The tomato OST1-VOZ1 module regulates drought-mediated flowering.

Flowering is a critical agricultural trait that substantially affects tomato fruit yield. Although drought stress influences flowering time, the molecular mechanism underlying drought-regulated flowering in tomato remains elusive. In this study, we demonstrated that loss of function of tomato OPEN STOMATA 1 (SlOST1), a protein kinase essential for abscisic acid (ABA) signaling and abiotic stress responses, lowers the tolerance of tomato plants to drought stress. slost1 mutants also exhibited a late flowering phenotype under both normal and drought stress conditions. We also established that SlOST1 directly interacts with and phosphorylates the NAC (NAM, ATAF and CUC)-type transcription factor VASCULAR PLANT ONE-ZINC FINGER 1 (SlVOZ1), at residue serine 67, thereby enhancing its stability and nuclear translocation in an ABA-dependent manner. Moreover, we uncovered several SlVOZ1 binding motifs from DNA affinity purification sequencing analyses and revealed that SlVOZ1 can directly bind to the promoter of the major flowering-integrator gene SINGLE FLOWER TRUSS to promote tomato flowering transition in response to drought. Collectively, our data uncover the essential role of the SlOST1-SlVOZ1 module in regulating flowering in response to drought stress in tomato and offer insights into a novel strategy to balance drought stress response and flowering.

Genome-wide systematic survey and analysis of the RNA helicase gene family and their response to abiotic stress in sweetpotato.

Sweetpotato (Ipomoea batatas (L.) Lam.) holds a crucial position as one of the staple foods globally, however, its yields are frequently impacted by environmental stresses. In the realm of plant evolution and the response to abiotic stress, the RNA helicase family assumes a significant role. Despite this importance, a comprehensive understanding of the RNA helicase gene family in sweetpotato has been lacking. Therefore, we conducted a comprehensive genome-wide analysis of the sweetpotato RNA helicase family, encompassing aspects such as chromosome distribution, promoter elements, and motif compositions. This study aims to shed light on the intricate mechanisms underlying the stress responses and evolutionary adaptations in sweetpotato, thereby facilitating the development of strategies for enhancing its resilience and productivity. 300 RNA helicase genes were identified in sweetpotato and categorized into three subfamilies, namely IbDEAD, IbDEAH and IbDExDH. The collinearity relationship between the sweetpotato RNA helicase gene and 8 related homologous genes from other species was explored, providing a reliable foundation for further study of the sweetpotato RNA helicase gene family's evolution. Furthermore, through RNA-Seq analysis and qRT-PCR verification, it was observed that the expression of eight RNA helicase genes exhibited significant responsiveness to four abiotic stresses (cold, drought, heat, and salt) across various tissues of ten different sweetpotato varieties. Sweetpotato transgenic lines overexpressing the RNA helicase gene IbDExDH96 were generated using A.rhizogenes-mediated technology. This approach allowed for the preliminary investigation of the role of sweetpotato RNA helicase genes in the response to cold stress. Notably, the promoters of RNA helicase genes contained numerous cis-acting elements associated with temperature, hormone, and light response, highlighting their crucial role in sweetpotato abiotic stress response.

Systematic identification and expression analysis of bHLH gene family reveal their relevance to abiotic stress response and anthocyanin biosynthesis in sweetpotato.

BACKGROUND: bHLH transcription factors play significant roles in regulating plant growth and development, stress response, and anthocyanin biosynthesis. Sweetpotato is a pivotal food and industry crop, but little information is available on sweetpotato bHLH genes. RESULTS: Herein, 227 putative IbbHLH genes were defined on sweetpotato chromosomes, and fragment duplications were identified as the dominant driving force for IbbHLH expansion. These IbbHLHs were divided into 26 subfamilies through phylogenetic analysis, as supported by further analysis of exon-intron structure and conserved motif composition. The syntenic analysis between IbbHLHs and their orthologs from other plants depicted evolutionary relationships of IbbHLHs. Based on the transcriptome data under salt stress, the expression of 12 IbbHLHs was screened for validation by qRT-PCR, and differential and significant transcriptions under abiotic stress were detected. Moreover, IbbHLH123 and IbbHLH215, which were remarkably upregulated by stress treatments, had obvious transactivation activity in yeasts. Protein interaction detections and yeast two-hybrid assays suggested an intricate interaction correlation between IbbHLHs. Besides, transcriptome screening revealed that multiple IbbHLHs may be closely related to anthocyanin biosynthesis based on the phenotype (purple vs. white tissues), which was confirmed by subsequent qRT-PCR analysis. CONCLUSIONS: These results shed light on the promising functions of sweetpotato IbbHLHs in abiotic stress response and anthocyanin biosynthesis.

