CAM7 and HY5 genetically interact to regulate root growth and abscisic acid responses

Calmodulin7: recent insights into emerging roles in plant development and stress

Analyses of the function of Arabidopsis Calmodulin7 (CAM7) in concert with multiple regulatory proteins involved in various signal transduction processes. Calmodulin (CaM) plays various regulatory roles in multiple signaling pathways in eukaryotes. Arabidopsis CALMODULIN 7 (CAM7) is a unique member of the CAM family that works as a transcription factor in light signaling pathways. CAM7 works in concert with CONSTITUTIVE PHOTOMORPHOGENIC 1 and ELONGATED HYPOCOTYL 5, and plays an important role in seedling development. Further, it is involved in the regulation of the activity of various Ca²⁺-gated channels such as cyclic nucleotide gated channel 6 (CNGC6), CNGC14 and auto-inhibited Ca²⁺ ATPase 8. Recent studies further indicate that CAM7 is also an integral part of multiple signaling pathways including hormone, immunity and stress. Here, we review the recent advances in understanding the multifaceted role of CAM7. We highlight the open-ended questions, and also discuss the diverse aspects of CAM7 characterization that need to be addressed for comprehensive understanding of its cellular functions.

The interaction of CaM7 and CNGC14 regulates root hair growth in Arabidopsis

Oscillations in cytosolic free calcium determine the polarity of tip-growing root hairs. The Ca²⁺ channel cyclic nucleotide gated channel 14 (CNGC14) contributes to the dynamic changes in Ca²⁺ concentration gradient at the root hair tip. However, the mechanisms that regulate CNGC14 are unknown. In this study, we detected a direct interaction between calmodulin 7 (CaM7) and CNGC14 through yeast two-hybrid and bimolecular fluorescence complementation assays. We demonstrated that the third EF-hand domain of CaM7 specifically interacts with the cytosolic C-terminal domain of CNGC14. A two-electrode voltage clamp assay showed that CaM7 completely inhibits CNGC14-mediated Ca²⁺ influx, suggesting that CaM7 negatively regulates CNGC14-mediated calcium signaling. Furthermore, CaM7 overexpressing lines phenocopy the short root hair phenotype of a cngc14 mutant and this phenotype is insensitive to changes in external Ca²⁺ concentrations. We, thus, identified CaM7-CNGC14 as a novel interacting module that regulates polar growth in root hairs by controlling the tip-focused Ca²⁺ signal.

Seed comparative genomics in three coffee species identify desiccation tolerance mechanisms in intermediate seeds

In contrast to desiccation-tolerant 'orthodox' seeds, so-called 'intermediate' seeds cannot survive complete drying and are short-lived. All species of the genus Coffea produce intermediate seeds, but they show a considerable variability in seed desiccation tolerance (DT), which may help to decipher the molecular basis of seed DT in plants. We performed a comparative transcriptome analysis of developing seeds in three coffee species with contrasting desiccation tolerance. Seeds of all species shared a major transcriptional switch during late maturation that governs a general slow-down of metabolism. However, numerous key stress-related genes, including those coding for the late embryogenesis abundant protein EM6 and the osmosensitive calcium channel ERD4, were up-regulated during DT acquisition in the two species with high seed DT, C. arabica and C. eugenioides. By contrast, we detected up-regulation of numerous genes involved in the metabolism, transport, and perception of auxin in C. canephora seeds with low DT. Moreover, species with high DT showed a stronger down-regulation of the mitochondrial machinery dedicated to the tricarboxylic acid cycle and oxidative phosphorylation. Accordingly, respiration measurements during seed dehydration demonstrated that intermediate seeds with the highest DT are better prepared to cease respiration and avoid oxidative stresses.

Frequent intra- and inter-species introgression shapes the landscape of genetic variation in bread wheat

BACKGROUND

Bread wheat is one of the most important and broadly studied crops. However, due to the complexity of its genome and incomplete genome collection of wild populations, the bread wheat genome landscape and domestication history remain elusive.

RESULTS

By investigating the whole-genome resequencing data of 93 accessions from worldwide populations of bread wheat and its diploid and tetraploid progenitors, together with 90 published exome-capture data, we find that the B subgenome has more variations than A and D subgenomes, including SNPs and deletions. Population genetics analyses support a monophyletic origin of domesticated wheat from wild emmer in northern Levant, with substantial introgressed genomic fragments from southern Levant. Southern Levant contributes more than 676 Mb in AB subgenomes and enriched in the pericentromeric regions. The AB subgenome introgression happens at the early stage of wheat speciation and partially contributes to their greater genetic diversity. Furthermore, we detect massive alien introgressions that originated from distant species through natural and artificial hybridizations, resulting in the reintroduction of ~ 709 Mb and ~ 1577 Mb sequences into bread wheat landraces and varieties, respectively. A large fraction of these intra- and inter-introgression fragments are associated with quantitative trait loci of important traits, and selection events are also identified.

CONCLUSION

We reveal the significance of multiple introgressions from distant wild populations and alien species in shaping the genetic components of bread wheat, and provide important resources and new perspectives for future wheat breeding.

