Blue Light-excited Light-Oxygen-Voltage-sensing Domain 2 (LOV2) Triggers a Rearrangement of the Kinase Domain to Induce Phosphorylation Activity in Arabidopsis Phototropin1.

Phototropin1 is a blue light (BL) receptor in plants and shows BL-dependent kinase activation. The BL-excited light-oxygen-voltage-sensing domain 2 (LOV2) is primarily responsible for the activation of the kinase domain; however, the molecular mechanism by which conformational changes in LOV2 are transmitted to the kinase domain remains unclear. Here, we investigated BL-induced structural changes of a minimum functional fragment of Arabidopsis phototropin1 composed of LOV2, the kinase domain, and a linker connecting the two domains using small-angle x-ray scattering (SAXS). The fragment existed as a dimer and displayed photoreversible SAXS changes reflected in the radii of gyration of 42.9 Å in the dark and 48.8 Å under BL irradiation. In the dark, the molecular shape reconstructed from the SAXS profiles appeared as two bean-shaped lobes in a twisted arrangement that was 170 Å long, 80 Å wide, and 50 Å thick. The molecular shape under BL became slightly elongated from that in the dark. By fitting the crystal structure of the LOV2 dimer and a homology model of the kinase domain to their inferred shapes, the BL-dependent change could be interpreted as the positional shift in the kinase domain relative to that of the LOV2 dimer. In addition, we found that lysine 475, a functionally important residue, in the N-terminal region of LOV2 plays a critical role in transmitting the structural changes in LOV2 to the kinase domain. The interface between the domains is critical for signaling, suitably changing the structure to activate the kinase in response to conformational changes in the adjoining LOV2.

Light-induced conformational changes of LOV1 (light oxygen voltage-sensing domain 1) and LOV2 relative to the kinase domain and regulation of kinase activity in Chlamydomonas phototropin.

Phototropin (phot), a blue light (BL) receptor in plants, has two photoreceptive domains named LOV1 and LOV2 as well as a Ser/Thr kinase domain (KD) and acts as a BL-regulated protein kinase. A LOV domain harbors a flavin mononucleotide that undergoes a cyclic photoreaction upon BL excitation via a signaling state in which the inhibition of the kinase activity by LOV2 is negated. To understand the molecular mechanism underlying the BL-dependent activation of the kinase, the photochemistry, kinase activity, and molecular structure were studied with the phot of Chlamydomonas reinhardtii. Full-length and LOV2-KD samples of C. reinhardtii phot showed cyclic photoreaction characteristics with the activation of LOV- and BL-dependent kinase. Truncation of LOV1 decreased the photosensitivity of the kinase activation, which was well explained by the fact that the signaling state lasted for a shorter period of time compared with that of the phot. Small angle x-ray scattering revealed monomeric forms of the proteins in solution and detected BL-dependent conformational changes, suggesting an extension of the global molecular shapes of both samples. Constructed molecular model of full-length phot based on the small angle x-ray scattering data proved the arrangement of LOV1, LOV2, and KD for the first time that showed a tandem arrangement both in the dark and under BL irradiation. The models suggest that LOV1 alters its position relative to LOV2-KD under BL irradiation. This finding demonstrates that LOV1 may interact with LOV2 and modify the photosensitivity of the kinase activation through alteration of the duration of the signaling state in LOV2.

Photosensitivity of kinase activation by blue light involves the lifetime of a cysteinyl-flavin adduct intermediate, S390, in the photoreaction cycle of the LOV2 domain in phototropin, a plant blue light receptor.

Phototropin (phot) is a light-regulated protein kinase that mediates a variety of photoresponses in plants, such as phototropism, chloroplast positioning, and stomata opening. Arabidopsis has two homologues, phot1 and phot2, that share physiological functions depending on light intensity. A phot molecule has two photoreceptive light oxygen voltage-sensing domains, LOV1 and LOV2, and a Ser/Thr kinase domain. The LOV domains undergo a photocycle upon blue light (BL) stimulation, including transient adduct formation between the chromophore and a conserved cysteine (S390 intermediate) that leads to activation of the kinase. To uncover the mechanism underlying the photoactivation of the kinase, we have introduced a kinase assay system composed of a phot1 LOV2-linker-kinase polypeptide as a light-regulated kinase and its N-terminal polypeptide as an artificial substrate (Okajima, K., Matsuoka, D., and Tokutomi, S. (2011) LOV2-linker-kinase phosphorylates LOV1-containing N-terminal polypeptide substrate via photoreaction of LOV2 in Arabidopsis phototropin1. FEBS Lett. 585, 3391-3395). In the present study, we extended the assay system to phot2 and compared the photochemistry and kinase activation by BL between phot1 and phot2 to gain insight into the molecular basis for the different photosensitivities of phot1 and phot2. Photosensitivity of kinase activation by BL and the lifetime of S390 of phot1 were 10 times higher and longer, respectively, than those of phot2. This correlation was confirmed by an amino acid substitution experiment with phot1 to shorten the lifetime of S390. The present results demonstrated that the photosensitivity of kinase activation in phot involves the lifetime of S390 in LOV2, suggesting that the lifetime is one of the key factors for the different photosensitivities observed for phot1 and phot2.

