BIG regulates stomatal immunity and jasmonate production in Arabidopsis

The Arabidopsis Rab protein RABC1 affects stomatal development by regulating lipid droplet dynamics

Lipid droplets (LDs) are evolutionarily conserved organelles that serve as hubs of cellular lipid and energy metabolism in virtually all organisms. Mobilization of LDs is important in light-induced stomatal opening. However, whether and how LDs are involved in stomatal development remains unknown. We show here that Arabidopsis thaliana LIPID DROPLETS AND STOMATA 1 (LDS1)/RABC1 (At1g43890) encodes a member of the Rab GTPase family that is involved in regulating LD dynamics and stomatal morphogenesis. The expression of RABC1 is coordinated with the different phases of stomatal development. RABC1 targets to the surface of LDs in response to oleic acid application in a RABC1GEF1-dependent manner. RABC1 physically interacts with SEIPIN2/3, two orthologues of mammalian seipin, which function in the formation of LDs. Disruption of RABC1, RABC1GEF1, or SEIPIN2/3 resulted in aberrantly large LDs, severe defects in guard cell vacuole morphology, and stomatal function. In conclusion, these findings reveal an aspect of LD function and uncover a role for lipid metabolism in stomatal development in plants.

Uncovering the mechanism of anthocyanin accumulation in a purple-leaved variety of foxtail millet (Setaria italica) by transcriptome analysis

Anthocyanin is a natural pigment that has a functional role in plants to attract pollinating insects and is important in stress response. Foxtail millet (Setaria italica) is known as a nutritional crop with high resistance to drought and barren. However, the molecular mechanism regulating anthocyanin accumulation and the relationship between anthocyanin and the stress resistance of foxtail millet remains obscure. In this study, we screened hundreds of germplasm resources and obtained several varieties with purple plants in foxtail millet. By studying the purple-leaved B100 variety and the control variety, Yugu1 with green leaves, we found that B100 could accumulate a large amount of anthocyanin in the leaf epiderma, and B100 had stronger stress tolerance. Further transcriptome analysis revealed the differences in gene expression patterns between the two varieties. We identified nine genes encoding enzymes related to anthocyanin biosynthesis using quantitative PCR validation that showed significantly higher expression levels in B100 than Yugu1. The results of this study lay the foundation for the analysis of the molecular mechanism of anthocyanin accumulation in foxtail millet, and provided genetic resources for the molecular breeding of crops with high anthocyanin content.

BIG Modulates Stem Cell Niche and Meristem Development via SCR/SHR Pathway in Arabidopsis Roots

BIG, a regulator of polar auxin transport, is necessary to regulate the growth and development of Arabidopsis. Although mutations in the BIG gene cause severe root developmental defects, the exact mechanism remains unclear. Here, we report that disruption of the BIG gene resulted in decreased quiescent center (QC) activity and columella cell numbers, which was accompanied by the downregulation of WUSCHEL-RELATED HOMEOBOX5 (WOX5) gene expression. BIG affected auxin distribution by regulating the expression of PIN-FORMED proteins (PINs), but the root morphological defects of big mutants could not be rescued solely by increasing auxin transport. Although the loss of BIG gene function resulted in decreased expression of the PLT1 and PLT2 genes, genetic interaction assays indicate that this is not the main reason for the root morphological defects of big mutants. Furthermore, genetic interaction assays suggest that BIG affects the stem cell niche (SCN) activity through the SCRSCARECROW (SCR)/SHORT ROOT (SHR) pathway and BIG disruption reduces the expression of SCR and SHR genes. In conclusion, our findings reveal that the BIG gene maintains root meristem activity and SCN integrity mainly through the SCR/SHR pathway.

Jasmonate Signaling Pathway Modulates Plant Defense, Growth, and Their Trade-Offs

Lipid-derived jasmonates (JAs) play a crucial role in a variety of plant development and defense mechanisms. In recent years, significant progress has been made toward understanding the JA signaling pathway. In this review, we discuss JA biosynthesis, as well as its core signaling pathway, termination mechanisms, and the evolutionary origin of JA signaling. JA regulates not only plant regeneration, reproductive growth, and vegetative growth but also the responses of plants to stresses, including pathogen as well as virus infection, herbivore attack, and abiotic stresses. We also focus on the JA signaling pathway, considering its crosstalk with the gibberellin (GA), auxin, and phytochrome signaling pathways for mediation of the trade-offs between growth and defense. In summary, JA signals regulate multiple outputs of plant defense and growth and act to balance growth and defense in order to adapt to complex environments.