The unique sweet potato NAC transcription factor IbNAC3 modulates combined salt and drought stresses.

Plants often simultaneously experience combined stresses rather than a single stress, causing more serious damage, but the underlying mechanisms remain unknown. Here, we identified the stress-induced IbNAC3 from sweet potato (Ipomoea batatas) as a nucleus-localized transcription activator. IbNAC3 contains a unique activation domain whose MKD sequence confers transactivation activities to multiple other TFs and is essential for the activated expression of downstream target genes. Ectopic expression of IbNAC3 conferred tolerance to single and combined salt and drought stresses in Arabidopsis (Arabidopsis thaliana), and a group of NAM, ATAF1/2, and CUC2 (NAC) TFs, including ANAC011, ANAC072, ANAC083, ANAC100, and NAP, interacted with IbNAC3, and the specific domains responsible for each interaction varied. Intriguingly, IbNAC3 repressed the interaction among the five NACs, and knockout or mutation of ANAC011 and ANAC072 dramatically impaired combined stress tolerance. IbNAC3-ANAC072 and IbNAC3-NAP modules synergistically activated the MICROTUBULE-RELATED E3 LIGASE57 (MREL57) gene. Consistently, mutation of MREL57 and overexpression of WAVE-DAM-PENED2-LIKE7, encoding a target protein of MREL57, both remarkably impaired combined stress tolerance. Moreover, transgenic plants displayed abscisic acid (ABA) hyposensitivity by directly promoting the transcription of ENHANCED RESPONSE TO ABA 1, a key negative regulator of ABA signaling. The data unravel the unique IbNAC3 TF functions as a pivotal component in combined stress tolerance by integrating multiple regulatory events and ubiquitin pathways, which is essential for developing high-tolerant plants in natural environments.

A systematical genome-wide analysis and screening of WRKY transcription factor family engaged in abiotic stress response in sweetpotato.

BACKGROUND: WRKY transcription factors play pivotal roles in regulating plant multiple abiotic stress tolerance, however, a genome-wide systematical analysis of WRKY genes in sweetpotato is still missing. RESULTS: Herein, 84 putative IbWRKYs with WRKY element sequence variants were identified in sweetpotato reference genomes. Fragment duplications, rather than tandem duplications, were shown to play prominent roles in IbWRKY gene expansion. The collinearity analysis between IbWRKYs and the related orthologs from other plants further depicted evolutionary insights into IbWRKYs. Phylogenetic relationships displayed that IbWRKYs were divided into three main groups (I, II and III), with the support of the characteristics of exon-intron structures and conserved protein motifs. The IbWRKY genes, mainly from the group Ib, displayed remarkable and diverse expression profiles under multiple abiotic stress (NaCl, PEG6000, cold and heat) and hormone (ABA, ACC, JA and SA) treatments, which were determined by RNA-seq and qRT-PCR assays, suggesting their potential roles in mediating particular stress responses. Moreover, IbWRKY58L could interact with IbWRKY82 as revealed by yeast two-hybrid based on the protein interaction network screening. And abiotic stress-remarkably induced IbWRKY21L and IbWRKY51 were shown to be localized in the nucleus and had no transactivation activities. CONCLUSION: These results provide valuable insights into sweetpotato IbWRKYs and will lay a foundation for further exploring functions and possible regulatory mechanisms of IbWRKYs in abiotic stress tolerance.

Genome-wide survey and expression analysis of GRAS transcription factor family in sweetpotato provides insights into their potential roles in stress response.

