The Dof transcription factor COG1 acts as a key regulator of plant biomass by promoting photosynthesis and starch accumulation.

Photosynthetic efficiency is the primary determinant of crop yield, including vegetative biomass and grain yield. Manipulation of key transcription factors known to directly control photosynthetic machinery can be an effective strategy to improve photosynthetic traits. In this study, we identified an Arabidopsis gain-of-function mutant, cogwheel1-3D, that shows a significantly enlarged rosette and increased biomass compared with wild-type plants. Overexpression of COG1, a Dof transcription factor, recapitulated the phenotype of cogwheel1-3D, whereas knocking out COG1 and its six paralogs resulted in a reduced rosette size and decreased biomass. Transcriptomic and quantitative reverse transcription polymerase chain reaction analyses demonstrated that COG1 and its paralogs were required for light-induced expression of genes involved in photosynthesis. Further chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that COG1 can directly bind to the promoter regions of multiple genes encoding light-harvesting antenna proteins. Physiological, biochemical, and microscopy analyses revealed that COG1 enhances photosynthetic capacity and starch accumulation in Arabidopsis rosette leaves. Furthermore, combined results of bioinformatic, genetic, and molecular experiments suggested that the functions of COG1 in increasing biomass are conserved in different plant species. These results collectively demonstrated that COG1 acts as a key regulator of plant biomass by promoting photosynthesis and starch accumulation. Manipulating COG1 to optimize photosynthetic capacity would create new strategies for future crop yield improvement.

The type-B response regulators ARR10, ARR12, and ARR18 specify the central cell in Arabidopsis.

In Arabidopsis thaliana, the female gametophyte consists of two synergid cells, an egg cell, a diploid central cell, and three antipodal cells. CYTOKININ INDEPENDENT 1 (CKI1), a histidine kinase constitutively activating the cytokinin signaling pathway, specifies the central cell and restricts the egg cell. However, the mechanism regulating CKI1-dependent central cell specification is largely unknown. Here, we showed that the type-B ARABIDOPSIS RESPONSE REGULATORS10, 12, and 18 (ARR10/12/18) localize at the chalazal pole of the female gametophyte. Phenotypic analysis showed that the arr10 12 18 triple mutant is female sterile. We examined the expression patterns of embryo sac marker genes and found that the embryo sac of arr10 12 18 plants had lost central cell identity, a phenotype similar to that of the Arabidopsis cki1 mutant. Genetic analyses demonstrated that ARR10/12/18, CKI1, and ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN2, 3, and 5 (AHP2/3/5) function in a common pathway to regulate female gametophyte development. In addition, constitutively activated ARR10/12/18 in the cki1 embryo sac partially restored the fertility of cki1. Results of transcriptomic analysis supported the conclusion that ARR10/12/18 and CKI1 function together to regulate the identity of the central cell. Our results demonstrated that ARR10/12/18 function downstream of CKI1-AHP2/3/5 as core factors to determine cell fate of the female gametophyte.

Regulation of Brassinosteroid Homeostasis in Higher Plants.

Brassinosteroids (BRs) are known as one of the major classes of phytohormones essential for various processes during normal plant growth, development, and adaptations to biotic and abiotic stresses. Significant progress has been achieved on revealing mechanisms regulating BR biosynthesis, catabolism, and signaling in many crops and in model plant Arabidopsis. It is known that BRs control plant growth and development in a dosage-dependent manner. Maintenance of BR homeostasis is therefore critical for optimal functions of BRs. In this review, updated discoveries on mechanisms controlling BR homeostasis in higher plants in response to internal and external cues are discussed.

CIK Receptor Kinases Determine Cell Fate Specification during Early Anther Development in Arabidopsis.

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.

Receptor-like protein kinases: Key regulators controlling root hair development in Arabidopsis thaliana.

Root hairs are tubular outgrowths specifically differentiated from epidermal cells in a differentiation zone. The formation of root hairs greatly increases the surface area of a root and maximizes its ability to absorb water and inorganic nutrients essential for plant growth and development. Root hair development is strictly regulated by intracellular and intercellular signal communications. Cell surface-localized receptor-like protein kinases (RLKs) have been shown to be important components in these cellular processes. In this review, the functions of a number of key RLKs in regulating Arabidopsis root hair development are discussed, especially those involved in root epidermal cell fate determination and root hair tip growth.

A group of receptor kinases are essential for CLAVATA signalling to maintain stem cell homeostasis.

Continuous organ initiation and outgrowth in plants relies on the proliferation and differentiation of stem cells maintained by the CLAVATA (CLV)-WUSCHEL (WUS) negative-feedback loop1-3. Leucine-rich repeat receptor-like protein kinases (LRR-RLKs), including CLV1, BARELY ANY MERISTEMS and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), a receptor-like protein CLV2 and a pseudokinase CORYNE (CRN) are involved in the perception of the CLV3 signal to repress WUS expression4-10. WUS, a homeodomain transcription factor, in turn directly activates CLV3 expression and promotes stem cell activity in the shoot apical meristem11,12. However, the signalling mechanism immediately following the perception of CLV3 by its receptors is poorly understood. Here, we show that a group of LRR-RLKs, designated as CLAVATA3 INSENSITIVE RECEPTOR KINASES (CIKs), have essential roles in regulating CLV3-mediated stem cell homeostasis. The cik1 2 3 4 quadruple mutant exhibits a significantly enlarged SAM, resembling clv mutants. Genetic analyses and biochemical assays demonstrated that CIKs function as co-receptors of CLV1, CLV2/CRN and RPK2 to mediate CLV3 signalling through phosphorylation. Our findings not only widen the understanding of the underlying mechanism of CLV3 signal transduction in regulating stem cell fate but also reveal a novel group of RLKs that function as co-receptors to possibly mediate multiple extrinsic and intrinsic signals during plant growth and development.

