Study of salt-stress tolerance and defensive mechanisms in Brassica rapa CAX1a TILLING mutants

Crop production is facing the increase of salinity in soils and irrigation waters. This is a widespread problem that affects crops with high economic importance and reduces yields. Salinity causes oxidative stress in plants which induce complex defensive mechanisms involving the antioxidant system and osmoprotector compounds. Ca2+ plays a pivotal role in counteracting salt stress, and the modification of the Ca2+ vacuolar transporter CAX1 could represent a potential method to improve tolerance to salinity. Three new CAX1 variants in Brassica rapa (BraA.cax1a) were generated using TILLING strategy. The objective of this work is to evaluate the tolerance of these mutants to saline conditions. For this, BraA.cax1a mutants and the parental line R-o-18 were grown in pots under control and saline conditions (150 mM NaCl). Parameters of biomass, ions accumulation, antioxidant compounds concentration and antioxidant enzyme activities, heat shock protein 70 (HSP70) occurrence, and proline (Pro) levels were measured. According to the results, BraA.cax1a-4 mutation provided a higher tolerance to salinity stress. Thus, BraA.cax1a-4 plants showed higher biomass; higher Ca2+ and K+ accumulation in the shoot, a lower Na+/K+ ratio, an enhanced ROS scavenging (increased superoxide dismutase and catalase activities) and an enhanced ascorbate redox state. BraA.cax1a-4 and BraA.cax1a-12 showed higher K+ transport to the shoot which could contribute to their lower lipid peroxidation. In contrast, BraA.cax1a-7 showed a lower antioxidant response. Besides, an increased occurrence of cytosolic and chloroplastic HSP70 isoforms and Pro levels could contribute to protect these B. rapa mutants from saline stress. Therefore, this study identifies a potential useful genotype that could be used to enhance salt tolerance of crops