BACKGROUND: The plant-specific GRAS transcription factors play pivotal roles in various adverse environmental conditions. Numerous GRAS genes have been explored and characterized in different plants, however, comprehensive survey on GRASs in sweetpotato is lagging. RESULTS: In this study, 72 putative sweetpotato IbGRAS genes with uneven distribution were isolated on 15 chromosomes and classified into 12 subfamilies supported by gene structures and motif compositions. Moreover, both tandem duplication and segmental duplication events played critical roles in the expansion of sweetpotato GRAS genes, and the collinearity between IbGRAS genes and the related orthologs from nine other plants further depicted evolutionary insights into GRAS gene family. RNA-seq analysis under salt stress and qRT-PCR detection of 12 selected IbGRAS genes demonstrated their significant and varying inductions under multiple abiotic stresses (salt, drought, heat and cold) and hormone treatments (ABA, ACC and JA). Consistently, the promoter regions of IbGRAS genes harbored a series of stress- and hormone-associated cis-acting elements. Among them, IbGRAS71, the potential candidate for breeding tolerant plants, was characterized as having transactivation activity in yeasts, while IbGRAS-2/-4/-9 did not. Moreover, a complex interaction relationship between IbGRASs was observed through the interaction network analysis and yeast two-hybrid assays. CONCLUSIONS: Our results laid a foundation for further functional identifications of IbGRAS genes, and multiple members may serve as potential regulators for molecular breeding of tolerant sweetpotato.

A domesticated Harbinger transposase forms a complex with HDA6 and promotes histone H3 deacetylation at genes but not TEs in Arabidopsis.

In eukaryotes, histone acetylation is a major modification on histone N-terminal tails that is tightly connected to transcriptional activation. HDA6 is a histone deacetylase involved in the transcriptional regulation of genes and transposable elements (TEs) in Arabidopsis thaliana. HDA6 has been shown to participate in several complexes in plants, including a conserved SIN3 complex. Here, we uncover a novel protein complex containing HDA6, several Harbinger transposon-derived proteins (HHP1, SANT1, SANT2, SANT3, and SANT4), and MBD domain-containing proteins (MBD1, MBD2, and MBD4). We show that mutations of all four SANT genes in the sant-null mutant cause increased expression of the flowering repressors FLC, MAF4, and MAF5, resulting in a late flowering phenotype. Transcriptome deep sequencing reveals that while the SANT proteins and HDA6 regulate the expression of largely overlapping sets of genes, TE silencing is unaffected in sant-null mutants. Our global histone H3 acetylation profiling shows that SANT proteins and HDA6 modulate gene expression through deacetylation. Collectively, our findings suggest that Harbinger transposon-derived SANT domain-containing proteins are required for histone deacetylation and flowering time control in plants.

High-throughput deep sequencing reveals the important role that microRNAs play in the salt response in sweet potato (Ipomoea batatas L.).

BACKGROUND: MicroRNAs (miRNAs), a class of small regulatory RNAs, have been proven to play important roles in plant growth, development and stress responses. Sweet potato (Ipomoea batatas L.) is an important food and industrial crop that ranks seventh in staple food production. However, the regulatory mechanism of miRNA-mediated abiotic stress response in sweet potato remains unclear. RESULTS: In this study, we employed deep sequencing to identify both conserved and novel miRNAs from salinity-exposed sweet potato cultivars and its untreated control. Twelve small non-coding RNA libraries from NaCl-free (CK) and NaCl-treated (Na150) sweet potato leaves and roots were constructed for salt-responsive miRNA identification in sweet potatoes. A total of 475 known miRNAs (belonging to 66 miRNA families) and 175 novel miRNAs were identified. Among them, 51 (22 known miRNAs and 29 novel miRNAs) were significantly up-regulated and 76 (61 known miRNAs and 15 novel miRNAs) were significantly down-regulated by salinity stress in sweet potato leaves; 13 (12 known miRNAs and 1 novel miRNAs) were significantly up-regulated and 9 (7 known miRNAs and 2 novel miRNAs) were significantly down-regulated in sweet potato roots. Furthermore, 636 target genes of 314 miRNAs were validated by degradome sequencing. Deep sequencing results confirmed by qRT-PCR experiments indicated that the expression of most miRNAs exhibit a negative correlation with the expression of their targets under salt stress. CONCLUSIONS: This study provides insights into the regulatory mechanism of miRNA-mediated salt response and molecular breeding of sweet potatoes though miRNA manipulation.

SlSTE1 promotes abscisic acid-dependent salt stress-responsive pathways via improving ion homeostasis and reactive oxygen species scavenging in tomato.

