You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Convergence of Light and ABA Signaling on the ABI5 Promoter}
Article:
Abstract:
Light is one of the most important environmental cues regulating multiple aspects of plant growth and development, and abscisic acid( ABA) is a plant hormone that plays important roles during many phases of the plant life cycle and in plants' responses to various environmental stresses. How plants integrate the external light signal with endogenous ABA pathway for better adaptation and survival remains poorly understood. Here, we show that BBX21( also known as SALT TOLERANCE HOMOLOG 2), a B-box( BBX) protein previously shown to positively regulate seedling photomorphogenesis, is also involved in ABA signaling. Our genetic data show that BBX21 may act upstream of several ABA INSENSITIVE( ABI) genes and ELONGATED HYPOCOTYL 5( HY5) in ABA control of seed germination. Previous studies showed that HY5 acts as a direct activator of ABI5 expression, and that BBX21 interacts with HY5. We further demonstrate that BBX21 negatively regulates ABI5 expression by interfering with HY5 binding to the ABI5 promoter. In addition, ABI5 was shown to directly activate its own expression, whereas BBX21 negatively regulates this activity by directly interacting with ABI5. Together, our study indicates that BBX21 coordinates with HY5 and ABI5 on the ABI5 promoter and that these transcriptional regulators work in concert to integrate light and ABA signaling in Arabidopsis thaliana.
Introduction:
Light is of utmost importance to sessile plants, not only as the primary energy source for photosynthesis, but also as an environmental signal regulating their growth and development throughout their life cycle. Light signals are perceived by several families of photoreceptors in plants, including phytochromes[primarily absorb red( R) and far-red( FR)], cryptochromes and phototropins[absorb blue( B) and ultraviolet-A( UV-A)], and a recently characterized ultraviolet-B( UV-B) photoreceptor UV RESISTANCE LOCUS 8( UVR8)[1]. Upon light irradiation, multiple photo-activated photoreceptors( except for UVR8) repress the activity of CONSTITUTIVELY PHOTOMORPHOGENIC 1( COP1), a central repressor of photomorphogenesis induced by visible-light[2]. COP1 is a conserved RING finger E3 ubiquitin ligase targeting several photomorphogenesis-promoting proteins for degradation, including ELONGATED HYPOCOTYL 5( HY5)[3] and HY5 HOMOLOG( HYH)[4]. HY5, a constitutively-nuclear bZIP protein, is the first known and most extensively studied transcription factor involved in promoting photomorphogenesis under a wide spectrum of wavelengths, including FR, R, B, and UV-B[3],[5],[6],[7]. It was shown that the abundance of HY5 protein is directly correlated with the extent of photomorphogenic development[3]. Recent chromatin immunoprecipitation( ChIP)-chip studies revealed that HY5 binds directly to a large number of genomic sites, mainly at the promoter regions of annotated genes[8]–[9]. Several recent studies demonstrated that B-box( BBX) family proteins act as important transcriptional regulators in response to light and circadian cues. The BBX family represents a subgroup of zinc finger proteins that contain one or more N-terminal zinc binding BBX domains[10]. In Arabidopsis thaliana, the BBX family consists of 32 proteins, and can be divided into five subfamilies according to its protein sequence[10]. CONSTANS( CO), a key player in the regulation of flowering by photoperiod, was the first identified member of BBX family in Arabidopsis[10]–[11]. CO and its five other similar proteins belong to Subfamily I, whose members all contain two BBX domains in the N terminus and a CCT domain in the C terminus[10]. However, for the eight members in Subfamily IV( BBX18 to BBX25), each BBX protein contains two BBX domains in the N terminus but lack the CCT domain[10],[12]. Interestingly, in this subfamily, BBX21/SALT TOLERANCE HOMOLOG 2( STH2) and BBX22/STH3/LIGHT-REGULATED ZINC FINGER PROTEIN 1( LZF1) function as positive regulators of photomorphogenesis, whereas BBX18/DBB1a, BBX19/DBB1b, BBX24/SALT TOLERANCE( STO) and BBX25/STH1 act as negative regulators of photomorphogenesis[13]–[22]. Notably, BBX21, BBX22, BBX24 and BBX25 were all shown to physically interact with HY5, and the B-box domains of these BBX proteins and the bZIP domain of HY5 mediate their interactions[13]–[17],[20]. In addition, it was shown that HY5 promotes the expression of BBX22 by directly binding to its