Fruit characteristics of sweet watermelon are largely the result of human selection. Here we report an improved watermelon reference genome and whole-genome resequencing of 414 accessions representing all extant species in the Citrullus genus. Population genomic analyses reveal the evolutionary history of Citrullus, suggesting independent evolutions in Citrullus amarus and the lineage containing Citrullus lanatus and Citrullus mucosospermus. Our findings indicate that different loci affecting watermelon fruit size have been under selection during speciation, domestication and improvement. A non-bitter allele, arising in the progenitor of sweet watermelon, is largely fixed in C. lanatus. Selection for flesh sweetness started in the progenitor of C. lanatus and continues through modern breeding on loci controlling raffinose catabolism and sugar transport. Fruit flesh coloration and sugar accumulation might have co-evolved through shared genetic components including a sugar transporter gene. This study provides valuable genomic resources and sheds light on watermelon speciation and breeding history.

Seed germination and flowering initiation are both transitions responding to similar seasonal cues. This study shows that ABSCISIC ACID-INSENSITIVE MUTANT 5 (ABI5), a bZIP transcription factor, which plays an important role in the abscisic acid (ABA)-arrested seed germination, is robustly associated with the floral transition in Arabidopsis. Under long-day conditions, overexpression of ABI5 could delay floral transition through upregulating FLOWERING LOCUS C (FLC) expression. In contrast, ectopically overexpressing FLC in an abi5 mutant reversed the earlier flowering phenotype. Further analysis indicated that transactivation of FLC could be promoted by ABI5 and/or other abscisic acid-responsive element (ABRE)-binding factors (ABFs). The expression of FLC that was promoted by ABI5 and/or other ABFs could be blocked in a triple SNF1-related protein kinase (SnRK) mutant, snrk2.2/2.3/2.6, despite the presence of ABA. In sharp contrast, when SnRK2.6 was coexpressed, the reduction of transactivity of FLC was reverted in mesophyll protoplasts of snrk2.2/2.3/2.6. Additional results from analysing transgenic plants carrying mutations of phosphoamino acids (ABI5 S42AS145AT201A), which are conserved in ABI5, suggested that SnRK2-mediated ABI5 and/or ABF phosphorylation may be crucial for promoting FLC expression. The transgenic plants ABI5 S42AS145AT201A were insensitive to ABA in seed germination, in addition to having an earlier flowering phenotype. Direct binding of ABI5 to the ABRE/G-box promoter elements existing in FLC was demonstrated by chromatin immunoprecipitation. Mutations at the ABRE/G-box regions in FLC promoter sequences abolished the ABI5-promoted transactivation of FLC. In summary, these results may decipher the inhibitory effect of ABA on floral transition in Arabidopsis.

Arginine is an important medium for the transport and storage of nitrogen, and arginase (also known as arginine amidohydrolase, ARGAH) is responsible for catalyse of arginine into ornithine and urea in plants. In this study, the impact of AtARGAHs on abiotic stress response was investigated by manipulating AtARGAHs expression. In the knockout mutants of AtARGAHs, enhanced tolerances were observed to multiple abiotic stresses including water deficit, salt, and freezing stresses, while AtARGAH1- and AtARGAH2-overexpressing lines exhibited reduced abiotic stress tolerances compared to the wild type. Consistently, the enhanced tolerances were confirmed by the changes of physiological parameters including electrolyte leakage, water loss rate, stomatal aperture, and survival rate. Interestingly, the direct downstream products of arginine catabolism including polyamines and nitric oxide (NO) concentrations significantly increased in the AtARGAHs-knockout lines, but decreased in overexpressing lines under control conditions. Additionally, the AtARGAHs-overexpressing and -knockout lines displayed significantly reduced relative arginine (% of total free amino acids) relative to the wild type. Similarly, reactive oxygen species accumulation was remarkably regulated by AtARGAHs under abiotic stress conditions, as shown from hydrogen peroxide (H2O2), superoxide radical () concentrations, and antioxidant enzyme activities. Taken together, this is the first report, as far as is known, to provide evidence that AtARGAHs negatively regulate many abiotic stress tolerances, at least partially, attribute to their roles in modulating arginine metabolism and reactive oxygen species accumulation. Biotechnological strategy based on manipulation of AtARGAHs expression will be valuable for future crop breeding.

SUMMARYAbscisic acid (ABA) and cytokinin are key hormones controlling plant development. How ABA and cytokinin interplay to control the transition from a dry seed into a young seedling remains elusive. Here we undertook a gain-of-function genetic screen to identify ABA-insensitive mutants during seed germination in Arabidopsis using an estradiol-inducible approach. In the presence of estradiol, one of these mutants gim1 (germination insensitive to ABA mutant 1) exhibited an elevated level of cytokinin that was attributed to the estradiolinduced expression of AtIPT8 that encodes an isopentenyltransferase for the biosynthesis of cytokinins. Our data on OE-2 and Com-1 transgenic plants carrying the ectopically expressing AtIPT8 gene indicated that the elevation of cytokinin level was responsible for the ABA-insensitivity of gim1 seed germination. Further analyses on alterations of gene transcriptomes in the gim1 mutant demonstrated that the expression of some ABA-inducible genes, including ABI5, was reduced, and could not be restored by exogenous ABA treatment. Moreover, we also failed to observe the ABA-mediated repression of a family of cytokinin signal transducers and transcription repressors called type-A ARR4, ARR5 and ARR6 in the gim1 seedlings. Further analysis demonstrated that type-A ARR4, ARR5 and ARR6 could negatively regulate ABI5 expression, and the physical interaction of ABI5 and type-A ARR4, ARR5 and ARR6 proteins was detected. In summary, our study suggests that the interaction of ABA and cytokinin during seed germination and seedling growth can be mediated by the interplay of transcriptional regulators in Arabidopsis.