The extensin protein SAE1 plays a role in leaf senescence and is targeted by the ubiquitin ligase SlSINA4 in tomato.

Extensins are hydroxyproline-rich glycoproteins (HRGPs) and generally play a structural role in cell wall integrity. In this study, we determined a novel role of tomato (Solanum lycopersicum) senescence-associated extensin1 (SAE1) in leaf senescence. Both gain-of-function and loss-of-function analyses suggest that SAE1 plays a positive role in leaf senescence in tomato. Transgenic tomato plants overexpressing the SAE1 gene (SAE1-OX) exhibited premature leaf senescence and enhanced dark-induced senescence, whereas SAE1 knock-out (SAE1-KO) plants displayed delayed development-dependent or dark-induced leaf senescence. Heterologous overexpression of SAE1 in Arabidopsis also led to premature leaf senescence and enhanced dark-induced senescence. In addition, the SAE1 protein interacted with the tomato ubiquitin ligase SlSINA4 and SlSINA4 promoted SAE1 degradation in a ligase-dependent manner when co-expressed in Nicotiana benthamiana leaves, suggesting SlSINA4 controls SAE1 protein levels via the ubiquitin-proteasome pathway (UPS). Consistently, introduction of the SlSINA4 overexpression construct into the SAE1-OX tomato completely eliminated accumulation of SAE1 protein and suppressed the phenotypes conferred by overexpression of SAE1. Taken together, our data suggest that the tomato extensin SAE1plays a positive role in leaf senescence and is regulated by the ubiquitin ligase SlSINA4.