Genome-wide identification and functional analysis of the cyclic nucleotide-gated channel gene family in Chinese cabbage

Cyclic nucleotide-gated channels (CNGCs) are a class of nonselective cationic channels that are widely found in animals and plants. Plant CNGCs participate in numerous biological functions that vary from development to stress tolerance. Most CNGC genes have been identified in plant genomes, but no such comprehensive study has yet been conducted on Chinese cabbage. In this study, thirty BrCNGC genes were identified, divided into five groups, and used for evolutionary analysis. We assigned names of all individual CNGC members on the basis of phylogenetic relationship with A. thaliana CNGCs. All BrCNGC genes were randomly distributed on chromosomes, and the A08 chromosome did not carry any CNGC gene. The CNGC genes of Chinese cabbage and A. thaliana from the same group displayed similar conserved motifs and gene structures. Especially the closer the homology, the higher the similarity. Quantitative expression analysis showed that most of the CNGC genes were expressed under four stresses, indicating that they play a key role in the stress response of Chinese cabbage. Expression patterns of 12 BrCNGC in the roots, stems, leaves, flowers, and siliques showed that BrCNGC8 and BrCNGC16 were specifically expressed only in flowers but not in other parts. This study lays a theoretical foundation for future research on the function of the CNGC gene family in Chinese cabbage.

Molecular cloning and functional analysis of a UV-B photoreceptor gene, BpUVR8 (UV Resistance Locus 8), from birch and its role in ABA response

As a photoreceptor specifically for UV-B light, UVR8 gene plays an important role in the photomorphogenesis and developmental growth of plants. In this research, we isolated the UVR8 gene from birch, named BpUVR8 (AHY02156). BpUVR8 overexpression rescued the uvr8 mutant phenotype using functional complementation assay of BpUVR8 in Arabidopsis uvr8 mutants, which showed that the function of UVR8 is conserved between Arabidopsis and birch. The expression analysis of BpUVR8 indicated that this gene is expressed in various tissues, but its expression levels in leaves are higher than in other organs. Moreover, abiotic stress factors, such as UV-B, salinity, and abscisic acid (ABA) can induce the expression of BpUVR8 gene. Interestingly, the analysis of promoter activity indicated that BpUVR8 promoter not only has the promoting activity but can also respond to the induction of abiotic stress and ABA signal. So, we analyzed its function in ABA response via transgenic UVR8 overexpression in Arabidopsis. The BpUVR8 enhances the susceptibility to ABA, which indicates that BpUVR8 is regulated by ABA and can inhibit seed germination. The root length of 20-day-old 35S::BpUVR8/WT transgenic plants was 18% reduced as compared to the wild-type under the ABA treatment. The membrane of the BpUVR8-overexpressing in Arabidopsis thaliana was the most damaged after ABA treatment and 35S::BpUVR8/WT transgenic plant was more sensitive to ABA than the wild type. These results showed that BpUVR8 is a positive regulator in the ABA signal transduction pathway. In the presence of low dose of UV-B, the sensitivity of wild-type and 35S::BpUVR8/WT plants to ABA was reduced. Moreover, BpUVR8 regulates the expression of a subset of ABA-responsive genes, both in Arabidopsis and Betula platyphylla, under the ABA treatment. Our data provide evidence that BpUVR8 is a positive regulator in the UV-B-induced photomorphogenesis in plants. Moreover, we propose from this research that BpUVR8 might have an important role in integrating plant growth and ABA signaling pathway.

Arabidopsis IQM4, a Novel Calmodulin-Binding Protein, Is Involved With Seed Dormancy and Germination in Arabidopsis

Seed dormancy and germination are regulated by complex mechanisms controlled by diverse hormones and environmental cues. Abscisic acid (ABA) promotes seed dormancy and inhibits seed germination and post-germination growth. Calmodulin (CaM) signals are involved with the inhibition of ABA during seed germination and seedling growth. In this study, we showed that Arabidopsis thaliana IQM4 could bind with calmodulin 5 (CaM5) both in vitro and in vivo, and that the interaction was the Ca2+-independent type. The IQM4 protein was localized in the chloroplast and the IQM4 gene was expressed in most tissues, especially the embryo and germinated seedlings. The T-DNA insertion mutants of IQM4 exhibited the reduced primary seed dormancy and lower ABA levels compared with wild type seeds. Moreover, IQM4 plays key roles in modulating the responses to ABA, salt, and osmotic stress during seed germination and post-germination growth. T-DNA insertion mutants exhibited ABA-insensitive and salt-hypersensitive phenotypes during seed germination and post-germination growth, whereas IQM4-overexpressing lines had ABA- and osmotic-hypersensitive, and salt-insensitive phenotypes. Gene expression analyses showed that mutation of IQM4 inhibited the expression of ABA biosynthetic genes NCED6 and NCED9, and seed maturation regulators LEC1, LEC2, ABI3, and ABI5 during the silique development, as well as promoted the expression of WRKY40 and inhibited that of ABI5 in ABA-regulated seed germination. These observations suggest that IQM4 is a novel Ca2+-independent CaM-binding protein, which is positively involved with seed dormancy and germination in Arabidopsis.