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.


Introduction
Intricate regulation of plant growth and development processes depends mainly on precise spatial and temporal control of gene expression mediated by chromatin modifications in reply to endogenous or external stimuli in the environment. Recognition of the transcriptional profile of genes encoding most plant-specific transcription factors and chromatin regulators is fundamental for understanding and elucidating many plant biological processes. Recent studies have demonstrated significant findings regarding B-box (BBX) proteins, representing a diverse group of zinc finger transcription factors and regulators based on their structure and functions.
The BBX genes have been present in all eukaryotic genomes analyzed so far, with the highest number of members within all kingdoms. The availability of complete plant genomic sequences has led to the identification of the B-box (BBX) gene family, consisting of 64 BBX representatives in apple, 37 in white pear, 32 in Arabidopsis, 30 in rice and potato, 29 in tomato, 25 in pear, and 24 in grapevine [1][2][3][4][5][6][7][8]. Regardless of the species, all BBX family members have one single B-box domain or two arranged in tandem, classified into two types, known as B-box1 (B1) and B-box2 (B2), depending on their consensus sequence and the distance between the zinc-binding residues. Some BBX proteins also possess CCT-domain and several conserved motifs localized outside the domains mentioned above [9,10].
In plants, especially in Arabidopsis, the BBX family has been significantly expanded and functionally well-characterized. Considering the importance of crops, the study of BBX proteins in these plants has become more intense. The diverse functions of BBX in plant growth and development range from the involvement in seedling photomorphogenesis [1,[10][11][12][13][14][15][16][17], seed germination, photoperiodic regulation of flowering [18,19],

Classification of BBX Genes in Crops
The BBX gene family can be divided into five structural groups depending on one or two B-box domains and the CCT domain ( Figure 1). The first and second groups consist of proteins with two B-box domains and a CCT domain. Additionally, the VP motif is composed of six amino acid residues, localized in C termini, and has been established as belonging in the first group. In the third group, proteins have one B-box and one CCT domain. The fourth group consists of two tandem B-box domains, and in the fifth group, proteins have a single B-box domain. Crocco and Botto [31] conducted a comprehensive evolutionary analysis of the BBX protein family in 12 plant species that started from green algae and ended with dicots. The results showed that each of the five BBX protein groups evolved independently during plant evolution. Some literature has distinguished BBX proteins in separate subfamilies, including COL (CO-like) and DBB (double B-box). Indeed, the COL family contains proteins with double B-box and CCT domains, and they are homologs of the CONSTANS protein. The DBB proteins lack the CCT domain and have two tandem-localized B-box domains in the sequence.

Cereal Crops
Rice was the first crop to have the whole B-box protein family identified [2]. In this plant, 30 OsBBX genes have been identified and named according to rice chromosomes position. OsBBX genes are distributed in all chromosomes, omitting chromosomes 10 and 11 (Table 1). A segmental duplication analysis showed that 18 OsBBXs are located in the chromosomes duplicated segmental regions [2].   37] genes in its genome. All BBX genes are distributed on 9 of 10 maize chromosomes, except chromosome 8, and their nomenclature refers to their position on chromosomes (Table 1). Eight ZmCOL gene pairs have been identified to be involved in segmental duplications. Simultaneously, the expansion of the maize ZmDBB gene family occurred at the same duplication event.

Rosaceae Species
According to phylogenetic relationships and domains, 64 BBX genes in the apple genome are divided into five groups. This number of genes is significantly large compared to BBX genes in other plants, and it suggests that tandem, segmental, or genome-wide duplication in apple might cause this phenomenon [6]. A total of 50 genes have been mapped into 15 of 17 apple chromosomes (Table 1). Identifying the chromosome position for the remaining 14 genes was not successful, probably caused by incorrect assembling of genomic sequences.
In the pear genome (Pyrus bretschneideri Rehd.), 25 BBX genes have been identified, clustered in five groups, and sequentially named [5]. All the PbBBX genes are distributed among 12 of the total 17 pear chromosomes. The presence of segmental duplication for 13 gene pairs and no single tandem duplication is characteristic of BBX genes in this species. By contrast, in other Pyrus species, Pyrus pyrifolia, a total of 39 BBX family members were identified and were named according to the chromosomal distribution [38].

Solanaceae Species
In tomato, 29 putative BBX genes have been identified and named according to their homology to Arabidopsis BBX genes. The whole family is distributed within all chromosomes except for chromosome 11. The nuclear location of most tomato BBX proteins have been envisaged using in silico analysis, and has been confirmed for seven of them by Arabidopsis mesophyll protoplast assay [3].
A comparable number of BBX genes, 30, have been discovered in potato, and numbered based on BBX and CCT domains length and presence. Except for chromosome 11, potato StBBX genes are widely distributed in the whole genome [4].