High salinity is one of the major limiting factors that reduces crop productivity and quality. Herein, we report that small SALT TOLERANCE ENHANCER1 (STE1) protein without any known conserved domains is required for tomato salt tolerance. Overexpression (OE) of SlSTE1 enhanced the tolerance to multiple chloride salts (NaCl, KCl, and LiCl) and oxidative stress, along with elevated antioxidant enzyme activities, increased abscisic acid (ABA) and chlorophyll contents, and reduced malondialdehyde (MDA) and reactive oxygen species (ROS) accumulations compared to that of wild-type (WT) plants. Moreover, decreased K+ efflux and increased H+ efflux were detected in the OE plants, which induced a higher K+ /Na+ ratio. In contrast, SlSTE1-RNAi plants displayed decreased tolerance to salt stress. RNA-seq data revealed 1 330 differentially expressed genes in the OE plants versus WT plants under salt stress, and the transcription of numerous and diverse genes encoding transcription factors, stress-related proteins, secondary metabolisms, kinases, and hormone synthesis/signaling-related proteins (notably ABA and 1-aminocyclopropane-1-carboxylate) was greatly elevated. Furthermore, SlSTE1-OE plants showed increased sensitivity to ABA, and the results suggest that SlSTE1 promotes ABA-dependent salt stress-responsive pathways by interacting with SlPYLs and SlSnRK2s. Collectively, our findings reveal that the small SlSTE1 protein confers salt tolerance via ABA signaling and ROS scavenging and improves ion homeostasis in tomato.

RNA-Seq and iTRAQ reveal multiple pathways involved in storage root formation and development in sweet potato (Ipomoea batatas L.).

BACKGROUND: Sweet potato (Ipomoea batatas L.) is the sixth most important food crop in the world. The formation and development of storage roots in sweet potato is a highly complicated and genetically programmed process. However, the underlying mechanisms of storage root development have not yet been elucidated. RESULTS: To better understand the molecular mechanisms involved in storage root development, a combined analysis of the transcriptome and proteome of sweet potato fibrous roots (F) and storage roots at four different stages (D1, D3, D5 and D10) was performed in the present study. A total of 26,273 differentially expressed genes were identified in a comparison between the fibrous root library and four storage root libraries, while 2558 proteins showed a 1.0-fold or greater expression difference as indicated by isobaric tags for relative and absolute quantitation (iTRAQ) analysis. The combination of the transcriptome and proteome analyses and morphological and physiological data revealed several critical pathways involved in storage root formation and development. First, genes/proteins involved in the development of meristems/cambia and starch biosynthesis were all significantly upregulated in storage roots compared with fibrous roots. Second, multiple phytohormones and the genes related to their biosynthesis showed differential expression between fibrous roots and storage roots. Third, a large number of transcription factors were differentially expressed during storage root initiation and development, which suggests the importance of transcription factor regulation in the development of storage roots. Fourth, inconsistent gene expression was found between the transcriptome and proteome data, which indicated posttranscriptional regulatory activity during the development of storage roots. CONCLUSION: Overall, these results reveal multiple events associated with storage root development and provide new insights into the molecular mechanisms underlying the regulatory networks involved in storage root development.

An AGAMOUS MADS-box protein, SlMBP3, regulates the speed of placenta liquefaction and controls seed formation in tomato.

AGAMOUS (AG) MADS-box transcription factors have been shown to play crucial roles in floral organ and fruit development in angiosperms. Here, we isolated a tomato (Solanum lycopersicum) AG MADS-box gene SlMBP3 and found that it is preferentially expressed in flowers and during early fruit developmental stages in the wild-type (WT), and in the Nr (never ripe) and rin (ripening inhibitor) mutants. Its transcripts are notably accumulated in the pistils; transcripts abundance decrease during seed and placental development, increasing again during flower development. SlMBP3-RNAi tomato plants displayed fleshy placenta without locular gel and extremely malformed seeds with no seed coat, while SlMBP3-overexpressing plants exhibited advanced liquefaction of the placenta and larger seeds. Enzymatic activities related to cell wall modification, and the contents of cell wall components and pigments were dramatically altered in the placentas of SlMBP3-RNAi compared with the WT. Alterations in these physiological features were also observed in the placentas of SlMBP3-overexpressing plants. The lignin content of mature seeds in SlMBP3-RNAi lines was markedly lower than that in the WT. RNA-seq and qRT-PCR analyses revealed that genes involved in seed development and the biosynthesis of enzymes related to cell wall modification, namely gibberellin, indole-3-acetic acid, and abscisic acid were down-regulated in the SlMBP3-RNAi lines. Taking together, our results demonstrate that SlMBP3 is involved in the regulation of placenta and seed development in tomato.

