CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 6: A Key Regulator of Na⁺ Homeostasis during Germination

Abstract

Salinity impairs seed germination and seedling establishment. We investigated the role of Arabidopsis (Arabidopsis thaliana) CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 6 (CAMTA6) in salinity stress responses during early germination. Compared with the wild type, the camta6-4 and camta6-5 mutants were more tolerant to NaCl and abscisic acid (ABA) and accumulated less Na +. In contrast, 4-to 11-d-old camta6 seedlings were more sensitive to NaCl. In camta6, expression of HIGH-AFFINITY K + TRANSPORTER1 (AtHKT1;1), encoding an Na + /K + transporter, was restricted to the radicles and was not enhanced by NaCl or ABA. During germination, the camta6 hkt1 double mutant was as sensitive as the wild type and hkt1 to NaCl, suggesting that HKT1;1 is crucial for the salt tolerance of camta6. An ABA response element in the HKT1;1 promoter was found to be indispensable for the enhanced expression of the gene in response to NaCl and to ABA. Transcriptome analysis of the wild type and camta6-5 with and without salt treatment revealed 1,020 up-regulated and 1,467 down-regulated saltresponsive genes in the wild type. Among these, 638 up-regulated and 1,242 down-regulated genes were classified as CAMTA6-dependent. Expression of several known salt stress-associated genes, including SALT OVERLY SENSITIVE1 and Na + /H + ANTIPORTER, was impaired in camta6 mutants. Bioinformatics analysis of the 59 upstream sequences of the saltresponsive CAMTA6-dependent up-regulated genes revealed the CACGTGTC motif as the most prominent element, representing an ABA response element and a potential CAMTA-binding site. We suggest that CAMTA6 regulates, directly or indirectly, the expression of most of the salt-responsive genes in germinating seeds, including genes that are crucial for Na + homeostasis and salt stress tolerance.

Calmodulin-binding transcription activator (CAMTA) genes family: Genome-wide survey and phylogenetic analysis in flax (Linum usitatissimum)

“…For instance, AtCAMTA1 and AtCAMTA3 play roles against freezing and drought stress as well as regulation of auxin and salicylic acid in plants [31][32][33][34][35]. AtCAMTA6 support the plants against salt stress, while AtCAMTA4 performs pivotal role in defense responses of plants against Puccinia triticina and low temperature stress [36,37].…”

Changing Gears in Salt Stress Tolerance: AtCAMTA6’s Role in Transcriptional Regulation of Ion Transport

“…This increase in germination could be explained by reduced sodium uptake by the germinating seed, observed for two independent camta6 alleles. Interestingly, Shkolnik et al (2019) observed that decreased sodium accumulation corresponded to the repressed expression of high-affinity K + transporter (AtHKT1) known to import sodium ions into the root pericycle from the vasculature, thereby limiting sodium accumulation in the shoot (Mäser et al, 2002;Møller et al, 2009). The levels of AtHKT1 were not induced in response to salt stress in camta6 lines, and AtHKT1 expression was also limited to the radicle and excluded from developing cotyledons.…”

“…Another exciting aspect of the work described by Shkolnik et al (2019) is that although camta6 seedlings outperformed wild-type lines during germination under salt stress, reduced salt stress tolerance was observed for the mutants at later developmental stages. This phenomenon could be explained by camta6 lines' inability to induce AtHKT1 expression.…”

Transcriptional Control Of Calmodulin By CAMTA Regulates Neural Excitability

“…Calmodulin-binding transcription activators (CAMTAs) are a highly conserved family of Ca 2+ -regulated transcription factors 1 . In plants, CAMTAs are transcriptional effectors of Ca 2+ /CaM signaling in response to biotic/abiotic stress [2][3][4][5][6] . Mammals encode two CAMTA proteins that are enriched in heart and brain tissue 7 .…”