Other Crops
In cotton, 42 GhCOL genes were identified in the genome, distributed unevenly along 18 different chromosomes. Phylogenetic analysis clustered them into three groups, whereby 14 COL genes in group I showed conserved structure compared with other plants. Analysis of gene expression patterns in group I concluded that these genes are potentially involved in photoperiodic flowering and light signaling regulation [40].
Comprehensive bioinformatics analysis of whole genomes of grapevines led to the detection of 24 BBX genes, of which 22 genes are evenly distributed in 11 of the 19 chromosomes, while the two genes are not assigned to any position [8].
The same number of BBX genes, 24, were detected on nine of the ten chromosomes in a wild peanut [41].
In bananas, 25 COL genes belong to group I-III. Nine genes from group I were investigated by Chaurasia et al. and the results showed that those genes are highly conserved in structure compared to members in other plants [42].
In soybeans, 26 CO-like genes are classified into three clades, comprising 13 homologous pairs [43]. On the contrary, only 17 putative COL genes were identified in leek, a herbaceous plant belonging to the Amaryllidaceae f amily [44]. Four of these leek COL genes show high sequence similarity with key factors modulating the heading date in barley and rice.
Sugar beets have been demonstrated to possess at least 10 CONSTANS-LIKE genes. However, these data are based on ESTs collection availability, whereas the sugar beet genome sequence was published a few years after that [45]. Therefore, it is expected that a larger number of genes may be identified.

Time to Switch from Vegetative to Generative Development
Strict regulation of flowering time is essential for plant reproductive success, enabling seed development completion in beneficial environmental conditions [46]. The photoperiodic flowering induction mechanism has been best recognized and characterized in Arabidopsis thaliana, where the FLOWERING LOCUS T (FT) and the CONSTANS (CO/BBX1) are the critical elements [47][48][49]. Research has shown that the AtBBX1, the first identified and characterized protein belonging to the BBX family, plays an essential role in regulating flowering time and flower development [19]. Besides, several other proteins belonging to the BBX family also perform a crucial role in regulatory networks, controlling floral transition and flower formation in Arabidopsis, including AtBBX4/COL3 [50], AtBX6/COL5 [51], AtBBX7/COL9 [52], AtBBX10/COL12 [53], and AtBBX17/COL8 [54].
Undoubtedly, less is known about the function of BBX proteins in crop growth and development. However, many BBX proteins in plants other than Arabidopsis are also likely to play a role in these processes. In rice, the short day (SD) plant, the GI-CO-FT regulatory pathway is conserved and flowering time is mutually regulated by two different photoperiodic pathways, in which several BBX members act as flowering inductors or repressors ( Figure 2). The rice CO ortholog, Hd1 (HEADING DATE 1)/OsBBX18, promotes flowering under inductive short-day conditions by regulating the Heading date 3a (Hd3a) and Rice FT-like 1 (RFT1) florigen genes [55]. Hd3a is also induced by another flowering activator, Ehd1 (Early heading date 1), which functions independently of Hd1 under SD conditions. Meanwhile, under noninductive long-day conditions, Hd1 turns into a flowering repressor and affects the expression of Hd3a. Another key repressor of flowering in rice is a small protein termed Ghd7 (Grain number, plant height, and heading date 7), which acts as an LD-specific repressor of EHd1 expression. So far, other proteins belonging to the BBX family in rice that may negatively affect flowering under two different photoperiodic conditions have been identified (Figure 2). Among them, OsBBX5(OsCOL4), OsBBX7(OsCOL9), Os-BBX10(OsCOL10), and OsBBX23(OsCOL13) repress flowering by reducing the expression of FT-like genes and heading date through Ehd1 (Early heading date 1) [56][57][58][59]. Moreover, some BBX proteins, including OsBBX10(OsCOL10) and OsBBX26(OsCOL15), act downstream of Ghd7 repressor, reducing expression of Ehd1 [58][59][60]. In maize, a typical SD plant, the most critical period in the whole development is the flowering time that determines the size of the cob formed by the plant and its filling with grain to a significant extent. In this species, the B-box-type gene corresponding to the Arabidopsis CO, called Conz1, activates the FT-like ZCN8, which functions as a floral inductor involved in photoperiod sensitivity in maize [61,62]. The AtCO gene homologs, SbHd1, HvCO1, and HvCO9, have also been found in other cereal crops, such as sorghum and barley, respectively, representing the long-day (LD) plants [63][64][65]. Under LD conditions, SbHd1 activates flowering by inducing SbCN8 and SbCN12 (orthologs of maize ZCN8 and ZCN12, respectively) [63], while HvCO1 and HvCO9 are involved in the activation of FT-like genes required for flowering induction in barley [65,66]. AtCO homologs in different species, including potato [74], ryegrass [75], grape [76], and alfalfa [77], are also presumably involved in photoperiodic flowering induction. Additionally, in potatoes, StCO regulates photoperiodic tuberization in a graft-transmissible manner [78]. The genes corresponding to the tomato FT homolog, designated StSP6A and StSPD3, have been identified in wild potato species S. tuberosum ssp. andigena [73,79,80]. As shown, the StSP6A encodes a protein promoting tuber formation [73], while StSPD3 encodes a protein promoting floral development [79,80]. Potato StCOL1 (BBX1) protein controls StSP6A expression through direct activation of an additional FT family member, while StSP5G, which acts as a repressor of StSP6A in leaves, mediates the strict short-day (SD) requirement of andigena plants for tuberization [73].
Understanding the flowering mechanisms and the role of B-box proteins in the photoperiodic flowering pathways in various crops is a crucial interest. Although the regulatory network triggering flowering is conserved in many species, the function of BBX acting downstream of the photoperiod response to accelerate or prevent floral initiation may vary significantly among plants.

