Physiological profile of CAX1a TILLING mutants of Brassica rapa exposed to different calcium doses

CAX control: multiple roles of vacuolar cation/H+ exchangers in metal tolerance, mineral nutrition and environmental signalling

Plant vacuolar transporters, particularly CAX (Cation/H+ Exchangers) responsible for Ca2+/H+ exchange on the vacuole tonoplast, play a central role in governing cellular pH, ion balance, nutrient storage, metal accumulation, and stress responses. Furthermore, CAX variants have been employed to enhance the calcium content of crops, contributing to biofortification efforts. Recent research has uncovered the broader significance of these transporters in plant signal transduction and element partitioning. The use of genetically encoded Ca2+ sensors has begun to highlight the crucial role of CAX isoforms in generating cytosolic Ca2+ signals, underscoring their function as pivotal hubs in diverse environmental and developmental signalling networks. Interestingly, it has been observed that the loss of CAX function can be advantageous in specific stress conditions, both for biotic and abiotic stressors. Determining the optimal timing and approach for modulating the expression of CAX is a critical concern. In the future, strategically manipulating the temporal loss of CAX function in agriculturally important crops holds promise to bolster plant immunity, enhance cold tolerance, and fortify resilience against one of agriculture's most significant challenges, namely flooding.

Evaluation of the alkalinity stress tolerance of three Brassica rapa CAX1 TILLING mutants

Alkalinity is an important environmental factor that affects crop production and will be exacerbated in the current climate change scenario. Thus, the presence of carbonates and high pH in soils negatively impacts nutrient assimilation and photosynthesis and causes oxidative stress. A potential strategy to improve tolerance to alkalinity could be the modification of cation exchanger (CAX) activity, given that these transporters are involved in calcium (Ca²⁺) signaling under stresses. In this study, we used three Brassica rapa mutants (BraA.cax1a-4, BraA.cax1a-7, and BraA.cax1a-12) from the parental line 'R-o-18' that were generated by Targeting Induced Local Lesions in Genomes (TILLING) and grown under control and alkaline conditions. The objective was to assess the tolerance of these mutants to alkalinity stress. Biomass, nutrient accumulation, oxidative stress, and photosynthesis parameters were analyzed. ^{The results showed} that BraA.cax1a-7 mutation was negative for alkalinity tolerance because it reduced plant biomass, increased oxidative stress, partially inhibited antioxidant response, and lowered photosynthesis performance. Conversely, the BraA.cax1a-12 mutation increased plant biomass and Ca²⁺ accumulation, reduced oxidative stress, and improved antioxidant response and photosynthesis performance. Hence, this study identifies BraA.cax1a-12 as a useful CAX1 mutation to enhance the tolerance of plants grown under alkaline conditions.

Micromorphology and Histology of the Secretory Apparatus of Diospyros villosa (L.) de Winter Leaves and Stem Bark

Diospyros villosa is a perennial species prominently acknowledged for its local medicinal applications. The native utilisation of this species in traditional medicine may be ascribed to the presence of secretory structures and their exudate (comprised of phytochemicals). However, the morphological nature and optical features of the secretory structures in D. villosa remain largely unclear. This study was directed to ascertain the occurrence and adaptive features of structures found within the leaves and stem bark of D. villosa using light and electron microscopy techniques. The current study notes the existence of trichomes, and other secretory structures were noted. SEM indicated the presence of non-glandular hirsute trichomes with bulky stalk on both leaves and stem surfaces. Transverse stem sections revealed the existence of crystal idioblasts. Moreover, the presence of the main phytochemical groups and their localisation within the foliage and stem bark was elucidated through various histochemical tests. The trichomal length and density were also assessed in leaves at different stages of development. The results indicated that the trichomal density at different stages of development of the D. villosa leaves and stem bark was not significantly different from one another, F_(3,39) = 1.183, p = 0.3297. The average length of the non-glandular trichomes in the emergent, young and mature leaves, as well as in the stem, was recorded to be 230 ± 30.6 µm, 246 ± 40.32 μm, 193 ± 27.55 µm and 164 ± 18.62 µm, respectively. The perimeter and circumference of the observed trichomes in the developmental stages of D. villosa leaf and the stem bark were not statistically different, F_(3,39) = 1.092, p = 0.3615. The results of histochemical tests showed the existence of phenols alkaloids, which are medicinally important and beneficial for treatment of diseases. The findings of this study, being reported for the first time may be considered in establishing microscopic and pharmacognostic measure for future identification and verification of natural herbal plant. Trichomal micromorphology and histological evaluations could be utilised as a tool for appropriate description for the assessment of this species.

