Molecular and functional characterization of SISPL-CNR in tomato fruit ripening and cell death

Lai, T., Wang, X., Ye, B., Jin, M., Chen, W., Wang, Y., Zhou, Y., Blank, A.M., Gu, M., Zhang, P., Zhang, X., Li, C., Wang, H., Liu, Y., Gallusci, P., Tör, M. ORCID: https://orcid.org/0000-0002-4416-5048 and Hong, Yiguo (2020) Molecular and functional characterization of SISPL-CNR in tomato fruit ripening and cell death. Journal of Experimental Botany, 71 (10). pp. 2995-3011. ISSN Online ISSN 1460-2431 - Print ISSN 0022-0957

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Abstract

SlSPL-CNR, an SBP-box transcription factor (TF) gene residing at the epimutant Colourless non-ripening (Cnr) locus, is involved in tomato ripening. This epimutant provides a unique model to investigate the (epi)genetic basis of fruit ripening. Here we report that SlSPL-CNR is a nucleus-localized protein with a distinct monopartite nuclear localization signal (NLS). It consists of four consecutive residues ‘30KRKR33’ at the N-terminal of the protein. Mutation of the NLS abolishes SlSPL-CNR to localize into nucleus. SlSPL-CNR comprises two zinc-finger motifs (ZFMs) within the C-terminal SBP-box domain. Both ZFMs contribute to zinc-binding activity. SlSPL-CNR can induce cell death in tomato and tobacco. Induction of cell death by SlSPL-CNR is dependent on its nuclear localization. However, the two ZFMs have differential impacts on SlSPL-CNR to induce severe necrosis or mild necrotic ringspot. NLS and ZFM mutants cannot complement Cnr fruits to ripen. SlSPL-CNR interacts with SlSnRK1. Virus-induced SlSnRK1 silencing leads to reduction in expression of ripening-related genes and inhibits ripening in tomato. We conclude that SlSPL-CNR is a multifunctional protein that consists of a distinct monopartite NLS, binds to zinc and interacts with SlSnRK1 to affect cell death and tomato fruit ripening.

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