Several geranylgeranyl diphosphate synthase isoforms supply metabolic substrates for carotenoid biosynthesis in tomato

Abstract

Geranylgeranyl diphosphate (GGPP) produced by GGPP synthase (GGPPS) serves as a precursor for many plastidial isoprenoids, including carotenoids. Phytoene synthase (PSY) converts GGPP into phytoene, the first committed intermediate of the carotenoid pathway.Here we used biochemical, molecular, and genetic tools to characterise the plastidial members of the GGPPS family in tomato (Solanum lycopersicum) and their interaction with PSY isoforms.The three tomato GGPPS isoforms found to localise in plastids (SlG1, 2 and 3) exhibit similar kinetic parameters. Gene expression analyses showed a preferential association of individual GGPPS and PSY isoforms when carotenoid biosynthesis was induced during root mycorrhization, seedling de-etiolation and fruit ripening. SlG2, but not SlG3, physically interacts with PSY proteins. By contrast, CRISPR-Cas9 mutants defective in SlG3 showed a stronger impact on carotenoid levels and derived metabolic, physiological and developmental phenotypes compared with those impaired in SlG2. Double mutants defective in both genes could not be rescued.Our work demonstrates that the bulk of GGPP production in tomato chloroplasts and chromoplasts relies on two cooperating GGPPS paralogues, unlike other plant species such as Arabidopsis thaliana, rice or pepper, which produce their essential plastidial isoprenoids using a single GGPPS isoform.

The Theta Rhythm of the Hippocampus: From Neuronal and Circuit Mechanisms to Behavior

“…Stimulation of areas connected to the MS-DbB could elicit reset with higher frequencies than the spontaneous θ (Buño et al, 1978;García-Sánchez et al, 1978;Gaztelu and Buño, 1982;Núñez et al, 1990b), suggesting independent θ generator systems, which can produce different rhythms (Kramis et al, 1975). Electrical stimulation of structures projecting to MS-DbB tended to induce phase reset of the field θ and of the rhythmic bursting neuronal activity.…”

“…Indeed, evidence has accumulated suggesting that θ arises from interactions between MS-DbB and intra-hippocampal neuronal and circuital oscillators (Stumpf et al, 1962;McLennan and Miller, 1974;Stewart and Fox, 1989;Hangya et al, 2009;Huh et al, 2010;Müller and Remy, 2018). Strong support to the above interpretation was provided by lessions of the MS-DbB that blocked hippocampal θ and rhythmic neuronal activity (Green and Arduini, 1954;Buño et al, 1978;Mitchell et al, 1982). However, the rules of innervation of hippocampus by diverse MS-DbB neurons are unknown, and the contribution of hippocampal neuronal populations and circuits in the genesis of θ are a matter of debate.…”

“…The rhythmic MS neurons that fire both with or without field θ could play a leading role in triggering the rhythmic activity of other MS neurons leading to the generation of hippocampal θ. In addition to the MS-DbB neurons that fire rhythmic bursts, there were also non-rhythmic MS-DbB neurons (Buño et al, 1978;Gaztelu and Buño, 1982;Barrenechea et al, 1995;Pedemonte et al, 1998).…”

“…Initially, it was proposed that the combined effects of a background excitatory influence provided by cholinergic MS-DbB inputs interacting with a precisely timed rhythmic inhibition provided the main excitatory and inhibitory inputs required to generate θ (Andersen and Eccles, 1962;Petsche et al, 1968;Smythe et al, 1992;Desikan et al, 2018). Data in support of the cholinergic contribution to hippocampal θ are: (i) increased release of acetylcholine (ACh) during θ (Dudar, 1977); (ii) induction of θ by ACh and by cholinergic agonist MacVicar and Tse, 1989;Bland and Colom, 1993); (iii) production of θ by the increased cholinergic activity caused by blockade of ACh hydrolysis (Macadar et al, 1970;Roig et al, 1970;Buño et al, 1978;Gaztelu and Buño, 1982); and (iv) blockade of θ by specific cholinergic antagonists FIGURE 1 | Rhythmic medial septum-diagonal band of Broca (MS-DbB) neurons and theta (θ): in vivo recordings. (A1) Intracellular recordings of a bursting neuron with large spontaneous θ-like membrane potential oscillations (lower) in the absence of θ in the hippocampal EEG (upper).…”

“…Phase reset of θ and rhythmical hippocampal units can be induced by electrical stimulation of hippocampal afferents both in anesthetized and in behaving rats, and during rapid eye movement (REM) in sleep (García-Sánchez et al, 1978;Gaztelu and Buño, 1982;Lerma and García-Austt, 1985;Núñez et al, 1990b;Vinogradova, 1995;Givens, 1996;McCartney et al, 2004;Jackson et al, 2008). Importantly, reset was consistently evoked by stimulation of structures with rich contacts with the MS-DbB, but abolished by destruction of the MS and fornix, suggesting that it could be induced by reset of MS-DbB neurons triggered by input from connected structures (Buño et al, 1978;Brazhnik et al, 1985). An alternative possibility is that direct inhibition of MS neurons by synchronized hippocampal output originally elicited by septal inputs may be the reset mechanism.…”