Crops BBX Genes in the Anthocyanins Biosynthesis
Anthocyanins are pigments responsible for the red to black color of plant organs, such as flowers or fruits. Besides visual effects and market value, the accumulation of anthocyanins in organs is connected with biotic or abiotic stress, such as viral pathogens, wounding, or drought [82,83]. Those pigments also take part in protection against photooxidative and heat damage [84]. A few apple BBX proteins, including MdBBX1, MdBBX20, and MdBBX33/MdCOL11 are regulator factors of anthocyanin synthesis. MdBBX33 is a close homolog of Arabidopsis AtBBX22, and its overexpression causes an increased anthocyanin level in Arabidopsis seedlings [85]. MdBBX33 protein regulates anthocyanin accumulation, influencing the red skin color of apples in a light and temperature-dependent manner. Both low temperature and UV-B light correlate with upregulation of MdBBX33 expression and positively affect anthocyanin accumulation in apple fruits [85]. Furthermore, the expression of two anthocyanin accumulation-responsible genes, MdMYBA and MdbHLH, increase in the fruit ripening stage, which is associated with the increase of MdBBX33 transcript level.
On the contrary, another BBX gene, MdBBX1, when overexpressed in apple, does not directly increase anthocyanin accumulation. However, MdBBX1 may activate two essential genes-an MYB activator, MYB10, and the anthocyanin biosynthetic gene DRF (DIHYDROFLAVONOL 4-REDUCTASE)-by binding to a CCAAT motif present in their promoter region, ultimately leading to increased anthocyanin levels [86]. Moreover, expression patterns of some other BBX genes in apples, such as MdBBX15, MdBBX17, MdBBX35, MdBBX51, and MdBBX54, are correlated with anthocyanin induction in apple fruit skin [86]. Transactivation assays on the MYB10 promoter revealed that these BBX proteins could function as activators via direct induction of the apple anthocyanin-regulating MYB10 [86].
Studies on apples revealed that ultraviolet treatments promote some BBX transcription factors, which activate the expression of main anthocyanin biosynthetic genes and ultimately lead to increased anthocyanin levels. One of them is MdBBX20, which stimulates anthocyanin accumulation under ultraviolet radiation and low-temperature conditions. Overexpression of MdBBX20 caused increased anthocyanin accumulation in transformed calli [87]. Furthermore, MdBBX20 interacts by its B-box2 domain with transcription factor HY5, and in complexes, it regulates transcription of MdMYB1/MdMYB10, the anthocyanin key regulator concentrations, by binding its G-box cis-element. MdBBX22 is another UVinducible protein that directly interacts with MdHY5 and enhances the binding to key anthocyanin synthesis factors, MdMYB10 and MdCHS. Overexpression of MdBBX22 has been shown to induce anthocyanin biosynthesis [88,89]. Interestingly, MdBBX24, MdBBX33, MdBBX37, and MdBBX48 also interact with MdHY5, suggesting that numerous BBX might be entangled with anthocyanins synthesis [88,89].
It is also worth mentioning that some BBX members in apples, including MdBBX20, MdBBX22, MdBBX23 MdBBX24 MdBBX25 MdBBX33, and MdBBX43, interact with MdBT2 protein, known as a negative regulator of the UV-B-induced anthocyanin biosynthesis. An et al. [88] revealed that MdBT2 degrades MdBBX22 protein through the 26S proteasome pathway and the other members of the BBX family might be ubiquitination substrates for MdBT2.
So far, two BBX proteins have been identified that act as positive regulators of anthocyanin accumulation in a red pear. One of them is nuclear-localized protein, PpBBX16, a close homolog of AtBBX22, that favorably controls anthocyanin production in light-induced conditions via activating PpMYB10 [38]. However, PpBBX16 cannot directly bind the promoter of PpMYB10 and requires the presence of PpHY5 to achieve complete functionality. Moreover, PpBBX16 can promote the expression level of anthocyanin-related genes, such as PpCHI, PpCHS, and PpDFR, as was shown in the dual-luciferase assay introduced in tobacco. Overexpression of PpBBX16 in Arabidopsis seedlings increased anthocyanin content in the hypocotyls and tops of flower stalks. Furthermore, other BBX protein PpBBX18 also physically interacted with PpHY5, thus inducing transcription of PpMYB10 and consequently regulating anthocyanin biosynthesis in Arabidopsis and pear [38].
Besides positive regulators of anthocyanins biosynthesis, BBXs also play a role as negative regulators. In apples, MdBBX37 was indicated as an inhibitor of anthocyanin biosynthesis. Its interactions with pivotal positive regulators MdMYB1 and MdMYB9 block the binding to their target genes. Also, it acts as a suppressor of MdHY5 expression by binding to its promoter [88,89]. Meanwhile, in pears, PpBBX21 protein directly interacts with PpBBX18 or PpHY5, inhibits PpBBX18-PpHY5 complex formation, and represses anthocyanin biosynthesis [90].
BBX proteins are involved in the precise control of anthocyanin synthesis by binding to HY5 and transcriptional regulation of MYB10. Likewise, modulation of expression of other essential genes involved in anthocyanin production provides new insights into the multifunctionality of these factors. However, many questions remain to be answered to fill the knowledge gaps on light-induced anthocyanin biosynthesis.

