The Role of Tubulin and Thaumatin Genes and Osmotic Factors in Salinity Tolerance of Tomato Plants

Abstract

Soil salinity is a drastic abiotic factor that affects many physiological processes and whole plants’ activities, as well as up- and down-regulating gene expression. Studying the effect of salinity on tubulin and thaumatin relative gene expression as DNA markers for salinity stress in tomato plants is a scarcely studied topic. Tubulin regulates and plays an important role in the immunolocalization of xylem and phloem fibers in stems and additionally maintains the concept of heavy microtubule contribution during cellulose microfibril confession in secondary cell walls under abiotic and biotic stresses. Like tubulin, thaumatin-like proteins are concomitant with plant defense responses against both biotic and abiotic stresses. The expression of the thaumatin gene can be meaningfully induced after plants' exposure to either drought, freezing, or salinity stresses. Thus, the present investigation was conducted to study the impact of different salinity levels (0, 75, 100, and 120 mM NaCl) on the tomato plants’ growth, osmotic adjustment, and relative gene expression of both antioxidant and salt tolerance genes. As the salt concentration intensified, the fresh and dry weight of the shoots and the roots reduced significantly, accompanied by a reduction in chlorophyll a and carotenoids. On the other hand, salinity stress significantly decreased the level of osmotica (e.g., soluble sugars and soluble proteins) in tomato tissues compared to non-saline-grown plants, while a significant accumulation of free amino acids was recorded. At the molecular level, it was observed that the relative expression of the polyphenol oxidase, peroxidase, and thaumatin genes was high at the level of 100 mM NaCl, but it was suppressed at 120 mM NaCl. In contrast, salinity down-regulated tubulin gene expression in stressed tomato plants relative to controls, revealing various mechanisms that instigated salinity tolerance, which is concentration-dependent. The study recommended the importance of amino acids as osmotica as well as the relative expressions of PPO, peroxidase, and thaumatin genes in conferring salt tolerance at low to moderate salt levels.