Research Progress of Betalain in Response to Adverse Stresses and Evolutionary Relationship Compared with Anthocyanin.

Betalains are applicable to many aspects of life, and their properties, characteristics, extraction and biosynthesis process have been thoroughly studied. Although betalains are functionally similar to anthocyanins and can substitute for them to provide pigments for plant color, it is rare to study the roles of betalains in plant responses to adverse environmental conditions. Owing to their antioxidant capability to remove excess reactive oxygen species (ROS) in plants and humans, betalains have attracted much attention due to their bioactivity. In addition, betalains can also act as osmotic substances to regulate osmotic pressure in plants and play important roles in plant responses to adverse environmental conditions. The study of the physiological evolution of betalains is almost complete but remains complicated because the evolutionary relationship between betalains and anthocyanins is still uncertain. In this review, to provide a reference for the in-depth study of betalains compared with anthocyanins, the biochemical properties, biosynthesis process and roles of betalains in response to environmental stress are reviewed, and the relationship between betalains and anthocyanins is discussed.

Basic leucine zipper transcription factor SlbZIP1 mediates salt and drought stress tolerance in tomato.

BACKGROUND: Basic region/leucine zipper (bZIP) transcription factors perform as crucial regulators in ABA-mediated stress response in plants. Nevertheless, the functions for most bZIP family members in tomato remain to be deciphered. RESULTS: Here we examined the functional characterization of SlbZIP1 under salt and drought stresses in tomato. Silencing of SlbZIP1 in tomato resulted in reduced expression of multiple ABA biosynthesis- and signal transduction-related genes in transgenic plants. In stress assays, SlbZIP1-RNAi transgenic plants exhibited reduced tolerance to salt and drought stresses compared with WT plants, as are evaluated by multiple physiological parameters associated with stress responses, such as decreased ABA, chlorophyll contents and CAT activity, and increased MDA content. In addition, RNA-seq analysis of transgenic plants revealed that the transcription levels of multiple genes encoding defense proteins related to responses to abiotic stress (e.g. endochitinase, peroxidases, and lipid transfer proteins) and biotic stress (e.g. pathogenesis-related proteins) were downregulated in SlbZIP1-RNAi plants, suggesting that SlbZIP1 plays a role in regulating the genes related to biotic and abiotic stress response. CONCLUSIONS: Collectively, the data suggest that SlbZIP1 exerts an essential role in salt and drought stress tolerance through modulating an ABA-mediated pathway, and SlbZIP1 may hold potential applications in the engineering of salt- and drought-tolerant tomato cultivars.

The SlFSR gene controls fruit shelf-life in tomato.

Fruit ripening represents a process that changes flavor and appearance and also a process that dramatically increases fruit softening. Fruit softening and textural variations mainly result from disruptions to the cell walls of the fruit throughout ripening, but the exact mechanisms and specific modifications of the cell wall remain unclear. Plant-specific GRAS proteins play a critical role in development and growth. To date, few GRAS genes have been functionally categorized in tomato. The expression of a novel GRAS gene described in this study and designated as SlFSR (fruit shelf-life regulator) specifically increased during fruit ripening, but was significantly decreased in the tomato mutant rin (ripening inhibitor). RNAi repression of SlFSR resulted in reduced expression of multiple cell wall modification-related genes, decreased the activities of PG (polygalacturonase), TBG (tomato ß-galactosidase), CEL (cellulase), and XYL (ß-D-xylosidase), and significantly prolonged fruit shelf-life. Furthermore, overexpression of SlFSR in mutant rin gave rise to up-regulated expression of multiple cell wall modification-related genes, such as PG, TBG4, CEL2, XYL1, PL, PE, MAN1, EXP1, and XTH5, and significantly shortened the fruit shelf-life. These findings reveal some of the genetic mechanisms underlying fruit cell wall metabolism and suggest that the SlFSR gene is another potential biotechnological target for the control of tomato fruit shelf-life.

