Regulatory Role of Circadian Clocks on ABA Production and Signaling, Stomatal Responses, and Water-Use Efficiency under Water-Deficit Conditions

Plants deploy molecular, physiological, and anatomical adaptations to cope with long-term water-deficit exposure, and some of these processes are controlled by circadian clocks. Circadian clocks are endogenous timekeepers that autonomously modulate biological systems over the course of the day–night cycle. Plants’ responses to water deficiency vary with the time of the day. Opening and closing of stomata, which control water loss from plants, have diurnal responses based on the humidity level in the rhizosphere and the air surrounding the leaves. Abscisic acid (ABA), the main phytohormone modulating the stomatal response to water availability, is regulated by circadian clocks. The molecular mechanism of the plant’s circadian clock for regulating stress responses is composed not only of transcriptional but also posttranscriptional regulatory networks. Despite the importance of regulatory impact of circadian clock systems on ABA production and signaling, which is reflected in stomatal responses and as a consequence influences the drought tolerance response of the plants, the interrelationship between circadian clock, ABA homeostasis, and signaling and water-deficit responses has to date not been clearly described. In this review, we hypothesized that the circadian clock through ABA directs plants to modulate their responses and feedback mechanisms to ensure survival and to enhance their fitness under drought conditions. Different regulatory pathways and challenges in circadian-based rhythms and the possible adaptive advantage through them are also discussed.Peer Reviewe

Beyond Arabidopsis: BBX Regulators in Crop Plants

B-box proteins represent diverse zinc finger transcription factors and regulators forming large families in various plants. A unique domain structure defines them—besides the highly conserved B-box domains, some B-box (BBX) proteins also possess CCT domain and VP motif. Based on the presence of these specific domains, they are mostly classified into five structural groups. The particular members widely differ in structure and fulfill distinct functions in regulating plant growth and development, including seedling photomorphogenesis, the anthocyanins biosynthesis, photoperiodic regulation of flowering, and hormonal pathways. Several BBX proteins are additionally involved in biotic and abiotic stress response. Overexpression of some BBX genes stimulates various stress-related genes and enhanced tolerance to different stresses. Moreover, there is evidence of interplay between B-box and the circadian clock mechanism. This review highlights the role of BBX proteins as a part of a broad regulatory network in crop plants, considering their participation in development, physiology, defense, and environmental constraints. A description is also provided of how various BBX regulators involved in stress tolerance were applied in genetic engineering to obtain stress tolerance in transgenic crops

Genome-wide survey of B-box proteins in potato (<i>Solanum tuberosum</i>)—Identification, characterization and expression patterns during diurnal cycle, etiolation and de-etiolation

<div><p>Plant B-box domain proteins (BBX) mediate many light-influenced developmental processes including seedling photomorphogenesis, seed germination, shade avoidance and photoperiodic regulation of flowering. Despite the wide range of potential functions, the current knowledge regarding BBX proteins in major crop plants is scarce. In this study, we identify and characterize the <i>StBBX</i> gene family in potato, which is composed of 30 members, with regard to structural properties and expression profiles under diurnal cycle, etiolation and de-etiolations. Based on domain organization and phylogenetic relationships, <i>StBBX</i> genes have been classified into five groups. Using real-time quantitative PCR, we found that expression of most of them oscillates following a 24-h rhythm; however, large differences in expression profiles were observed between the genes regarding amplitude and position of the maximal and minimal expression levels in the day/night cycle. On the basis of the time-of-day/time-of-night, we distinguished three expression groups specifically expressed during the light and two during the dark phase. In addition, we showed that the expression of several <i>StBBX</i> genes is under the control of the circadian clock and that some others are specifically associated with the etiolation and de-etiolation conditions. Thus, we concluded that StBBX proteins are likely key players involved in the complex diurnal and circadian networks regulating plant development as a function of light conditions and day duration.</p></div

Regulation of <i>StBBX</i> family genes expression during etiolation and de-etiolation.

<p>Analysis of <i>StBBX</i> transcript abundances in 2-week-old phytotron-grown <i>S</i>. <i>tuberosum</i> plants under a 14-h photoperiod—control plants (solid blue line) and a continuous dark followed by a 14-h photoperiod (black bold line). Samples were collected at the indicated time points. qRT-PCR was performed as described in Material and Methods.</p

Diurnal and circadian regulation of <i>StBBX</i> family genes expression.

<p>Analysis of <i>StBBX</i> transcript abundances in 2-week-old phytotron-grown <i>S</i>. <i>tuberosum</i> plants under a 14-h photoperiod (solid blue line) and continuous light (dashed red line) for 42 h during the subjective light and dark phases. Samples were collected at the indicated time points. qRT-PCR was performed as described in Material and Methods. To determine statistical significance between the maximum and minimum levels of the corresponding transcripts oscillation of the <i>StBBX</i> genes, we applied the Student's T-test.</p

Classification of <i>StBBX</i> genes from <i>Solanum tuberosum</i>, cv. Desiree in terms of their expression in diurnal and circadian cycle and etiolation and de-etiolation conditions.

<p>Classification of <i>StBBX</i> genes from <i>Solanum tuberosum</i>, cv. Desiree in terms of their expression in diurnal and circadian cycle and etiolation and de-etiolation conditions.</p

Phylogenetic analysis of the potato B-box family.

<p>Full-length proteins of the 30 B-box members were aligned using MUSCLE in MEGA 6.06 software with default parameters [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177471#pone.0177471.ref049" target="_blank">49</a>]. The achieved alignment was used as a input to construct the phylogenetic tree with 1000 bootstrap replicates.</p

Structural classification of the StBBX family proteins.

<p>The name and the length of corresponding StBBX protein and the position of characteristic B-box 1, B-box 2, CCT domains is shown on the right. The location and order of B-box 1, B-box 2 and CCT domains and VP motif within each protein is presented on the diagram.</p

Locations of 30 <i>BBX</i> genes on 12 potato chromosomes.

<p>The scale on the left is in megabases. Chromosome numbers are indicated at the top of each bar. The gene names on the left and the right side of each chromosome correspond to the approximate locations of each <i>BBX</i> gene. An additional copy of <i>StBBX*</i> gene identified using potato pseudomolecule database v 4.04 was marked in red.</p