Genetic Insight Into the Insect Resistance in Bread Wheat Exploiting the Untapped Natural Diversity

Climate change is an undeniable threat to sustainable wheat production in the future as an increased temperature will significantly increase grain loss due to the increased number of generations per season of multivoltine species that are detrimental to plants. Among insects, orange wheat blossom midge (OWBM), yellow wheat blossom midge (YWBM), saddle gall midge (SGM), thrips, and frit fly (FF) are important wheat pests in the European environments, which can be managed by the development of resistant cultivars. This involves the identification, confirmation, and incorporation of insect resistance sources into new high-yielding cultivars. We used two diverse and unrelated wheat [winter wheat (WW) and spring wheat (SW)] panels to associate single-nucleotide polymorphism (SNP) markers with the mentioned pests using the tools of association mapping. All in all, a total of 645 and 123 significant associations were detected in WW and SW, respectively, which were confined to 246 quantitative trait loci. Many candidate genes were identified using the BLAST analysis of the sequences of associated SNPs. Some of them are involved in controlling the physical structures of plants such as stomatal immunity and closure, cuticular wax in leaf blade, whereas others are involved in the production of certain enzymes in response to biotic and abiotic stresses. To our knowledge, this is the first detailed investigation that deals with YWBM, SGM, thrips, and FF resistance genetics using the natural variation in wheat. The reported germplasm is also readily available to breeders across the world that can make rational decisions to breed for the pest resilience of their interest by including the resistant genotypes being reported.

Overexpressing the N-terminus of CATALASE2 enhances plant jasmonic acid biosynthesis and resistance to necrotrophic pathogen Botrytis cinerea B05.10

Salicylic acid (SA) acts antagonistically to jasmonic acid (JA) in plant immunity. We previously reported that CATALASE2 (CAT2) promotes JA-biosynthetic acyl-CoA oxidase (ACX) activity to enhance plant resistance to necrotrophic Botrytis cinerea, and SA represses JA biosynthesis through inhibiting CAT2 activity, while the underlying mechanism remains to be further elucidated. Here, we report that the truncated CAT2 N-terminus (CAT2-N) interacts with and promotes ACX2/3, and CAT2-N-overexpressing plants have increased JA accumulation and enhanced resistance to B. cinerea B05.10, but compromised antagonism of SA on JA. Catalase inhibitor treatment or mutating CAT2 active amino acids abolished CAT2 H₂ O₂ -decomposing activity but did not affect its promotion of ACX2/3 activity via interaction. CAT2-N, a truncated protein with no catalase activity, interacted with and promoted ACX2/3. Overexpressing CAT2-N in Arabidopsis plants resulted in increased ACX activity, higher JA accumulation, and stronger resistance to B. cinerea B05.10 infection. Additionally, SA dramatically repressed JA biosynthesis and resistance to B. cinerea in the wild type but not in the CAT2-N-overexpressing plants. Together, our study reveals that CAT2-N can be utilized as an accelerator for JA biosynthesis during plant resistance to B. cinerea B05.10, and this truncated protein partly relieves SA repression of JA biosynthesis in plant defence responses.

Contribution of phenylpropanoid metabolism to plant development and plant-environment interactions

Phenylpropanoid metabolism is one of the most important metabolisms in plants, yielding more than 8,000 metabolites contributing to plant development and plant-environment interplay. Phenylpropanoid metabolism materialized during the evolution of early freshwater algae that were initiating terrestrialization and land plants have evolved multiple branches of this pathway, which give rise to metabolites including lignin, flavonoids, lignans, phenylpropanoid esters, hydroxycinnamic acid amides, and sporopollenin. Recent studies have revealed that many factors participate in the regulation of phenylpropanoid metabolism, and modulate phenylpropanoid homeostasis when plants undergo successive developmental processes and are subjected to stressful environments. In this review, we summarize recent progress on elucidating the contribution of phenylpropanoid metabolism to the coordination of plant development and plant-environment interaction, and metabolic flux redirection among diverse metabolic routes. In addition, our review focuses on the regulation of phenylpropanoid metabolism at the transcriptional, post-transcriptional, post-translational, and epigenetic levels, and in response to phytohormones and biotic and abiotic stresses.

Involvement of Arabidopsis BIG protein in cell death mediated by Myo-inositol homeostasis

Programmed cell death (PCD) is essential for several aspects of plant life. We previously identified the mips1 mutant of Arabidopsis thaliana, which is deficient for the enzyme catalysing myo-inositol synthesis, and that displays light-dependent formation of lesions on leaves due to Salicylic Acid (SA) over-accumulation. Rationale of this work was to identify novel regulators of plant PCD using a genetic approach. A screen for secondary mutations that abolish the mips1 PCD phenotype identified a mutation in the BIG gene, encoding a factor of unknown molecular function that was previously shown to play pleiotropic roles in plant development and defence. Physiological analyses showed that BIG is required for lesion formation in mips1 via SA-dependant signalling. big mutations partly rescued transcriptomic and metabolomics perturbations as stress-related phytohormones homeostasis. In addition, since loss of function of the ceramide synthase LOH2 was not able to abolish cell death induction in mips1, we show that PCD induction is not fully dependent of sphingolipid accumulation as previously suggested. Our results provide further insights into the role of the BIG protein in the control of MIPS1-dependent cell death and also into the impact of sphingolipid homeostasis in this pathway.