promoter[13], whereas BBX24 and BBX25 repress BBX22 expression by interfering with HY5 transcriptional activity[17]. The plant hormone abscisic acid( ABA) regulates several aspects of plant growth and development, plays an essential role in plants' adaptive responses to environmental stress, and is a key regulator of stomatal aperture[23]–[24]. Genetic studies in Arabidopsis have identified several factors that participate in ABA signal transduction, including ABA INSENSITIVE 1( ABI1) to ABI5. ABI1 and ABI2, group A type protein phosphatases 2C( PP2Cs), negatively regulate SNF1-related protein kinase 2( SnRK2) proteins which phosphorylate downstream targets such as various AREB/ABFs[25]–[26]. ABI3, ABI4, and ABI5 belong to three distinct classes of transcription factors, i. e. the basic B3, AP2/ERF, and the basic leucine zipper( bZIP) families, respectively, and share overlapping functions in ABA signaling during seed germination and early seedling development[27]–[29]. It was reported that ABI5 acts downstream of ABI3 and is involved in postgerminational developmental arrest and repression of germination[30]–[32]. Interestingly, a recent study showed that HY5 directly binds to the promoter of ABI5 and is required for the expression of ABI5 and ABI5-targeted genes[33]. In addition, HY5 binding to the ABI5 promoter is enhanced by ABA treatment, and overexpression of ABI5 restores ABA sensitivity in hy5 mutants and leads to enhanced light responses in the wild type plants[33]. Thus, HY5 regulation of ABI5 expression represents an integration of light and ABA signaling during seed germination and early seedling development. Accumulating evidence indicates that BBX proteins work in concert with HY5 in regulating photomorphogenesis[14]–[17],[20]. Since HY5 directly activates ABI5 expression, it is interesting to investigate whether BBX proteins are also involved in ABA signaling. In this paper, we report that bbx21 mutants showed hypersensitive phenotypes to ABA and NaCl treatments, and our genetic data suggest that BBX21 may act upstream of several ABI genes and HY5. Furthermore, we show that BBX21 negatively regulates ABI5 expression by interfering with HY5 binding to the ABI5 promoter. In addition, ABI5 binds to its own promoter and activate its expression, whereas BBX21 negatively regulates this activity. Taken together, our data reveal a complicated but delicate control of crosstalk between light and ABA signaling in Arabidopsis.
Discussion:
Light and ABA are the external environmental and the endogenous hormonal cues, respectively, that both play important roles in the control of seed germination and seedling development. The ability of plants to integrate external signals with internal regulatory pathways is vital for their survival[38]–[39]. However, the molecular mechanisms underlying the cross-talk between light and ABA are just starting to be understood. It was reported that HY5, a well-studied transcription factor involved in promoting seedling photomorphogenesis, mediates ABA responses in seed germination, early seedling growth, and root development in Arabidopsis by directly binding to the ABI5 promoter and activating its expression[33]. A recent study reported that FAR-RED ELONGATED HYPOCOTYL3( FHY3) and FAR-RED IMPAIRED RESPONSE1( FAR1), two key transcription factors in the phytochrome A pathway, are positive regulators of ABA signaling by directly activating ABI5 expression[40]. In addition, it was shown that ABI5 is also a direct target of PIF1( also known as PIL5), a phytochrome-interacting bHLH transcription factor[41]. Therefore, it is evident that multiple transcription factors in light signaling pathway may regulate ABA responses by regulating ABI5 expression. In this study, we show that BBX21, another transcriptional regulator implicated in regulating seedling photomorphogenesis, negatively regulates ABI5 expression by interfering with HY5 binding to the ABI5 promoter. Interestingly, ABI5 was also shown to directly activate its own expression, whereas BBX21 negatively regulates this activation by directly interacting with ABI5. Therefore, BBX21 acts as a transcriptional repressor of ABI5, possibly by modulating the binding activities of both HY5 and ABI5 to the ABI5 promoter( Figure 10). On the other hand, several components of ABA signaling pathway have been shown to be implicated in photomorphogenesis. For example, overexpression of ABI5 in wild type background led to shorter hypocotyls of the transgenic seedlings in