory mechanism of grain filling is helpful to cultivate high-quality and high-yield maize. In the past few years, the regulatory network of grain filling has been explored through multiple means, including transcriptomic, proteomic and functional genomic techniques. Many genes that control grain filling through different mechanisms have been cloned, such as CTLP1 (Choline Transporter-like Protein 1), OS1 ( Opaque Endosperm and Small Germ 1 ), and MN6 ( Miniature Seed6 ). To identify new genes involved in maize grain filling, ethyl methanesulfonate (EMS) was used to induce mutations, and the ehsp90.6 mutant, which exhibited abnormal kernel development, was isolated by bulked segregant analysis RNA sequencing (BSR). Question: Why does the single-amino acid mutation of HSP90.6 affect grain size, and how does the loss of HSP90.6 affect grain filling? Findings: A single-amino acid mutant ( ehsp90.6 ) and knockout mutant ( hsp90.6 ) were obtained. We found that HSP90-6 was involved in the regulation of maize grain filling. A single-single amino acid mutation in the HATPase_c domain reduced the ATPase activity of HSP90.6, resulting in smaller grains. The functional loss of HSP90.6 resulted in the expression of amino acid biosynthesis- and carbon metabolism-related genes being significantly downregulated in hsp90.6 . We indicated that HSP90.6 is involved in the 26S proteasome degradation pathway, which affects nitrogen recycling to regulate amino acid synthesis by interacting with the 26S proteasome subunits RPN6 and PBD2 (PRC2). Moreover, HSP90.6 was found to interact with the 14-3-3 protein GF14-6 to participate in carbon metabolism. Next steps: To further verify that the interaction between HSP90.6 and 26S proteasome subunits and GF14-6 affects grain filling, knockout validation of RPN6, PBD2 (PRC2) and GF14-6 will be performed. In addition, since GF14-6 interacts with the phosphorylated proteins, we will determine the phosphorylation site of HSP90.6. Due to the important role of HSP90 family proteins in plant development, there are other regulatory pathways that need to be explored.