Salinity tolerance is an important physiological index for crop breeding.  Roots are typically the first plant tissue to withstand salt stress.  In this study, we found that the tomato (Solanum lycopersicum) trehalose-6-phosphate phosphatase (SlTPP4) gene is induced by abscisic acid (ABA) and salt, and is mainly expressed in roots.  Overexpression of SlTPP4 in tomato enhanced tolerance to salt stress, resulting in better growth performance.  Under saline conditions, SlTPP4 overexpression plants demonstrated enhanced sucrose metabolism, as well as increased expression of genes related to salt tolerance.  At the same time, expression of genes related to ABA biosynthesis and signal transduction was enhanced or altered, respectively.  In-depth exploration demonstrated that SlTPP4 enhances Casparian band development in roots to restrict the intake of Na⁺.  Our study thus clarifies the mechanism of SlTPP4-mediated salt tolerance, which will be of great importance for the breeding of salt-tolerant tomato crops.

The tetracycline (Tet)-off gene expression regulation system based on the TetR-VP16/Top10 construct has not been widely utilized in plants, for its highly expressed TetR-VP16 activator is toxic to some plants and repeatedly replenishing tetracycline to turn off the constitutively active system is a tedious process.  To solve these problems, a Tet-off and heat shock (HS)-on gene expression regulation system was constructed in this study.  This system is composed of a chimeric transactivator gene TetR-HSF that is derived from a Tet repressor (TetR) and a HS transcription factor (HSF) controlled by a HS promoter HSP70m, and a Tet operator containing hybrid promoter, Om35S, that drives expression of the β-glucuronidase (GUS) gene.  The resultant system yields a GUS expression pattern similar to that of the HSP70m promoter under inducing temperatures and at 35 and 40°C drives GUS expression to a similar level as the Cauliflower mosaic virus (CaMV) 35S promoter.  Further examination revealed that the TetR-HSF and GUS genes were induced by HS, reaching peak expression after 1 and 6 h treatment, respectively, and the HS induction of the expression system could be inhibited by Tet.  This system will provide a useful tool for transgenic studies of plants in the laboratory and in the field, including transgene function analysis, agronomic trait improvement, biopharmaceutical protein production and others.