FR, R and B light conditions[33]. Consistent with this observation, disruption of RPN10 and KEG, which stabilize the ABI5 protein, both caused decreased hypocotyl growth of the mutants[42]–[43]. Another study proposed that seeds in shallow soil could sense their light environment through phytochrome B, so as to down-regulate ABI3 expression and promote hypocotyl growth[44]. In darkness, the hypocotyl length was slightly longer in abi3 mutants but shorter in ABI3 overexpression plants[44]. Therefore, both ABI3 and ABI5 may play positive roles in photomorphogenesis, and ABA seems to inhibit hypocotyl elongation of seedlings, which may confer better adaptation of seedlings to environmental stresses. It is interesting that several families of transcription factors directly bind to the ABI5 promoter and regulate its expression. FHY3/FAR1 bind to the FHY3/FAR1-binding site( FBS; CACGCGC)[40], whereas ABI5 binds to the G-box motifs which are ∼130 bp upstream of FBS in the ABI5 promoter( Figure 8). In addition, HY5 presumably binds to a typical G-box motif in the ABI5 promoter[33] which is ∼500 bp upstream of the ABI5-binding sites. Although both HY5 and ABI5 belong to the bZIP transcription factor family and preferentially bind to G-box motif, they apparently bind to different G-box motifs in the ABI5 promoter, suggesting that additional sequences around the core G-box motif may be important for selective binding of these bZIP transcription factors. The binding site of PIF1( which also binds to G-box motif) in the ABI5 promoter remains to be determined. Moreover, ABI4, an AP2/ERF family transcription factor, was shown to bind to CE1-like element( CACCG) in the 5′-untranslated region of ABI5[45]. Therefore, at least FHY3/FAR1, ABI5, HY5 and ABI4 directly bind to the ABI5 promoter, and they seem to occupy different regions of the ABI5 promoter. Interestingly, based on our data and those reported in previous studies, all of these transcription factors function as activators of ABI5. How their activities are coordinated and whether they could regulate each other on the ABI5 promoter remain to be elucidated. Since BBX21 is the only negative regulator identified so far on the ABI5 promoter, it remains unknown whether BBX21 could also regulate the binding activities of other transcription factors, such as FHY3/FAR1 and ABI4. In addition, it was shown that ABI3, a B3-domain containing transcription factor, functions as essential regulator upstream of ABI5[31]. ABI5 expression is greatly reduced in the abi3 mutants, and 35S:ABI5 could rescue ABA insensitivity of abi3[31]. Whether ABI3 is also a direct regulator of ABI5 awaits further investigation. Together, the ABI5 promoter may represent a converging point on which the transcriptional regulators of light and ABA signaling pathways integrate the signals of both pathways by fine-tuning ABI5 expression. In addition, it was documented that light could affect ABA metabolism in germinating seeds, and that PIF1 represses seed germination by regulating the expression of ABA biosynthetic and catabolic genes[46]–[48]. Moreover, PIF1 interacts with ABI3 to activate the expression of SOMNUS( SOM) in imbibed seeds[49], suggesting that light and ABA signals also converge on other gene promoters. Together, it is evident that light and ABA integrate at multiple layers to fine-tune seed germination and seedling development, which may ensure the young seedlings to better adapt to their environmental stresses and dynamic light conditions.
Many factors such as light , phytohormone abscisic acid ( ABA ) , etc . , regulate multiple developmental processes throughout the plants' life cycle . Light promotes seed germination and ABA maintains seed dormancy . However , little is known about how light and ABA signaling pathways interact with each other . It was previously reported that Arabidopsis HY5 , a well-known bZIP transcription factor involved in promoting seedling photomorphogenesis , is involved in ABA signaling by directly activating ABI5 expression . Here , we report that the B-box protein BBX21 negatively regulates ABI5 expression by interfering with HY5 binding to the ABI5 promoter . Interestingly , ABI5 was shown to directly bind to its own promoter and activate its expression , whereas BBX21 also negatively regulates this activity by interacting with ABI5 . Together , our study shows that light and ABA signaling pathways converge on the ABI5 promoter , on which BBX21 acts as a negative regulator of ABI5 expression .