Genomics Armed With Diversity Leads the Way in Brassica Improvement in a Changing Global Environment

Meeting the needs of a growing world population in the face of imminent climate change is a challenge; breeding of vegetable and oilseed Brassica crops is part of the race in meeting these demands. Available genetic diversity constituting the foundation of breeding is essential in plant improvement. Elite varieties, land races, and crop wild species are important resources of useful variation and are available from existing genepools or genebanks. Conservation of diversity in genepools, genebanks, and even the wild is crucial in preventing the loss of variation for future breeding efforts. In addition, the identification of suitable parental lines and alleles is critical in ensuring the development of resilient Brassica crops. During the past two decades, an increasing number of high-quality nuclear and organellar Brassica genomes have been assembled. Whole-genome re-sequencing and the development of pan-genomes are overcoming the limitations of the single reference genome and provide the basis for further exploration. Genomic and complementary omic tools such as microarrays, transcriptomics, epigenetics, and reverse genetics facilitate the study of crop evolution, breeding histories, and the discovery of loci associated with highly sought-after agronomic traits. Furthermore, in genomic selection, predicted breeding values based on phenotype and genome-wide marker scores allow the preselection of promising genotypes, enhancing genetic gains and substantially quickening the breeding cycle. It is clear that genomics, armed with diversity, is set to lead the way in Brassica improvement; however, a multidisciplinary plant breeding approach that includes phenotype = genotype × environment × management interaction will ultimately ensure the selection of resilient Brassica varieties ready for climate change.

CAX3 (cation/proton exchanger) mediates a Cd tolerance by decreasing ROS through Ca elevation in Arabidopsis

KEY MESSAGE

Over-expression of CAX3 encoding a cation/proton exchanger enhances Cd tolerance by decreasing ROS (Reactive Oxygen Species) through activating anti-oxidative enzymes via elevation of Ca level in Arabidopsis CAXs (cation/proton exchangers) are involved in the sequestration of cations such as Mn, Li, and Cd, as well as Ca, from cytosol into the vacuole using proton gradients. In addition, it has been reported that CAX1, 2 and 4 are involved in Cd tolerance. Interestingly, it has been reported that CAX3 expressions were enhanced by Cd in Cd-tolerant transgenic plants expressing Hb1 (hemoglobin 1) or UBC1 (Ub-conjugating enzyme 1). Therefore, to investigate whether CAX3 plays a role in increasing Cd tolerance, CAX3 of Arabidopsis and tobacco were over-expressed in Arabidopsis thaliana. Compared to control plants, both transgenic plants displayed an increase in Cd tolerance, no change in Cd accumulation, and enhanced Ca levels. In support of these, AtCAX3-Arabidopsis showed no change in expressions of Cd transporters, but reduced expressions of Ca exporters and lower rate of Ca efflux. By contrast, atcax3 knockout Arabidopsis exhibited a reduced Cd tolerance, while the Cd level was not altered. The expression of Δ90-AtCAX3 (deletion of autoinhibitory domain) increased Cd and Ca tolerance in yeast, while AtCAX3 expression did not. Interestingly, less accumulation of ROS (H2O2 and O2-) was observed in CAX3-expressing transgenic plants and was accompanied with higher antioxidant enzyme activities (SOD, CAT, GR). Taken together, CAX3 over-expression may enhance Cd tolerance by decreasing Cd-induced ROS production by activating antioxidant enzymes and by intervening the positive feedback circuit between ROS generation and Cd-induced spikes of cytoplasmic Ca.