Involvement of the BBX Proteins in Stress Response and Hormonal Pathways
Many reports have indicated that BBX proteins are involved in the signaling pathway induced by abiotic stresses, including low temperature, high salinity, drought, and heat. Some BBX proteins might be engaged in responses to several abiotic stress factors. In Arabidopsis, AtBBX18 negatively regulates thermotolerance through modulation of the expression of heat-stress-responsive genes, such as DGD1, Hsp70, Hsp101, and APX2 [25]. Another Arabidopsis B-box protein, AtBBX24/STO, enhances the growth of roots in high salt conditions [24]. Regulation of gene expression at the transcriptional level is mostly mediated by sequence-specific binding of transcription factors to the cis-acting promoter elements. Numerous BBX genes contain several putative stress-related cis-acting elements, such as MBS, ARE, LTR, and HSE. The transcript level of many BBX genes are altered under different stress conditions, as shown by transcription profiling (Table 2). Thus BBX proteins seem to be essential factors that integrate various signal transduction pathways, replying to diverse stresses and engaging in many cellular processes. However, only a few BBX proteins have been proven to be associated with responses to stress factors so far. IbBBX24 MH813941 + Ipomoea batatas [96] Solanum lycopersicum BBX genes with the same type of responses are marked with asterisks: Plants in nature are also exposed to biotic stresses covering a broad spectrum of plant pathogens. Present knowledge indicates that BBX regulators may also participate in the control of plant defense responses. Unfortunately, the understanding of the role of BBX proteins in this process is still in its infancy. The expression of a rice gene, OsCOL9, encoding a BBX protein belonging to group II of the COL protein family, has been shown to be enhanced at the mRNA level after Magnaporthe oryzae infection. Moreover, transgenic OsCOL9 knock-out rice plants showed increased pathogen susceptibility [97]. The expression of a banana gene, MaCOL1, increased after infection by Colletotrichum musae [95]. Overexpression of IbBBX24 gene significantly increased Fusarium wilt disease resistance in cultivated sweet potatoes [96].
Some BBX family members also play essential roles in hormone signaling pathways. There are many reports documenting the response of Arabidopsis BBX genes to plant hormones and the involvement of these proteins in many hormonal pathways [98]. Moreover, the transcript accumulation of several BBX genes in crops is elevated in response to exogenous treatment of phytohormones, including ABA, GA, JA, and SA ( Table 2). Most of these genes possess one or more well-defined hormone-responsive elements in their promoter sequences, like ABRE (ABA-responsive element), ERE (ethylene responsive element), CGTCA-motif and TGACG-motif (MeJA responsive elements), which respond to different hormonal pathways [2,3]. Interestingly, in bananas, MaCOL1 protein can mediate cross-talk between signaling pathways in response to biotic and abiotic stresses since the accumulation of MaCOL1 transcript was enhanced by chilling and pathogen infection [95].
Thus, transcriptomic analyses using macro-and microarray approaches are excellent tools for identifying new genes related to plant responses to different stresses and exogenous hormone treatments. However, the alternations in gene expression are frequently not reflected at the protein level. Therefore, the dynamic coordination of transcription seems essential to verify observed changes in expression profiles in response to external and internal signals.

