Effects of Endogenous Salicylic Acid During Calcium Deficiency-Induced Tipburn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Inhibition of NPR1 Leads to Shoot Growth Improvement under Low-Calcium Conditions in Arabidopsis

Under low-Ca conditions, plants accumulate salicylic acid (SA) and induce SA-responsive genes. However, the relationship between SA and low-Ca tolerance remains unclear. Here, we demonstrated that the inhibition or suppression of nonexpressor of pathogenesis-related 1 (NPR1) activity, a major regulator of the SA signaling pathway in the defense response, improves shoot growth under low-Ca conditions. Furthermore, mutations in phytoalexin-deficient 4 (PAD4) or enhanced disease susceptibility 1 (EDS1), which are upstream regulators of NPR1, improved shoot growth under low-Ca conditions, suggesting that NPR1 suppressed growth under low-Ca conditions. In contrast, growth of SA induction-deficient 2-2 (sid2-2), which is an SA-deficient mutant, was sensitive to low Ca levels, suggesting that SA accumulation by SID2 was not related to growth inhibition under low-Ca conditions. Additionally, npr1-1 showed low-Ca tolerance, and the application of tenoxicam-an inhibitor of the NPR1-mediated activation of gene expression-also improved shoot growth under low Ca conditions. The low-Ca tolerance of double mutants pad4-1, npr1-1 and eds1-22 npr1-1 was similar to that of the single mutants, suggesting that PAD4 and EDS1 are involved in the same genetic pathway in suppressing growth under low-Ca conditions as NPR1. Cell death and low-Ca tolerance did not correlate among the mutants, suggesting that growth improvement in the mutants was not due to cell death inhibition. In conclusion, we revealed that NPR1 suppresses plant growth under low-Ca conditions and that the other SA-related genes influence plant growth and cell death.

Genome-Wide Identification of BrCAX Genes and Functional Analysis of BrCAX1 Involved in Ca2+ Transport and Ca2+ Deficiency-Induced Tip-Burn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Calcium (Ca2+) plays essential roles in plant growth and development. Ca2+ deficiency causes a physiological disorder of tip-burn in Brassiceae crops and is involved in the regulation of cellular Ca2+ homeostasis. Although the functions of Ca2+/H+ exchanger antiporters (CAXs) in mediating transmembrane transport of Ca2+ have been extensively characterized in multiple plant species, the potential roles of BrCAX genes remain unclear in Chinese cabbage. In this study, eight genes of the BrCAX family were genome-widely identified in Chinese cabbage. These BrCAX proteins contained conserved Na_Ca_ex domain and belonged to five members of the CAX family. Molecular evolutionary analysis and sequence alignment revealed the evolutionary conservation of BrCAX family genes. Expression profiling demonstrated that eight BrCAX genes exhibited differential expression in different tissues and under heat stress. Furthermore, Ca2+ deficiency treatment induced the typical symptoms of tip-burn in Chinese cabbage seedlings and a significant decrease in total Ca2+ content in both roots and leaves. The expression changes in BrCAX genes were related to the response to Ca2+ deficiency-induced tip-burn of Chinese cabbage. Specially, BrCAX1-1 and BrCAX1-2 genes were highly expressed gene members of the BrCAX family in the leaves and were significantly differentially expressed under Ca2+ deficiency stress. Moreover, overexpression of BrCAX1-1 and BrCAX1-2 genes in yeast and Chinese cabbage cotyledons exhibited a higher Ca2+ tolerance, indicating the Ca2+ transport capacity of BrCAX1-1 and BrCAX1-2. In addition, suppression expression of BrCAX1-1 and BrCAX1-2 genes reduced cytosolic Ca2+ levels in the root tips of Chinese cabbage. These results provide references for functional studies of BrCAX genes and to investigate the regulatory mechanisms underlying Ca2+ deficiency disorder in Brassiceae vegetables.