A new tomato NAC (NAM/ATAF1/2/CUC2) transcription factor, SlNAC4, functions as a positive regulator of fruit ripening and carotenoid accumulation.

Fruit ripening in tomato (Solanum lycopersicum) is a complicated development process affected by both endogenous hormonal and genetic regulators and external signals. Although the role of NOR, a member of the NAC domain family, in mediating tomato fruit ripening has been established, its underlying molecular mechanisms remain unclear. To explore further the role of NAC transcription factors in fruit ripening, we characterized a new tomato NAC domain protein, named SlNAC4, which shows high accumulation in sepal and at the onset of fruit ripening. Various stress treatments including wounding, NaCl, dehydration and low temperature significantly increased the expression of SlNAC4. Reduced expression of SlNAC4 by RNA interference (RNAi) in tomato resulted in delayed fruit ripening, suppressed Chl breakdown and decreased ethylene synthesis mediated mainly through reduced expression of ethylene biosynthesis genes of system-2, and reduced carotenoids by alteration of the carotenoid pathway flux. Transgenic tomato fruits also displayed significant down-regulation of multiple ripening-associated genes, indicating that SlNAC4 functions as a positive regulator of fruit ripening by affecting ethylene synthesis and carotenoid accumulation. Moreover, we also noted that SlNAC4 could not be induced by ethylene and may function upstream of the ripening regulator RIN and positively regulate its expression. Yeast two-hybrid assay further revealed that SlNAC4 could interact with both RIN and NOR protein. These results suggested that ethylene-dependent and -independent processes are regulated by SlNAC4 in the fruit ripening regulatory network.

A tomato MADS-box transcription factor, SlMADS1, acts as a negative regulator of fruit ripening.

MADS-box genes encode a highly conserved gene family of transcriptional factors that regulate numerous developmental processes in plants. In this study, a tomato (Solanum lycopersicum) MADS-box gene, SlMADS1, was cloned and its tissue-specific expression profile was analyzed. The real-time polymerase chain reaction results showed that SlMADS1 was highly expressed in sepals and fruits; its expression level was increased with the development of sepals, while the transcript of SlMADS1 decreased significantly in accordance with fruit ripening. To further explore the function of SlMADS1, an RNA interference (RNAi) expression vector targeting SlMADS1 was constructed and transformed into tomato plants. Shorter ripening time of fruit was observed in SlMADS1-silenced tomatoes. The accumulation of carotenoid and the expression of PHYTOENE SYNTHETASE1 were enhanced in RNAi fruits. Besides, ethylene biosynthetic genes, including 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE1A, 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE6, 1-AMINOCYCLOPROPANE-1-CARBOXYLATE OXIDASE1, and 1-AMINOCYCLOPROPANE-1-CARBOXYLATE OXIDASE3, and the ethylene-responsive genes E4 and E8, which were involved in fruit ripening, were also up-regulated in silenced plants. SlMADS1 RNAi fruits showed approximately 2- to 4-fold increases in ethylene production compared with the wild type. Furthermore, SlMADS1-silenced seedlings displayed shorter hypocotyls and were more sensitive to 1-aminocyclopropane-1-carboxylate than the wild type. Additionally, a yeast two-hybrid assay revealed a clear interaction between SlMADS1 and SlMADS-RIN. These results suggest that SlMADS1 plays an important role in fruit ripening as a repressive modulator.

The abiotic stress-responsive NAC-type transcription factor SlNAC4 regulates salt and drought tolerance and stress-related genes in tomato (Solanum lycopersicum).

