SlWHIRLY1 maintains leaf photosynthetic capacity in tomato by regulating the expression of SlRbcS1 under chilling stress.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which consists of eight large subunits (RBCL) and eight small subunits (RBCS), is a major photosynthetic enzyme that is sensitive to chilling stress. Howeverit is largely unclear how plants maintain high Rubisco content under low temperature conditions. Here, we report that tomato WHIRLY1 (SlWHY1) positively regulates the Rubisco level under chilling stress by directly binding to the promoter region of SlRbcS1, resulting in the activation of SlRbcS1 expression. SlRbcS1 overexpressing lines had higher Rubisco contents and were more resistant to chilling stress compared with WT. qRT-PCR analyses showed that, among the five RbcS genes, only SlRbcS1 expression is up-regulated by chilling treatment. These results indicate that SlWHIRLY1 specifically enhances the levels of SlRbcS1 and confers tolerance to chilling stress. Amino acid sequence of SlRBCS1 shows 92.67% identity with those of other two RBCS proteins and four residues (R57, R105, and NS113) are specifically found in SlRBCS1. However, mutation of these residues to alanine in SlRBCS1 does not influence its function during cold adaptation. Thus, we conclude that high levels of Rubisco, but not the specific residues in SlRBCS1, play important roles in tolerance to chilling stress in tomato.