BBX21 (also known as SALT TOLERANCE HOMOLOG 2), a B-box (BBX)-containing protein, has been previously identified as a positive regulator of light signaling; however, the precise role of BBX21 in regulating seedling photomorphogenesis remains largely unclear. In this study, we report that CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) interacts with BBX21 in vivo and is able to ubiquitinate BBX21 in vitro. Thus, BBX21 is targeted for 26S proteasome-mediated degradation in dark-grown Arabidopsis seedlings in a COP1-dependent manner. Moreover, we show that BBX21 binds to the T/G-box in the ELONGATED HYPOCOTYL 5 (HY5) promoter and directly activates HY5 expression in the light. Transgenic seedlings overexpressing BBX21 exhibit dramatically shortened hypocotyls in the light, and this phenotype is dependent on a functional HY5. Taken together, our data suggest a molecular base underlying BBX21-mediated seedling photomorphogenesis, indicating that BBX21 is a pivotal component involved in the COP1-HY5 regulatory hub.P lant seedlings undergo two distinct developmental processes dependent on the presence and absence of light, termed photomorphogenesis and skotomorphogenesis, respectively (1). CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) is a central repressor of seedling photomorphogenesis, and mutants impaired in COP1 display constitutively photomorphogenic phenotypes in darkness (2, 3). COP1 acts as an E3 ubiquitin ligase targeting a subset of substrates for degradation in darkness, including ELONGATED HYPOCOTYL 5 (HY5), HY5 HOMOLOG (HYH), LONG AFTER FAR-RED LIGHT 1 (LAF1), LONG HYPOCOTYL IN FAR RED (HFR1), B-BOX PROTEIN 22/ SALT TOLERANCE HOMOLOG 3 (BBX22/STH3), and PHYTOCHROME INTERACTING FACTOR 3-LIKE1 (PIL1) (4-11). Of these, HY5 is considered a key signal integration point from dark to light transition (10, 12, 13). Its abundance is directly correlated with the extent of seedling photomorphogenic growth, but inversely correlated with the nuclear abundance of COP1 (4, 10, 14). As a b-ZIP type transcription factor, HY5 specifically interacts with the ACGT-containing elements (ACEs) and might directly bind to approximately one-third of the Arabidopsis genes (12, 13). Thus, HY5 ensures proper expression of a large number of downstream regulatory genes, which in turn eventually promotes photomorphogenesis in the light. Recent studies have shown that HY5 expression is regulated by HY5 itself, as well as by HYH and CALMODULIN7 (CAM7) (15,16).A total of 32 B-box (BBX)-containing proteins have been identified in the Arabidopsis genome (17), among which a number of BBX proteins have been shown to be involved in COP1-and HY5-mediated seedling photomorphogenesis (18). BBX4, BBX20, BBX22, BBX24, and BBX25 genetically and physically interact with COP1 and undergo COP1-mediated degradation in the dark (6,(19)(20)(21)(22)(23)(24). In addition, BBX22, BBX24, and BBX25 physically interact with HY5, and, interestingly, BBX22 acts as a coactivator of HY5 action (6), whereas BBX24 and BBX25 repress the transcriptional activating activity of HY5 ...

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.

We previously identified a salt and drought stressresponsive TFIIIA-type zinc finger protein gene ZFP252 from rice. Here we report the functional analysis of ZFP252 using gain-and loss-of-function strategies. We found that overexpression of ZFP252 in rice increased the amount of free proline and soluble sugars, elevated the expression of stress defense genes and enhanced rice tolerance to salt and drought stresses, as compared with ZFP252 antisense and non-transgenic plants. Our findings suggest that ZFP252 plays an important role in rice response to salt and drought stresses and is useful in engineering crop plants with enhanced tolerance to salt and drought stresses.

8At the end of 2019, the SARS-CoV-2 induces an ongoing outbreak of pneumonia in 9 China 1 , even more spread than SARS-CoV infection 2 . The entry of SARS-CoV into