Comparative Transcriptome and Co-Expression Network Analyses Reveal the Molecular Mechanism of Calcium-Deficiency-Triggered Tipburn in Chinese Cabbage (Brassica rapa L. ssp. Pekinensis)

Chinese cabbage tipburn is characterized by the formation of necrotic lesions on the margin of leaves, including on the insides of the leafy head. This physiological disorder is associated with a localized calcium deficiency during leaf development. However, little information is available regarding the molecular mechanisms governing Ca-deficiency-triggered tipburn. This study comprehensively analysed the transcriptomic comparison between control and calcium treatments (CK and 0 mM Ca) in Chinese cabbage to determine its molecular mechanism in tipburn. Our analysis identified that the most enriched gene ontology (GO) categories are photosynthesis, thylakoid and cofactor binding. Moreover, the KEGG pathway was most enriched in photosynthesis, carbon metabolism and carbon fixation. We also analyzed the co-expression network by functional categories and identified ten critical hub differentially expressed genes (DEGs) in each gene regulatory network (GRN). These DEGs might involve abiotic stresses, developmental processes, cell wall metabolism, calcium distribution, transcription factors, plant hormone biosynthesis and signal transduction pathways. Under calcium deficiency, CNX1, calmodulin-binding proteins and CMLs family proteins were downregulated compared to CK. In addition, plant hormones such as GA, JA, BR, Auxin and ABA biosynthesis pathways genes were downregulated under calcium treatment. Likewise, HATs, ARLs and TCP transcription factors were reported as inactive under calcium deficiency, and potentially involved in the developmental process. This work explores the specific DEGs' significantly different expression levels in 0 mM Ca and the control involved in plant hormones, cell wall developments, a light response such as chlorophylls and photosynthesis, transport metabolism and defence mechanism and redox. Our results provide critical evidence of the potential roles of the calcium signal transduction pathway and candidate genes governing Ca-deficiency-triggered tipburn in Chinese cabbage.

Exogenous brassinosteroids alleviate calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis in mini Chinese Cabbage

Tip-burn has seriously affected the yield, quality and commodity value of mini Chinese cabbage. Calcium (Ca²⁺) deficiency is the main cause of tip-burn. In order to investigate whether exogenous brassinosteroids (BRs) can alleviate tip-burn induced by calcium (Ca²⁺) deficiency and its mechanism, in this study, Ca²⁺ deficiency in nutrient solution was used to induced tip-burn, and then distilled water and BRs were sprayed on leaves to observe the tip-burn incidence of mini Chinese cabbage. The tip-burn incidence and disease index, leaf area, fluorescence parameters (Fv/Fm, NPQ, qP andφPSII) and gas exchange parameters (Tr, Pn, Gs and Ci), pigment contents, cell wall components, mesophyll cell ultrastructure and the expression of genes related to chlorophyll degradation were measured. The results showed that exogenous BRs reduced the tip-burn incidence rate and disease index of mini Chinese cabbage, and the tip-burn incidence rate reached the highest on the ninth day after treatment. Exogenous BRs increased the contents of cellulose, hemifiber, water-soluble pectin in Ca²⁺ deficiency treated leaves, maintaining the stability of cell wall structure. In addition, BRs increased photosynthetic rate by increasing the activities of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose 1,6-bisphosphatase (FBPase) related to Calvin cycle, maintaining relatively complete chloroplast structure and higher chlorophyll content via down-regulating the expression of BrPPH1 and BrPAO1 genes related to chlorophyll degradation. In conclusion, exogenous BRs alleviated calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis.

Analysis of miRNAs responsive to long-term calcium deficiency in tef (Eragrostis tef (Zucc.) Trotter)

MicroRNAs (miRNAs) play an important role in growth, development, stress resilience, and epigenetic modifications of plants. However, the effect of calcium (Ca²⁺) deficiency on miRNA expression in the orphan crop tef (Eragrostis tef) remains unknown. In this study, we analyzed expression of miRNAs in roots and shoots of tef in response to Ca²⁺ treatment. miRNA-seq followed by bioinformatic analysis allowed us to identify a large number of small RNAs (sRNAs) ranging from 17 to 35 nt in length. A total of 1380 miRNAs were identified in tef experiencing long-term Ca²⁺ deficiency while 1495 miRNAs were detected in control plants. Among the miRNAs identified in this study, 161 miRNAs were similar with those previously characterized in other plant species and 348 miRNAs were novel, while the remaining miRNAs were uncharacterized. Putative target genes and their functions were predicted for all the known and novel miRNAs that we identified. Based on gene ontology (GO) analysis, the predicted target genes are known to have various biological and molecular functions including calcium uptake and transport. Pairwise comparison of differentially expressed miRNAs revealed that some miRNAs were specifically enriched in roots or shoots of low Ca²⁺-treated plants. Further characterization of the miRNAs and their targets identified in this study may help in understanding Ca²⁺ deficiency responses in tef and related orphan crops.