Brassinosteroid Biosynthesis Is Modulated via a Transcription Factor Cascade of COG1, PIF4, and PIF5.

Brassinosteroids (BRs) are essential phytohormones regulating various developmental and physiological processes during normal growth and development. cog1-3D (cogwheel1-3D) was identified as an activation-tagged genetic modifier of bri1-5, an intermediate BR receptor mutant in Arabidopsis (Arabidopsis thaliana). COG1 encodes a Dof-type transcription factor found previously to act as a negative regulator of the phytochrome signaling pathway. cog1-3D single mutants show an elongated hypocotyl phenotype under light conditions. A loss-of-function mutant or inducible expression of a dominant negative form of COG1 in the wild type results in an opposite phenotype. A BR profile assay indicated that BR levels are elevated in cog1-3D seedlings. Quantitative reverse transcription-polymerase chain reaction analyses showed that several key BR biosynthetic genes are significantly up-regulated in cog1-3D compared with those of the wild type. Two basic helix-loop-helix transcription factors, PIF4 and PIF5, were found to be transcriptionally up-regulated in cog1-3D Genetic analysis indicated that PIF4 and PIF5 were required for COG1 to promote BR biosynthesis and hypocotyl elongation. Chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that COG1 binds to the promoter regions of PIF4 and PIF5, and PIF4 and PIF5 bind to the promoter regions of key BR biosynthetic genes, such as DWF4 and BR6ox2, to directly promote their expression. These results demonstrated that COG1 regulates BR biosynthesis via up-regulating the transcription of PIF4 and PIF5.

RGF1 INSENSITIVE 1 to 5, a group of LRR receptor-like kinases, are essential for the perception of root meristem growth factor 1 in Arabidopsis thaliana.

RGF1, a secreted peptide hormone, plays key roles in root meristem development in Arabidopsis. Previous studies indicated that a functional RGF1 needs to be sulfated at a tyrosine residue by a tyrosylprotein sulfotransferase and that RGF1 regulates the root meristem activity mainly via two downstream transcription factors, PLETHORA 1 (PLT1) and PLT2. How extracellular RGF1 is perceived by a plant cell, however, is unclear. Using genetic approaches, we discovered a clade of leucine-rich repeat receptor-like kinases, designated as RGF1 INSENSITIVE 1 (RGI1) to RGI5, serving as receptors of RGF1. Two independent rgi1 rgi2 rgi3 rgi4 rgi5 quintuple mutants display a consistent short primary root phenotype with a small size of meristem. An rgi1 rgi2 rgi3 rgi4 quadruple mutant shows a significantly reduced sensitivity to RGF1, and the quintuple mutant is completely insensitive to RGF1. The expression of PLT1 and PLT2 is almost undetectable in the quintuple mutant. Ectopic expression of PLT2 driven by an RGI2 promoter in the quintuple mutant greatly rescued its root meristem defects. One of the RGIs, RGI1, was subsequently analyzed biochemically in detail. In vitro dot blotting and pull-down analyses indicated that RGI1 can physically interact with RGF1. Exogenous application of RGF1 can quickly and simultaneously induce the phosphorylation and ubiquitination of RGI1, indicating that RGI1 can perceive and transduce the RGF1 peptide signal. Yet, the activated RGI1 is likely turned over rapidly. These results demonstrate that RGIs, acting as the receptors of RGF1, play essential roles in RGF1-PLT-mediated root meristem development in Arabidopsis thaliana.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis.

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

Somatic embryogenesis receptor kinases control root development mainly via brassinosteroid-independent actions in Arabidopsis thaliana.

Brassinosteroids (BRs), a group of plant steroidal hormones, play critical roles in many aspects of plant growth and development. Previous studies showed that BRI1-mediated BR signaling regulates cell division and differentiation during Arabidopsis root development via interplaying with auxin and other phytohormones. Arabidopsis somatic embryogenesis receptor-like kinases (SERKs), as co-receptors of BRI1, were found to play a fundamental role in an early activation step of BR signaling pathway. Here we report a novel function of SERKs in regulating Arabidopsis root development. Genetic analyses indicated that SERKs control root growth mainly via a BR-independent pathway. Although BR signaling pathway is completely disrupted in the serk1 bak1 bkk1 triple mutant, the root growth of the triple mutant is much severely damaged than the BR deficiency or signaling null mutants. More detailed analyses indicated that the triple mutant exhibited drastically reduced expression of a number of genes critical to polar auxin transport, cell cycle, endodermis development and root meristem differentiation, which were not observed in null BR biosynthesis mutant cpd and null BR signaling mutant bri1-701.