Appropriate cell division and differentiation ensure normal anther development in angiosperms. BARELY ANY MERISTEM 1/2 (BAM1/2) and RECEPTOR-LIKE PROTEIN KINASE2 (RPK2), two groups of leucine-rich repeat receptor-like protein kinases, are required for early anther cell specification. However, little is known about the molecular mechanisms underlying these two RLK-mediated signaling pathways. Here, we show that CLAVATA3 INSENSITIVE RECEPTOR KINASEs (CIKs), a group of novel coreceptor protein kinase-controlling stem cell homeostasis, play essential roles in BAM1/2- and RPK2-regulated early anther development in Arabidopsis thaliana. The archesporial cells of cik1/2/3 triple and cik1/2/3/4 quadruple mutant anthers perform anticlinal division instead of periclinal division. Defective cell division and specification of the primary and inner secondary parietal cells occur in these mutant anthers. The disordered divisions and specifications of anther wall cells finally result in excess microsporocytes and a lack of one to three parietal cell layers in mutant anthers, resembling rpk2 or bam1/2 mutant anthers. Genetic and biochemical analyses indicate that CIKs function as coreceptors of BAM1/2 and RPK2 to regulate archesporial cell division and determine the specification of anther parietal cells.

Significance: BAK1 plays a key role in multiple PRR-triggered immune signaling pathways. Double mutants generated by BAK1 and its paralog BKK1 show spontaneous cell death, which is not seen in any known PRR mutants. We discovered that the ADR1 class of helper nucleotide-binding leucine-rich repeat proteins (NLRs) is required for the autoimmune responses of bak1 bkk1 . Knocking out three ADR1 s can significantly suppress the cell death of bak1-3 bkk1-1 , suggesting the autoimmune responses of bak1 bkk1 are caused by NLR activation. Furthermore, expression of HopB1, an effector derived from Pseudomonas syringae that cleaves activated BAK1 and its paralogs, leads to cell death similar to bak1 bkk1 , which requires ADR1 s. Our results indicate BAK1 and its paralogs serve as guardees for NLRs.

Nitrate (⁠ $NO3−$⁠) and ammonium (⁠ $NH4+$⁠) are major inorganic nitrogen (N) supplies for plants, but $NH4+$ as the sole or dominant N source causes growth inhibition in many plants, known as ammonium toxicity. Small amounts of $NO3−$ can significantly mitigate ammonium toxicity, and the anion channel SLAC1 homolog 3 (SLAH3) is involved in this process, but the mechanistic detail of how SLAH3 regulates nitrate-dependent alleviation of ammonium toxicity is still largely unknown. In this study, we identified SnRK1.1, a central regulator involved in energy homeostasis, and various stress responses, as a SLAH3 interactor in Arabidopsis (Arabidopsis thaliana). Our results suggest that SNF1-related protein kinase 1 (SnRK1.1) functions as a negative regulator of SLAH3. Kinase assays indicate SnRK1.1 strongly phosphorylates the C-terminal of SLAH3 at the site S601. Under high- $NH4+$/low-pH condition, phospho-mimetic and phospho-dead mutations in SLAH3 S601 result in barely rescued phenotypes and fully complemented phenotypes in slah3. Furthermore, SnRK1.1 migrates from cytoplasm to nucleus under high- $NH4+$/low-pH conditions. The translocation of SnRK1.1 from cytosol to nucleus under high-ammonium stress releases the inhibition on SLAH3, which allows SLAH3-mediated $NO3−$ efflux leading to alleviation of high- $NH4+$/low-pH stress. Our study reveals that the C-terminal phosphorylation also plays important role in SLAH3 regulation and provides additional insights into nitrate-dependent alleviation of ammonium toxicity in plants.

Ammonium (NH₄⁺) and nitrate (NO₃⁻) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH₄⁺ often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO₃⁻ usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations, referred to as nitrate‐dependent alleviation of ammonium toxicity. Our previous studies indicated a NO₃⁻ efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO₃⁻‐mediated NH₄⁺ detoxification is unknown. Previously, mutations in NO₃⁻ transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH₄⁺. Whereas, in this study, we found when the high‐NH₄⁺ medium was supplemented with low concentrations of NO₃⁻, nrt1.1 mutant plants showed hyper‐sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high‐NH₄⁺/low‐NO₃⁻ condition, suggesting NRT1.1 regulates ammonium toxicity by facilitating H⁺ uptake. Moreover, NRT1.1 was shown to interact with SLAH3 to form a transporter‐channel complex. Interestingly, SLAH3 appeared to affect NO₃⁻ influx while NRT1.1 influenced NO₃⁻ efflux, suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels. Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate‐dependent alleviation of ammonium toxicity through regulating NO₃⁻ transport and balancing rhizosphere acidification.

ROOT MERISTEM GROWTH FACTORs (RGFs), a group of peptide hormones, play key roles in root apical meristem development. In Arabidopsis (Arabidopsis thaliana), there are 11 members of RGFs, in which at least RGF1, RGF2, and RGF3 are expressed at the root tip and are involved in root stem cell niche maintenance. RGFs are perceived by five functionally redundant receptor-like protein kinases, RGF1 INSENSITIVE 1 (RGI1) to RGI5, to maintain the expression of two downstream APETALA 2 (AP2) transcription factor genes, PLETHORA 1 (PLT1) and PLT2, and to stabilize PLT2. RGI1 to RGI3 were also named RGF RECEPTOR 1 (RGFR1) to RGFR3, respectively. Although previous studies have suggested that BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and its paralogs, SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASEs (SERKs), may act as coreceptors of RGIs, comprehensive genetic and biochemical analyses have not been well documented. Here, we report that single, double, and triple mutants of SERKs show various degrees of short root phenotypes and insensitivity to exogenously applied RGF1. The interaction between RGIs and BAK1 and their mutual phosphorylation are RGF1 dependent. We also found that RGF1-induced MAPK activation relies on both RGIs and SERKs. We demonstrate that RGIs play redundant roles in regulating root apical meristem development. Therefore, we genetically and biochemically substantiated that SERKs, as coreceptors, play essential roles in the RGF1-mediated signaling pathway.