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In natural environments, nitrogen (N) concentration in the soil fluctuates greatly and is often limiting for plant growth and development. The ability of plants to respond to changes in N availability is therefore essential for adaptation. The response of plants to N variations consists in particular of adjusting root N uptake systems and root architecture. To do so, plants integrate local sensing and signaling of external N availability with systemic sensing and signaling of their internal N status, in order to tune the functional and structural properties of the root system in accordance with the N demand for growth of the whole plant. Transcriptional regulation of gene expression is one of the most important processes plants use to adapt the properties of the root system in response to local and long-distance N pathways. This review focuses on the mechanisms that give rise to transcriptional responses in Arabidopsis roots under N fluctuations, with an emphasis on those associated with the regulation of nitrate uptake and transport systems. We discuss the putative long-distance signals triggering the gene expression responses, as well as the molecular players that locally induce transcriptional changes. We also highlight several observations revealing the importance of adopting an integrative approach in the regulation of N signaling. [ABSTRACT FROM AUTHOR]

The molecular mechanisms of abscisic acid (ABA) signalling have been studied for many years; however, how mitochondria-localized proteins play roles in ABA signalling remains unclear. Here an Arabidopsis mitochondria-localized protein RRL (RETARDED ROOT GROWTH-LIKE) was shown to function in ABA signalling. A previous study had revealed that the Arabidopsis mitochondria-localized protein RRG (RETARDED ROOT GROWTH) is required for cell division in the root meristem. RRL shares 54% and 57% identity at the nucleotide and amino acid sequences, respectively, with RRG; nevertheless, RRL shows a different function in Arabidopsis. In this study, disruption of RRL decreased ABA sensitivity whereas overexpression of RRL increased ABA sensitivity during seed germination and seedling growth. High expression levels of RRL were found in germinating seeds and developing seedlings, as revealed by ß-glucuronidase (GUS) staining of ProRRL-GUS transgenic lines. The analyses of the structure and function of mitochondria in the knockout rrl mutant showed that the disruption of RRL causes extensively internally vacuolated mitochondria and reduced ABA-stimulated reactive oxygen species (ROS) production. Previous studies have revealed that the expression of alternative oxidase (AOX) in the alternative respiratory pathway is increased by mitochondrial retrograde regulation to regain ROS levels when the mitochondrial electron transport chain is impaired. The APETALA2 (AP2)-type transcription factor ABI4 is a regulator of ALTERNATIVE OXIDASE1a (AOX1a) in mitochondrial retrograde signalling. This study showed that ABA-induced AOX1a and ABI4 expression was inhibited in the rrl mutant, suggesting that RRL is probably involved in ABI4-mediated mitochondrial retrograde signalling. Furthermore, the results revealed that ABI4 is a downstream regulatory factor in RRL-mediated ABA signalling in seed germination and seedling growth. [ABSTRACT FROM AUTHOR]

In flowering plant reproduction, pollen tube reception is the signaling system that results in pollen tube discharge, synergid degeneration, and successful delivery of male gametes (two sperm cells) to the site where they can fuse with female gametes (egg cell and central cell). Some molecules required for this complex and essential signaling exchange have been identified; however, fundamental questions about the nature of the interactions between the pollen tube and the synergid cells remain to be clarified. Here, we monitor pollen tube arrival, pollen tube discharge, and synergid degeneration in Arabidopsis (Arabidopsis thaliana) wild type and in male and female gametophytic mutants that disrupt development and function of the gametophytes. By combining assays used previously to study these interactions and an assay that facilitates simultaneous analysis of pollen tube discharge and synergid degeneration, we find that synergid degeneration could be initiated without pollen tube discharge. Our data support the hypothesis that pollen tube-synergid contact, or signaling via secreted molecules, initiates receptive synergid degeneration. We also find that when pollen tubes successfully burst, they always discharge into a degenerated synergid. In addition to this pollen tube-dependent promotion of synergid degeneration, we also show that a basal developmental pathway mediates synergid degeneration in the absence of pollination. Our results are consistent with the model that a complex set of interactions between the pollen tube and synergid cells promote receptive synergid degeneration. [ABSTRACT FROM AUTHOR]

The root meristem of Arabidopsis thaliana harbours a pool of stem cells, which divide to give rise to the differentiated cells of the various root tissues. Regulatory networks of inter-cellular signalling molecules control the homeostasis of stem cell number and position so that both stem and differentiated cells are consistently available. This work focuses on the transcription factor WUSCHEL-RELATED HOMEOBOX 5 (WOX5), the signalling peptide CLAVATA3/EMBRYOSURROUNDING REGION 40 (CLE40) and the feedback loops involving them, which maintain the columella stem cells (CSCs). WOX5 signals from the quiescent centre (QC) to promote stem cell fate, while CLE40 is secreted from the differentiated columella cells (CCs) to promote differentiation. Our analyses of mathematical models of this network show that, when cell fate is controlled primarily by antagonistic factors, homeostasis requires a spatial component and inter-cellular signalling. We have also found that WOX5 contributes to, but is not absolutely necessary for, CSC maintenance. Furthermore, our modelling led us to postulate an additional signalling molecule that promotes CSC maintenance. We propose that this WOX5-independent signal originates in the QC, is targeted by CLE40 signalling and is capable of maintaining CSCs. [ABSTRACT FROM AUTHOR]

During the last decade, significant research progress has been made in Arabidopsis thaliana in defining the molecular mechanisms behind the plant circadian clock. The circadian clock must have the ability to integrate both external light and ambient temperature signals into its transcriptional circuitry to regulate its function properly. We previously showed that transcription of a set of clock genes including LUX (LUX ARRHYTHMO), GI (GIGANTEA), LNK1 (NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED GENE 1), PRR9 (PSEUDO-RESPONSE REGULATOR 9) and PRR7 is commonly regulated through the evening complex (EC) night-time repressor in response to both moderate changes in temperature (Δ6°C) and differences in steady-state growth-compatible temperature (16–28°C). Here, we further show that a night-time-light signal also feeds into the circadian clock transcriptional circuitry through the EC night-time repressor, so that the same set of EC target genes is up-regulated in response to a night-time-light pulse. This light-induced event is dependent on phytochromes, but not cryptochromes. Interestingly, both the warm-night and night-time-light signals negatively modulate the activity of the EC night-time repressor in a synergistic manner. In other words, an exponential burst of transcription of the EC target genes is observed only when these signals are simultaneously fed into the repressor. Taken together, we propose that the EC night-time repressor plays a crucial role in modulating the clock transcriptional circuitry to keep track properly of seasonal changes in photo- and thermal cycles by conservatively double-checking the external light and ambient temperature signals. [ABSTRACT FROM AUTHOR]