FERONIA, the kinase that phosphorylates PhyB.

The phosphorylation status of phyB changes dynamically in response to environmental conditions and critically governs the corresponding plant's responses. However, the kinase(s) that phosphorylates phyB is/are still unknown. Liu et al. have not only identified the kinase that phosphorylates phyB but also revealed its biological implications during salt stress.

BBX24 Interacts with JAZ3 to Promote Growth by Reducing DELLA Activity in Shade Avoidance.

Shade avoidance syndrome (SAS) is a strategy of major adaptive significance and typically includes elongation of the stem and petiole, leaf hyponasty, reduced branching and phototropic orientation of the plant shoot toward canopy gaps. Both cryptochrome 1 and phytochrome B (phyB) are the major photoreceptors that sense the reduction in the blue light fluence rate and the low red:far-red ratio, respectively, and both light signals are associated with plant density and the resource reallocation when SAS responses are triggered. The B-box (BBX)-containing zinc finger transcription factor BBX24 has been implicated in the SAS as a regulator of DELLA activity, but this interaction does not explain all the observed BBX24-dependent regulation in shade light. Here, through a combination of transcriptional meta-analysis and large-scale identification of BBX24-interacting transcription factors, we found that JAZ3, a jasmonic acid signaling component, is a direct target of BBX24. Furthermore, we demonstrated that joint loss of BBX24 and JAZ3 function causes insensitivity to DELLA accumulation, and the defective shade-induced elongation in this mutant is rescued by loss of DELLA or phyB function. Therefore, we propose that JAZ3 is part of the regulatory network that controls the plant growth in response to shade, through a mechanism in which BBX24 and JAZ3 jointly regulate DELLA activity. Our results provide new insights into the participation of BBX24 and JA signaling in the hypocotyl shade avoidance response in Arabidopsis.

Phytochromes A and B Mediate Light Stabilization of BIN2 to Regulate Brassinosteroid Signaling and Photomorphogenesis in Arabidopsis.

Phytochromes A and B (phyA and phyB) are the far-red and red lights photoreceptors mediating many light responses in Arabidopsis thaliana. Brassinosteroid (BR) is a pivotal phytohormone regulating a variety of plant developmental processes including photomorphogenesis. It is known that phyB interacts with BES1 to inhibit its DNA-binding activity and repress BR signaling. Here, we show that far-red and red lights modulate BR signaling through phyA and phyB regulation of the stability of BIN2, a glycogen synthase kinase 3 (GSK3)-like kinase that phosphorylates BES1/BZR1 to inhibit BR signaling. The BIN2 gain-of-function mutant bin2-1 displays an enhanced photomorphogenic phenotype in both far-red and red lights. phyA-enhanced accumulation of BIN2 promotes the phosphorylation of BES1 in far-red light. BIN2 acts genetically downstream from PHYA to regulate photomorphogenesis under far-red light. Both phyA and phyB interact directly with BIN2, which may promote the interaction of BIN2 with BES1 and induce the phosphorylation of BES1. Our results suggest that far-red and red lights inhibit BR signaling through phyA and phyB stabilization of BIN2 and promotion of BES1 phosphorylation, which defines a new layer of the regulatory mechanism that allows plants to coordinate light and BR signaling pathways to optimize photomorphogenesis.

Corrigendum: Phytochromes A and B mediate light stabilization of BIN2 to regulate brassinosteroid signaling and photomorphogenesis in Arabidopsis.

[This corrects the article DOI: 10.3389/fpls.2022.865019.].

Alternative Splicing Regulation During Light-Induced Germination of Arabidopsis thaliana Seeds.