Tolerance to cadmium toxicity and phytoremediation potential of three Brassica rapa CAX1a TILLING mutants

Cadmium (Cd) is one of the most toxic heavy metals that reduces crop productivity and is a threat to all the food chain including human health. Phytoremediation is an environmentally friendly strategy to clean up soil contaminated with heavy metals. Researchers are selecting new varieties with an enhanced capacity for phytoremediation purposes. Three Brassica rapa mutants for CAX1 transporter were obtained through TILLING. The objective of this work is to evaluate the tolerance of these mutants to Cd toxicity and its potential for Cd phytoremediation. For this, the mutants and the parental R-o-18 were grown under control and Cd toxicity conditions (100 μM CdCl₂) and growth, Cd accumulation and physiological parameters were analyzed. The results show that BraA.cax1a mutation provides greater Cd uptake capacity although only BraA.cax1a-12 would be useful for phytoremediation because it registered more than three-fold the Cd content of R-o-18 and presented greater Cd tolerance. This tolerance could be due to the higher Ca and Mg accumulations, the maintaining of photosynthesis performance, the enhanced ROS detoxification and AsA/GSH and TCA cycles, the higher malate, and GA4 concentrations and the lower ethylene levels. Briefly, this study identifies BraA.cax1a-12 as a potential mutant for phytoremediation of Cd contaminated soil and identifies possible physiological elements that contribute to this capacity.

Characterization and Expression Analysis of the Ca2+/Cation Antiporter Gene Family in Tomatoes

The Ca²⁺/cation antiporter (CaCA) superfamily plays an important role in the regulation of the essential element Ca²⁺ and cation concentrations. Characterization and expression analyses of CaCA superfamily genes were performed in the tomato (Solanum lycopersicum) as a representative of dicotyledonous plants and fruit crops. Sixteen CaCA candidate genes were found and identified as tomato CaCA, SlCaCA, by a domain search. In a phylogenetic analysis of the SlCaCA superfamily, the 16 genes were classified into SlCAX, SlNCL, SlCCX, and SlMHX families. Among them, Solyc12g011070, belonging to the SlCAX family, had four splice variants, three of which were predicted to be nonfunctional because of a lack of important motifs. EF-hand domains were only found in SlNCL, in addition to consensus Na_Ca_ex domains, and the region containing EF-hand domains was characteristically long in some members of SlNCL. Furthermore, four genes of the SlCCX family were found to be intronless. As for intracellular localization, one SlCCX member was predicted to be localized to the plasma membrane, while other SlCCXs, SlCAXs, and SlMHXs were predicted to be localized to the vacuolar membrane. The expression patterns of SlCaCAs in various organs, including during several developmental stages of fruit, were classified into four groups. Genes involved in each of the SlCAX, SlNCL, and SlCCX gene families were categorized into three or four groups according to expression patterns, suggesting role sharing within each family. The main member in each subfamily and the members with characteristic fruit expression patterns included genes whose expression was regulated by sugar or auxin and that were highly expressed in a line having metabolite-rich fruit.

Effect of CAX1a TILLING mutations and calcium concentration on some primary metabolism processes in Brassica rapa plants

Cation/H⁺ exchanger transporters (CAXs) are crucial in Ca²⁺ homeostasis and in the generation of Ca²⁺ profiles involved in signalling processes. Given the crucial role of CAX1 in Ca²⁺ homeostasis, CAX1 modifications could have effects on plant metabolism. Three Brassica rapa mutants for CAX1 were obtained through TILLING. The aim of this work is to assess the effect of the different mutations and different Ca²⁺ doses on plant metabolism. For this, the mutants and the parental line were grown under low, control and high Ca²⁺ doses and parameters related to nitrogen (N) and tricarboxylic acid (TCA) metabolisms, and amino acid (AAs) and phytohormone profiles were measured. The results show that BraA.cax1a mutations affect metabolism especially under high Ca²⁺ dose. Thus, BraA.cax1a-7 inhibited some N metabolism enzymes and activated photorespiration activity. On the opposite side, BraA.cax1a-12 mutation provides a better tolerance to high Ca²⁺ dose. This tolerance could be provided by an improved N and TCA metabolisms enzymes, and a higher glutamate, malate, indole-3-acetic acid and abscisic acid concentrations. Therefore, BraA.cax1a-12 mutation could be used for B. rapa improving; the metabolomics changes observed in this mutant could be responsible for a better tolerance to high Ca²⁺.