Modification of tomato breeding traits and plant hormone signaling by target-AID, the genome-editing system inducing efficient nucleotide substitution.

Target activation-induced cytidine deaminase (Target-AID), a novel CRISPR/Cas9-based genome-editing tool, confers the base-editing capability on the Cas9 genome-editing system. It involves the fusion of cytidine deaminase (CDA), which catalyzes cytidine (C) to uridine (U) substitutions, to the mutated nickase-type nCas9 or deactivated-type dCas9. To confirm and extend the applicability of the Target-AID genome-editing system in tomatoes (Solanum lycopersicum L.), we transformed the model tomato cultivar "Micro-Tom" and commercial tomato cultivars using this system by targeting SlDELLA, which encodes a negative regulator of the plant phytohormone gibberellic acid (GA) signaling pathway. We confirmed that the nucleotide substitutions were induced by the Target-AID system, and we isolated mutants showing high GA sensitivity in both "Micro-Tom" and the commercial cultivars. Moreover, by successfully applying this system to ETHYLENE RECEPTOR 1 (SlETR1) with single sgRNA targeting, double sgRNA targeting, as well as dual-targeting of both SlETR1 and SlETR2 with a single sgRNA, we demonstrated that the Target-AID genome-editing system is a promising tool for molecular breeding in tomato crops. This study highlights an important aspect of the scientific and agricultural potential of the combinatorial use of the Target-AID and other base-editing systems.

Light-induced movement of the LOV2 domain in an Asp720Asn mutant LOV2-kinase fragment of Arabidopsis phototropin 2.

Phototropin, a blue-light receptor protein of plants, triggers phototropic responses, chloroplast relocation, and opening of stomata to maximize the efficiency of photosynthesis. Phototropin is composed of two light-oxygen-voltage sensing domains (LOV1 and LOV2) that absorb blue light and a serine/theroine kinase domain responsible for light-dependent autophosphorylation leading to cellular signaling cascades. Although the light-activated LOV2 domain is primarily responsible for subsequent activation of the kinase domain, it is unclear how conformational changes in the former transmit to the latter. To understand this molecular mechanism in Arabidopsis phototropin 2, we performed small-angle X-ray scattering analysis on a fragment composed of the LOV2 and kinase domains, which contained an Asp720Asn mutation that led to an absence of ATP binding activity. The scattering data were collected up to a resolution of 25 Å. The apparent molecular weight of the fragment estimated from scattering intensities demonstrated that the fragment existed in a monomeric form in solution. The fragment exhibited photoreversible changes in the scattering profiles, and the radii of gyration under dark and blue-light irradiation conditions were 32.4 and 34.8 Å, respectively. In the dark, the molecular shape restored from the scattering profile appeared as an elongated shape of 110 Å in length and 45 Å in width. The homology modeled LOV2 and kinase domains could be fitted to the molecular shape and appeared to make slight contact. However, under blue-light irradiation, a more extended molecular shape was observed. The changes in the molecular shape and radius of gyration were interpreted as a light-dependent positional shift of the LOV2 domain of approximately 13 Å from the kinase domain. Because the region connecting the LOV2 and kinase domains was categorized as a naturally unfolded polypeptide, we propose that the light-activated LOV2 domain triggers conformational changes in the linker region to separate the LOV2 and kinase domains.

Corrigendum: Loss-of-Function of a Tomato Receptor-Like Kinase Impairs Male Fertility and Induces Parthenocarpic Fruit Set.

[This corrects the article DOI: 10.3389/fpls.2019.00403.].

Loss-of-Function of a Tomato Receptor-Like Kinase Impairs Male Fertility and Induces Parthenocarpic Fruit Set.

