Comparative Physiological and Transcriptomic Analyses of Oat (Avena sativa) Seedlings under Salt Stress Reveal Salt Tolerance Mechanisms

Soil salinity is a major abiotic stress limiting crop production globally. Oat (<i>Avena sativa</i>) is an annual cereal with a strong salt tolerance, a high yield, and nutritional quality, although the mechanisms underlying its salt stress response remain largely unknown. We examined the physiological and transcriptomic responses of <i>A. sativa</i> seedlings to salt stress in tolerant cultivar Qingyongjiu 195 and sensitive cultivar 709. Under salt stress, Qingyongjiu 195 maintained a higher photosynthetic efficiency, antioxidant enzymes activity, and leaf K⁺ accumulation but a lower Na⁺ uptake than 709. RNA-seq revealed 6616 differentially expressed genes (DEGs), including 4265 up- and 2351 downregulated. These were enriched in pathways like plant-pathogen interaction, phenylpropanoid biosynthesis, and MAPK signaling. We specifically highlight DEGs involved in photosynthesis (<i>chlG</i>, <i>CP47 psbB</i>, <i>COX2</i>, <i>LHCB</i>) and antioxidants (<i>trxA</i>, <i>GroES</i>). Qingyongjiu 195 also appeared to enhance K⁺ uptake via <i>KAT1</i> and <i>AKT2</i> and sequester Na⁺ in vacuoles via <i>NHX2</i>. Additionally, <i>HKT</i> restricted Na⁺ while promoting K⁺ transport to shoots, maintaining K⁺/Na⁺. The expression levels of <i>CAX</i>, <i>ACA</i>, <i>CML</i>, <i>CaM</i>, and <i>CDPK</i> in Qingyongjiu 195 were higher than those in 709. Oats regulated Ca²⁺ concentration through CAX and ACA after salt stress, decoded Ca²⁺ signals through CML, and then transferred Ca²⁺ signals to downstream receptors through the Ca²⁺ sensors CaM and CDPK, thereby activating K⁺/Na⁺ transporters, such as SOS1 and NHX, etc. Our results shed light on plant salt stress response mechanisms and provide transcriptomic resources for molecular breeding in improving salt tolerance in oats.