Seed dormancy and germination are relevant processes for a successful seedling establishment in the field. Light is one of the most important environmental factors involved in the relief of dormancy to promote seed germination. In Arabidopsis thaliana seeds, phytochrome photoreceptors tightly regulate gene expression at different levels. The contribution of alternative splicing (AS) regulation in the photocontrol of seed germination is still unknown. The aim of this work is to study gene expression modulated by light during germination of A. thaliana seeds, with focus on AS changes. Hence, we evaluated transcriptome-wide changes in stratified seeds irradiated with a pulse of red (Rp) or far-red (FRp) by RNA sequencing (RNA-seq). Our results show that the Rp changes the expression of ∼20% of the transcriptome and modifies the AS pattern of 226 genes associated with mRNA processing, RNA splicing, and mRNA metabolic processes. We further confirmed these effects for some of the affected AS events. Interestingly, the reverse transcriptase-polymerase chain reaction (RT-PCR) analyses show that the Rp modulates the AS of splicing-related factors (At-SR30, At-RS31a, At-RS31, and At-U2AF65A), a light-signaling component (At-PIF6), and a dormancy-related gene (At-DRM1). Furthermore, while the phytochrome B (phyB) is responsible for the AS pattern changes of At-U2AF65A and At-PIF6, the regulation of the other AS events is independent of this photoreceptor. We conclude that (i) Rp triggers AS changes in some splicing factors, light-signaling components, and dormancy/germination regulators; (ii) phyB modulates only some of these AS events; and (iii) AS events are regulated by R and FR light, but this regulation is not directly associated with the intensity of germination response. These data will help in boosting research in the splicing field and our understanding about the role of this mechanism during the photocontrol of seed germination.

VASCULAR PLANT ONE-ZINC FINGER1 and VOZ2 repress the FLOWERING LOCUS C clade members to control flowering time in Arabidopsis.

Floral transition is regulated by environmental and endogenous signals. Previously, we identified VASCULAR PLANT ONE-ZINC FINGER1 (VOZ1) and VOZ2 as phytochrome B-interacting factors. VOZ1 and VOZ2 redundantly promote flowering and have pivotal roles in the downregulation of FLOWERING LOCUS C (FLC), a central repressor of flowering in Arabidopsis. Here, we showed that the late-flowering phenotypes of the voz1 voz2 mutant were suppressed by vernalization in the Columbia and FRIGIDA (FRI)-containing accessions, which indicates that the late-flowering phenotype of voz1 voz2 mutants was caused by upregulation of FLC. We also showed that the other FLC clade members, MADS AFFECTING FLOWERING (MAF) genes, were also a downstream target of VOZ1 and VOZ2 as their expression levels were also increased in the voz1 voz2 mutant. Our results suggest that the FLC clade genes integrate signals from VOZ1/VOZ2 and vernalization to regulate flowering.

Transcriptional programs related to phytochrome A function in Arabidopsis seed germination.

In Arabidopsis seeds, germination is promoted only by phytochromes, principally phytochrome B (phyB) and phytochrome A (phyA). Despite the abundant information concerning the molecular basis of phyB signaling downstream of PIF1/PIL5, the signaling network inducing germination by phyA is poorly known. Here, we describe the influence of phyA on the transcriptome of Arabidopsis seeds when germination is induced by a far-red (FR) pulse. The expression of 11% of the genome was significantly regulated by phyA. Most of the genes were up-regulated and the changes noted late (i.e. 5 h after a FR pulse), whereas changes in down-regulated genes were more abundant earlier (i.e. 0.5 h after a FR pulse). Auxin- and GA-associated elements were overrepresented in the genes that were modified by phyA. A significant number of genes whose expression was affected by phyA had not been previously reported to be dependent on PIL5. Among them, homozygotic mutant seeds of MYB66, a SAUR-like protein, PIN7, and GASA4 showed an impaired promotion of germination by phyA. Natural variation at the transcriptional level was found in early signaling and GA metabolic genes, but not in ABA metabolic and expansin genes between Columbia and Landsberg erecta accessions. Although phyA and phyB/PIL5 signaling pathways share some molecular components, our data suggest that phyA signaling is partially independent of PIL5 when germination is promoted by very low fluences of light.