Calcium signaling triggers early high humidity responses in <i>Arabidopsis thaliana</i>

Plants need to adapt to fluctuating atmospheric humidity and respond to both high and low humidity. Despite our substantial understanding of plant responses to low humidity, molecular mechanisms underlying the high humidity (HH) response are much less well understood. In this study, we investigated early responses to HH in <i>Arabidopsis</i>. Expression of <i>CYP707A3</i>, encoding an abscisic acid (ABA) 8'-hydroxylase, is induced by HH within 10 min, which leads to a decrease in foliar ABA level. We identified that the combined action of CAMTA3 and CAMTA2 transcription factors regulate this response. This regulation requires a calmodulin (CaM)-binding domain of CAMTA3. Transcriptomes of HH-regulated genes are enriched in those related to calcium signaling, including cyclic nucleotide-gated ion channels (CNGCs). Moreover, HH induces CNGC2- and CNGC4-mediated increases in cytosolic Ca²⁺ concentrations in leaves within a few minutes. We also found that CNGC2, CNGC4, and CAMTAs participate in HH-induced hyponastic movement of petioles. Taken together, our results indicate that CNGC2/CNGC4-Ca²⁺-CaM-CAMTA3/CAMTA2 acts as a primary regulatory module to trigger downstream HH responses.