Tomato PIF1a functions beyond ripening: evidence for a role in senescence

Phytochrome-Interacting Factors (PIFs) are a family of basic helix–loop–helix (bHLH) transcription factors that play different roles during plant development. In Arabidopsis thaliana, PIFs are involved in seedling deetiolation, shade avoidance, flowering, chloroplast development, elongation growth, or leaf senescence, among other light-regulated processes. However, little is known about the function of PIFs in other plants. Previous work in our lab showed that one member of the PIF family in tomato, PIF1a, is involved is the regulation of carotenoid accumulation during tomato fruit ripening. A careful examination of transgenic lines with a constitutively silenced PIF1a gene further suggested that this transcription factor might be involved in senescence. First, tomato fruit with reduced PIF1a activity did not show senescence symptoms when incubated in the dark. Second, transient expression experiments in Nicotiana benthamiana leaves confirmed that the PIF1a protein (but not other tomato PIF homologues) is able to promote senescence. In contrast with these results, the Arabidopsis pif1 mutant shows no senescence-associated phenotypes, whereas mutants for PIF3, PIF4, and PIF5 showed a delayed senescence of dark-incubated leaves. Experiments to address the molecular basis of such differences between Arabidopsis and tomato PIF homologues are in progress.