KEY MESSAGE: SlNAC4 functions as a stress-responsive transcription factor and might be useful for crop salt and drought tolerance improvement. Abiotic stresses, especially salinity and drought, are major factors that significantly limit crop growth and productivity. Plant-specific NAC transcription factors play crucial roles in various stress responses. However, to date only little information regarding stress-related NAC genes is available in tomato. Previously, we reported that tomato SlNAC4-SlNAC10 genes are involved in response of various abiotic stresses. Expression analysis revealed that SlNAC4 was also induced significantly by MeJA, but not by ABA. To further unravel the function of SlNAC4 in response to abiotic stress, we investigated the effects of salt and drought stress on wild-type and SlNAC4-RNAi transgenic tomato plants. The results demonstrated that the root and shoot growth of RNAi plants was more inhibited by salt stress than that of wild-type at post-germination stage. The leaf salt assay also showed less tolerance in transgenic plants by retaining lower chlorophyll content compared with wild-type plants. In addition, transgenic plants became less tolerant to salt and drought stress in soil, which were demonstrated by lower levels of water and chlorophyll contents, and higher water loss rate in their leaves as compared to wild-type plants under stressed conditions. Notably, the expressions of multiple stress-related genes were downregulated in SlNAC4-RNAi plants under both control and salt-stressed conditions. Collectively, these results highlight the important role of SlNAC4 functions as a stress-responsive transcription factor in positive modulation of abiotic stress tolerance through an ABA-independent signaling networks and possibly in response to biotic stress, and may hold promising applications in the engineering of salt- and drought-tolerant tomato.

A SEPALLATA MADS-Box Transcription Factor, SlMBP21, Functions as a Negative Regulator of Flower Number and Fruit Yields in Tomato.

MADS-box transcription factors act as the crucial regulators in plant organ differentiation. Crop yields are highly influenced by the flower number and fruit growth. However, flower identification is a very complex biological process, which involves many cascade regulations. The molecular mechanisms underlying the genetic regulation of flower identification in cultivated plants, such as tomato, are intricate and require further exploration. In this study, we investigated the vital function of a SEPALLATA (SEP) MADS-box gene, SlMBP21, in tomato sympodial inflorescence meristem (SIM) development for the conversion from SIMs to floral meristems (FMs). SlMBP21 transcripts were primarily accumulated in young inflorescence meristem, flowers, sepals, and abscission zones. The Ailsa Craig (AC++) tomato plants with suppressed SlMBP21 mRNA levels using RNAi exhibited a large increase in flower number and fruit yields in addition to enlarged sepals and inhibited abscission zone development. Scanning electron microscopy (SEM) revealed that the maturation of inflorescence meristems (IMs) was repressed in SlMBP21-RNAi lines. RNA-seq and qRT-PCR analyses showed that numerous genes related to the flower development, plant hormone signal transduction, cell cycle, and cell proliferation et al. were dramatically changed in SlMBP21-RNAi lines. Yeast two-hybrid assay exhibited that SlMBP21 can respectively interact with SlCMB1, SFT, JOINTLESS, and MC, which play key roles in inflorescence meristems or FM development. In summary, our data demonstrate that SlMBP21 functions as a key regulator in SIM development and the conversion from SIMs to FMs, through interacting with other regulatory proteins to control the expression of related genes.

Transcriptome- and genome-wide systematic identification of expansin gene family and their expression in tuberous root development and stress responses in sweetpotato (Ipomoea batatas).

Introduction: Expansins (EXPs) are essential components of the plant cell wall that function as relaxation factors to directly promote turgor-driven expansion of the cell wall, thereby controlling plant growth and development and diverse environmental stress responses. EXPs genes have been identified and characterized in numerous plant species, but not in sweetpotato. Results and methods: In the present study, a total of 59 EXP genes unevenly distributed across 14 of 15 chromosomes were identified in the sweetpotato genome, and segmental and tandem duplications were found to make a dominant contribution to the diversity of functions of the IbEXP family. Phylogenetic analysis showed that IbEXP members could be clustered into four subfamilies based on the EXPs from Arabidopsis and rice, and the regularity of protein motif, domain, and gene structures was consistent with this subfamily classification. Collinearity analysis between IbEXP genes and related homologous sequences in nine plants provided further phylogenetic insights into the EXP gene family. Cis-element analysis further revealed the potential roles of IbEXP genes in sweetpotato development and stress responses. RNA-seq and qRT-PCR analysis of eight selected IbEXPs genes provided evidence of their specificity in different tissues and showed that their transcripts were variously induced or suppressed under different hormone treatments (abscisic acid, salicylic acid, jasmonic acid, and 1-aminocyclopropane-1-carboxylic acid) and abiotic stresses (low and high temperature). Discussion: These results provide a foundation for further comprehensive investigation of the functions of IbEXP genes and indicate that several members of this family have potential applications as regulators to control plant development and enhance stress resistance in plants.