ROS of Distinct Sources and Salicylic Acid Separate Elevated CO2-Mediated Stomatal Movements in Arabidopsis

Elevated CO2 (eCO2) often reduces leaf stomatal aperture and density thus impacts plant physiology and productivity. We have previously demonstrated that the Arabidopsis BIG protein distinguishes between the processes of eCO2-induced stomatal closure and eCO2-inhibited stomatal opening. However, the mechanistic basis of this action is not fully understood. Here we show that eCO2-elicited reactive oxygen species (ROS) production in big mutants was compromised in stomatal closure induction but not in stomatal opening inhibition. Pharmacological and genetic studies show that ROS generated by both NADPH oxidases and cell wall peroxidases contribute to eCO2-induced stomatal closure, whereas inhibition of light-induced stomatal opening by eCO2 may rely on the ROS derived from NADPH oxidases but not from cell wall peroxidases. As with JA and ABA, SA is required for eCO2-induced ROS generation and stomatal closure. In contrast, none of these three signals has a significant role in eCO2-inhibited stomatal opening, unveiling the distinct roles of plant hormonal signaling pathways in the induction of stomatal closure and the inhibition of stomatal opening by eCO2. In conclusion, this study adds SA to a list of plant hormones that together with ROS from distinct sources distinguish two branches of eCO2-mediated stomatal movements.

The BIG gene controls size of shoot apical meristems in Arabidopsis thaliana

BIG regulates the shoot stem cell population. The shoot apical meristem (SAM) contains a population of self-renewing cells, and provides daughter cells for initiation and development of aerial parts of plants. However, the underlying mechanisms of SAM size regulation remain largely unclear. Here, we identified a mutant that displayed a large SAM, designated big-shoot meristem (big-m), in Arabidopsis thaliana. The phenotype of big-m is caused by a new T-DNA insertion allele of BIG, causing a loss of function. The big-m mutant had more stem cells in the SAM than in the wild type. Expression of WUSCHEL (WUS) and SHOOTMERISTEMLESS (STM) was promoted in big-m compared with the wild type, showing that BIG functions upstream of WUS and STM. Therefore, BIG is an important regulator of the stem cell population in the SAM. Furthermore, genetic analysis indicated that BIG acts synergistically with PIN-FORMED1 (PIN1) in controlling SAM size. Our results suggest that BIG plays an important role in controlling Arabidopsis thaliana SAM growth via PIN1-mediated auxin homeostasis.

BIG regulates sugar response and C/N balance in Arabidopsis

Mutations in BIG gene not only produce pleiotropic phenotypes of plant development but also impair plant adaptive responses under various stresses. However, the role of BIG gene in sugar signaling is not known. In this study, we first found that BIG deficiency significantly sensitized the sugar-induced anthocyanin accumulation and the sugar-inhibited primary root growth, suggesting BIG is an important component of the sugar signaling pathway. Then we found that big mutant plants had higher sugar levels compared with the wild type, indicating the involvement of BIG gene in regulating plant sugar homeostasis. Importantly, we also found that the relative ratio of carbon to nitrogen (C/N) was greatly enhanced by BIG deficiency. Overall, our work expands the known functionality of BIG and reveals its role in regulating sugar response and C/N balance. It is likely that BIG connects nutrient, light, and hormone signaling networks for regulating plant development and adaptive responses.

Rice BIG gene is required for seedling viability

Arabidopsis BIG (AtBIG) gene encodes an enormous protein that is required for auxin transport. Loss of AtBIG function not only profoundly changes plant architecture but also alters plant adaptability to environmental stimuli. A putative homolog of AtBIG exists in the rice genome, but no function has been ascribed to it. In this study, we focus on the characterization of the gene structure and function of OsBIG. Sequence and phylogenetic analysis shows that the homologs of OsBIG have high amino acid conservation in several domains across species. Transgenic rice plants in which the expression of OsBIG was disrupted through the CRISPR/Cas9 system-mediated genome editing were used for phenotypic analysis. The Osbig/- plants show high levels of cell death, enhanced electrolyte leakage and membrane lipid peroxidation, and reduced chlorophyll content, which likely accounted for the seedling lethality. Moreover, gene expression between Osbig/- and wild-type plants analyzed by RNA-seq indicates that a number of metabolic and hormonal pathways including ribosome, DNA replication, photosynthesis, and chlorophyll metabolism were significantly perturbed by OsBIG deficiency. In summary, OsBIG gene is integral to the normal growth and development in rice.