ABA-insensitivity of alfalfa (Medicago sativa L.) during seed germination associated with plant drought tolerance

Abscisic acid (ABA) is a vital stress hormone of plants in coping with adverse environmental conditions, such as drought stress. Sensitivity of seed germination to exogenous ABA treatment could be linked to varied drought stress responses in different plant species. Polyploid and outcrossing nature of alfalfa makes it an extensive reservoir of genetic variation, and each single alfalfa seed may have a different genetic background. We speculate the alfalfa seeds resistant to exogenous ABA treatment during germination could lead to a strong plant drought tolerance. Here, we selected the alfalfa seedlings (S0–50) from seeds germinated under 50 μM ABA treatment. The S0–50 alfalfa plants showed more sensitivity in stomatal closure to exogenous ABA and PEG treatments, and also stronger drought tolerance than the plants of ABA-sensitive seeds during germination (S0–0). Leaf ABA content analysis indicated that the S0–50 plants had a higher ABA content in normal and under drought stress growth conditions than the S0–0 plants. The seeds of the S0–50 plants (S1–50) showed significantly higher germination percentage under 50 μM ABA treatment and had longer roots after 15% PEG6000 treatment than the segregated ABA-sensitive seeds (S1–0). We found a cytosol-nucleus dual-localized PPR protein gene MsSOAR1 was significantly highly expressed in the S0–50 than in the S0–0 plants. Overexpression of AtSOAR1, a negative regulator in ABA-mediated seed germination inhibition in Arabidopsis and a homologous gene of MsSOAR1, led to significantly improved drought tolerance, increased branch number and plant height, and reduced expression of the ABA receptor genes MsPYL5 and MsPYL6 of AtSOAR1 transgenic alfalfa plants (TGs). The results suggest that ABA-insensitivity during seed germination is associated with the impaired ABA signaling transduction. Screening alfalfa seedlings during seed germination with exogenous ABA is a way to obtain drought tolerant alfalfa germplasm.