Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxin-dependent manner

RESEARCH ARTICLE Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxin-dependent manner Alessandra Cavalcanti Duarte Lupi1☯, Bruno Silvestre Lira1☯, Giovanna Gramegna1, Bruna Trench1, Frederico Rocha Rodrigues Alves1, Diego Demarco ID1, Lazaro Eustáquio Pereira Peres2, Eduardo Purgatto3, Luciano Freschi1, Magdalena Rossi ID1* a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 Departamento de Botânica, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil, 2 Departamento de Ciências Biológicas, Escola Superior de Agricultura ¨Luiz de Queiroz¨, Universidade de São Paulo, Piracicaba, SP, Brazil, 3 Departamento de Alimentos e Nutrição Experimental, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, São Paulo, SP, Brazil ☯ These authors contributed equally to this work. * Abstract OPEN ACCESS Citation: Lupi ACD, Lira BS, Gramegna G, Trench B, Alves FRR, Demarco D, et al. (2019) Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxindependent manner. PLoS ONE 14(2): e0212224. https://doi.org/10.1371/journal.pone.0212224 Editor: Hiroshi Ezura, University of Tsukuba, JAPAN Received: October 5, 2018 Accepted: January 29, 2019 Published: February 12, 2019 Copyright: © 2019 Lupi et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the manuscript and its Supporting Information files. Funding: B.S.L., G.G., F.R.R.A. and B.T. were recipients of FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) fellowships. M. R. and D.D. were funded by a fellowship from CNPq (Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico). A.C.D.L. was recipient of a CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nı́vel Superior) fellowship. This work Plastids are organelles responsible for essential aspects of plant development, including carbon fixation and synthesis of several secondary metabolites. Chloroplast differentiation and activity are highly regulated by light, and several proteins involved in these processes have been characterised. Such is the case of the GOLDEN 2-LIKE (GLK) transcription factors, which induces the expression of genes related to chloroplast differentiation and photosynthesis. The tomato (Solanum lycopersicum) genome harbours two copies of this gene, SlGLK1 and SlGLK2, each with distinct expression patterns. While the former predominates in leaves, the latter is mainly expressed in fruits, precisely at the pedicel region. During tomato domestication, the selection of fruits with uniform ripening fixed the mutation Slglk2, nowadays present in most cultivated varieties, what penalised fruit metabolic composition. In this study, we investigated how SlGLK2 is regulated by light, auxin and cytokinin and determined the effect of SlGLK2 on tocopherol (vitamin E) and sugar metabolism, which are components of the fruit nutritional and industrial quality. To achieve this, transcriptional profiling and biochemical analysis were performed throughout fruit development and ripening from SlGLK2, Slglk2, SlGLK2-overexpressing genotypes, as well as from phytochrome and hormonal deficient mutants. The results revealed that SlGLK2 expression is regulated by phytochrome-mediated light perception, yet this gene can induce chloroplast differentiation even in a phytochrome-independent manner. Moreover, auxin was found to be a negative regulator of SlGLK2 expression, while SlGLK2 enhances cytokinin responsiveness. Additionally, SlGLK2 enhanced chlorophyll content in immature green fruits, leading to an increment in tocopherol level in ripe fruits. Finally, SlGLK2 overexpression resulted in higher total soluble solid content, possibly by the regulation of sugar metabolism enzyme-encoding genes. The results obtained here shed light on the regulatory network that interconnects SlGLK2, phytohormones and light signal, promoting the plastidial activity and consequently, influencing the quality of tomato fruit. PLOS ONE | https://doi.org/10.1371/journal.pone.0212224 February 12, 2019 1 / 22 SlGLK2 effect on fruit quality was partially supported by grants from FAPESP (Brazil; 2016/01128-9, 2014/10651-1), CNPq (Brazil), CAPES (Finance Code 001, Brazil) and USP (Universidade de São Paulo, Brazil). Competing interests: The authors have declared that no competing interests exist. Introduction Plastids are organelles with a great diversity of shapes and functions that are found in all photosynthetic eukaryotes. Besides photosynthesis, these organelles are responsible for diverse functions, such as synthesis and storage of some secondary metabolites [1]. Proplastids are found in meristematic regions of the plant and are the precursor of the other plastid types, whose differentiation is tightly regulated by endogenous and exogenous signals. Several hormones have been described to play a role in the control of chloroplast biogenesis, such as auxins and cytokinins. While the first is known to have an inhibitory role in chloroplast differentiation in roots, the latter stimulates this process [2, 3, 4]. In contrast, in tomato (Solanum lycopersicum) fruits, auxins promote chloroplast biogenesis, as evidenced by the increment in the abundance of fully developed chloroplasts when the auxin response repressor SlARF4 (AUXIN RESPONSIVE FACTOR 4) is downregulated [5, 6]. Among the exogenous signals, light is paramount as it ensures that chloroplast differentiation only occurs in appropriate conditions for photosynthetic activity. The red/far-red photoreceptors PHYTOCHROMES (PHYs) play a significant role in this process. In the darkness, PHYs are inactive in the cytoplasm, and upon red light exposure an isomeric alteration of the chromophore leads to a rearrangement of the apoprotein structure that exposes the nuclear signalling domain, leading to their translocation into the nucleus [7]. In the nucleus, PHYs promote the degradation of PHYTOCHROME INTERACTING FACTORS (PIFs), which repress, among other positive regulators of photomorphogenesis, the expression of ELONGATED HYPOCOTYL 5 (HY5) and GOLDEN 2-LIKE (GLKs) transcription factors [8]. GLKs are essential for chloroplast differentiation and maintenance [9, 10] and in Arabidopsis thaliana are positively regulated by HY5 [11]. As in most plants, tomato genome harbours two GLK copies, SlGLK1 and SlGLK2. While SlGLK1 is mostly expressed in cotyledons, sepals and leaves, SlGLK2 is predominantly expressed in fruits, more specifically at the pedicellar portion, originating the so-called green shoulder phenotype [12]. This phenotype was lost along tomato domestication by the fixation of a non-functional truncated SlGLK2 coding allele (Slglk2), resulting in a (...truncated)