Stress Response of Transgenic Plants Overexpressing the BBX Regulators
Recognition of plants' genetic and molecular resistance mechanisms to environmental stimuli allows researchers to design the new strategies to improve plants' stress tolerance. Although abiotic stress tolerance is a polygenic trait, single genes encoding crucial transcriptional regulators can improve plant adaptation to various stresses by turning regulatory gene networks on and off. The significance of some BBX proteins in stress tolerance has been revealed by manipulating the genes encoding such proteins in transgenic economically essential plants to obtain desirable agronomic characteristics and stress resistance. Many studies revealed the potential of manipulating BBX genes to confer enhanced tolerance to various stresses. Changing the BBX gene expression enhanced stress tolerance in Arabidopsis, chrysanthemums, apples, and rice (Table 3)   Ghd2(OsBBX8) Os02g0731700 Oryza sativa Oryza sativa Drought tolerance [94] nd-no data available.
Although analyses of BBX gene overexpression in response to defined stress are very informative, studies focusing on crop productivity will provide answers regarding the transgenic plants' improvements in stress tolerance and yield under field conditions.

The Interplay between BBX Proteins and the Circadian Clock
Most processes in living organisms evolve cyclically. The rhythmic course of phenomena is the result of organisms' adaptation to periodically changing conditions on Earth. In plants, the synchronization of the development cycle with cyclical changes in the environment is possible by developing an endogenous mechanism of the biological clock, which generates rhythms of a~24 h period [102]. Since the expression of numerous genes in crops is controlled at the transcript level by the biological clock, it indicates that the circadian oscillator affects agricultural importance traits. Several oscillator components have been identified as essential determinants of yield-related traits [103].
Many genes, the expressions of which are controlled by the biological clock, encode proteins containing the B-box zinc finger domain [2,4,26,27]. In fact, in Arabidopsis, some BBX proteins involved in flowering are under circadian clock control. Thus the expressions of AtBBX1/CONSTANS and AtBBX32 are regulated by the biological clock [27,50]. Moreover, transcriptional analysis of other BBX genes in Arabidopsis revealed circadiandependent regulation of AtBBX18, AtBBX19, AtBBX22, AtBBX24, and AtBBX25 [17,27]. In the promoter regions of clock controlled genes, the specific cis-elements "CAANNNATC" associated with the circadian regulation were found [4,104]. The transcription factor StZPR1, belonging to the zinc finger family type C 4 , has been identified recently, which binds to the "CAACAGCATC" motive defined by the term CIRC (circadian regulated) in the StBBX24 gene promoter in Solanum tuberosum. Moreover, in potato transgenic plants with silenced StZPR1 expression, there are disturbances of some BBX genes daily oscillations, such as StBBX5, StBBX9, StBBX18, StBBX24 and StBBX27 [105]. It is also noteworthy that the circadian clock is able to interrupt an effect of external stimuli on some BBX expressions. This interruption allows plants perform temporal gating in response to environmental constraints, thus triggering appropriate reactions for stress at a more suitable time of a day [26].
The full extent of the mechanisms by which plant keep the clock is still under investigation. Moreover, understanding the multiplatform link between the clock genes and cell-level circadian responses involving large BBX gene networks remains unexplained.

Summary and Prospects
BBX proteins constitute a complex regulatory network in planta [17,21,98,106]. Despite considerable progress in understanding B-box proteins' function in growth and development and stress responses in crops, the physiological role and the molecular mechanisms for many of them remain still unknown. Knowledge of protein partners for B-box proteins under different circadian cycles and environmental conditions and identifying critical regulators of their transcription will provide insight into molecular relationships between structure and function of this family. More information regarding the functions of BBX might help to understand the complexity of signaling pathways generated by the biological clock. However, to provide new insights into the role of BBX proteins in plants, more time-consuming experimental in vivo data, as gene overexpression and knock-outs, are required.

Conflicts of Interest:
The authors declare no conflict of interest.