Callose Synthesis Suppresses Cell Death Induced by Low-Calcium Conditions in Leaves

Despite the importance of preventing calcium (Ca) deficiency disorders in agriculture, knowledge of the molecular mechanisms underlying plant adaptations to low-Ca conditions is limited. In this study, we provide evidence for a crucial involvement of callose synthesis in the survival of Arabidopsis (Arabidopsis thaliana) under low-Ca conditions. A mutant sensitive to low-Ca conditions, low calcium sensitive3 (lcs3), exhibited high levels of cell death in emerging leaves and had defects in its expanding true leaves under low-Ca conditions. Further analyses showed that the causal mutation was located in a putative β-1,3-glucan (callose) synthase gene, GLUCAN SYNTHASE-LIKE10 (GSL10). Yeast complementation assay results showed that GSL10 encodes a functional callose synthase. Ectopic callose significantly accumulated in wild-type plants under low-Ca conditions, but at a low level in lcs3 The low-Ca sensitivity of lcs3 was phenocopied by the application of callose synthase inhibitors in wild-type plants, which resulted in leaf expansion failure, cell death, and reduced ectopic callose levels under low-Ca conditions. Transcriptome analyses showed that the expression of genes related to cell wall and defense responses was altered in both wild-type plants under low-Ca conditions and in lcs3 under normal-Ca conditions, suggesting that GSL10 is required for the alleviation of both cell wall damage and defense responses caused by low Ca levels. These results suggest that callose synthesis is essential for the prevention of cell death under low-Ca conditions and plays a key role in plants' survival strategies under low-Ca conditions.

Natural variation in a calreticulin gene causes reduced resistance to Ca2+ deficiency-induced tipburn in Chinese cabbage (Brassica rapa ssp. pekinensis)

Tipburn is an irreversible physiological disorder of Chinese cabbage that decreases crop value. Because of a strong environmental component, tipburn-resistant cultivars are the only solution, although tipburn resistance genes are unknown in Chinese cabbage. We studied three populations of Chinese cabbage over four growing seasons under field conditions: (a) 194 diverse inbred lines, (b) a doubled haploid (DH100) population, and (c) an F₂ population. The 194 lines were genotyped using single nucleotide polymorphism markers, and genome-wide-association mapping showed that 24 gQTLs were significantly associated with tipburn disease index. Analysis of the DH100 and F₂ populations identified a shared tipburn-associated locus, gqbTRA06, that was found to cover the region defined by one of the 24 gQTLs. Of 35 genes predicted in the 0.14-Mb quantitative trait locus region, Bra018575 (calreticulin family protein, BrCRT2) showed higher expression levels during disease development. We cloned the two BrCRT2 alleles from tipburn-resistant (BrCRT2^R ) and tipburn-susceptible (BrCRT2^S ) lines and identified a 51-bp deletion in BrCRT2^S . Overexpression of BrCRT2^R increased Ca²⁺ storage in the Arabidopsis crt2 mutant and also reduced cell death in leaf tips and margins under Ca²⁺ -depleted conditions. Our results suggest that BrCRT2 is a possible candidate gene for controlling tipburn in Chinese cabbage.

Transcriptome-Wide Identification and Characterization of Circular RNAs in Leaves of Chinese Cabbage (Brassica rapa L. ssp. pekinensis) in Response to Calcium Deficiency-Induced Tip-burn

Circular RNA (circRNA) is a newly discovered non-coding RNA, which play significant roles in the function and transcriptional regulation of microRNA. To date, in Chinese cabbage, the functional characteristic of circRNAs in response to calcium deficiency-induced tip-burn have not been reported. In this study, 730 circRNAs were isolated from Chinese cabbage leaves, of which 23 and 22 were differentially expressed in different calcium deficiency stages compared with the control. Forty-six host genes of the differentially expressed circRNAs were identified, and one circRNA was found to act as miRNAs sponges. Based on the functional analysis of host genes and target mRNAs of the corresponding miRNAs, the identified circRNAs might participated in response to stimulus, electron carrier activity, ATPase activity, cell wall metabolism, transcription factors and plant hormone signal transduction. ABF2, a positive regulator of the abiotic stress response in the abscisic acid (ABA) pathway, may play a role in calcium deficiency tolerance through a circRNA regulatory pathway. Correspondingly, the concentration of ABA is also increased during the Ca²⁺ deficiency stress. Our results suggest that circRNAs participate in a broad range of biological processes and physiological functions in the response to calcium deficiency-induced tip-burn and provide a basis for further studies of the biological roles that circRNAs play in the plant stress response.