Parthenocarpy arises when an ovary develops into fruit without pollination/fertilization. The mechanisms involved in genetic parthenocarpy have attracted attention because of their potential application in plant breeding and also for their elucidation of the mechanisms involved in early fruit development. We have isolated and characterized a novel small parthenocarpic fruit and flower (spff) mutant in the tomato (Solanum lycopersicum) cultivar Micro-Tom. This plant showed both vegetative and reproductive phenotypes including dwarfism of floral organs, male sterility, delayed flowering, altered axillary shoot development, and parthenocarpic production of small fruits. Genome-wide single nucleotide polymorphism array analysis coupled with mapping-by-sequencing using next generation sequencing-based high-throughput approaches resulted in the identification of a candidate locus responsible for the spff mutant phenotype. Subsequent linkage analysis and RNA interference-based silencing indicated that these phenotypes were caused by a loss-of-function mutation of a single gene (Solyc04g077010), which encodes a receptor-like protein kinase that was expressed in vascular bundles in young buds. Cytological and transcriptomic analyses suggested that parthenocarpy in the spff mutant was associated with enlarged ovarian cells and with elevated expression of the gibberellin metabolism gene, GA20ox1. Taken together, our results suggest a role for Solyc04g077010 in male organ development and indicate that loss of this receptor-like protein kinase activity could result in parthenocarpy.

Application and development of genome editing technologies to the Solanaceae plants.

Genome editing technology using artificial nucleases, including zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and clustered regulatory interspaced short palindromic repeats (CRISPR)-Cas9, can mutagenize the target sites of genes of interest. This technology has been successfully applied in several crops, including the Solanaceae plants, such as tomato, potato, tobacco, and petunia. Among the three nucleases, CRISPR-Cas9 is the best for breeding, crop improvement, and the functional analysis of genes of interest, because of its simplicity and high efficiency. Although the technology is useful for reverse genetics, its use in plants is limited due to a lack of regeneration protocols and sequence information. In this review, the present status of genome editing technology in Solanaceae plants is described, and techniques that may improve genome editing technologies are discussed.

Targeted base editing in rice and tomato using a CRISPR-Cas9 cytidine deaminase fusion.

We applied a fusion of CRISPR-Cas9 and activation-induced cytidine deaminase (Target-AID) for point mutagenesis at genomic regions specified by single guide RNAs (sgRNAs) in two crop plants. In rice, we induced multiple herbicide-resistance point mutations by multiplexed editing using herbicide selection, while in tomato we generated marker-free plants with homozygous heritable DNA substitutions, demonstrating the feasibility of base editing for crop improvement.

The linker between LOV2-Jα and STK plays an essential role in the kinase activation by blue light in Arabidopsis phototropin1, a plant blue light receptor.

Phototropin (phot), a blue light receptor in plants, is composed of several domains: LOV1, LOV2, and a serine/threonine kinase (STK). LOV2 is the main regulator of light activation of STK. However, the detailed mechanism remains unclear. In this report, we focused on the linker region between LOV2 and STK excluding the Jα-helix. Spectroscopy and a kinase assay for the substituents in the linker region of Arabidopsis phot1 LOV2-STK indicated that the linker is involved in the activation of STK. A putative module in the middle of the linker would be critical for intramolecular signaling and/or regulation of STK.

Essential role of the A'α/Aß gap in the N-terminal upstream of LOV2 for the blue light signaling from LOV2 to kinase in Arabidopsis photototropin1, a plant blue light receptor.

Phototropin (phot) is a blue light (BL) receptor in plants and is involved in phototropism, chloroplast movement, stomata opening, etc. A phot molecule has two photo-receptive domains named LOV (Light-Oxygen-Voltage) 1 and 2 in its N-terminal region and a serine/threonine kinase (STK) in its C-terminal region. STK activity is regulated mainly by LOV2, which has a cyclic photoreaction, including the transient formation of a flavin mononucleotide (FMN)-cysteinyl adduct (S390). One of the key events for the propagation of the BL signal from LOV2 to STK is conformational changes in a Jα-helix residing downstream of the LOV2 C-terminus. In contrast, we focused on the role of the A'α-helix, which is located upstream of the LOV2 N-terminus and interacts with the Jα-helix. Using LOV2-STK polypeptides from Arabidopsis thaliana phot1, we found that truncation of the A'α-helix and amino acid substitutions at Glu474 and Lys475 in the gap between the A'α and the Aß strand of LOV2 (A'α/Aß gap) to Ala impaired the BL-induced activation of the STK, although they did not affect S390 formation. Trypsin digested the LOV2-STK at Lys603 and Lys475 in a light-dependent manner indicating BL-induced structural changes in both the Jα-helix and the gap. The digestion at Lys603 is faster than at Lys475. These BL-induced structural changes were observed with the Glu474Ala and the Lys475Ala substitutes, indicating that the BL signal reached the Jα-helix as well as the A'α/Aß gap but could not activate STK. The amino acid residues, Glu474 and Lys475, in the gap are conserved among the phots of higher plants and may act as a joint to connect the structural changes in the Jα-helix with the activation of STK.