SlKUF7 Regulates Fruit Chloroplast Development in Tomato by Affecting the Stability of SlGLK2

Fruit chloroplasts are crucial for the formation of fruit quality. However, the molecular regulatory network of fruit chloroplast development remains obscure. Here, we characterized a tomato homolog (SlKUF7) of KARRIKIN UP-REGULATED F-BOX 1. SlKUF7 is a nucleus-localized F-box protein with Kelch repeats. SlKUF7-overexpressing (OE-SlKUF7) tomato fruits exhibited a pale green appearance, while SlKUF7 knockout tomato fruits did not significantly change. Overexpression of SlKUF7 inhibited chlorophyll content and chloroplast ultrastructure formation in tomato fruits. Immunoprecipitation tandem mass spectrometry analysis revealed the GOLDEN2-LIKE transcription factor SlGLK2 as a potential substrate of SlKUF7. Interaction and protein stability assays showed that SlKUF7 interacted with SlGLK2, a master regulator of fruit chloroplast development in tomato, and mediated its degradation via the ubiquitin-proteasome pathway. Transcriptome profiling revealed that chlorophyll a/b-binding protein genes responsible for light harvesting were down-regulated in OE-SlKUF7 tomato fruits. Chromatin immunoprecipitation sequencing combined with RNA sequencing identified the transcription factor gene SlERF.D3 as a potential target of SlGLK2. SlGLK2 directly activated the expression of SlERF.D3 encoding a repressor for abscisic acid (ABA) accumulation. Furthermore, the content of ABA significantly increased in OE-SlKUF7 tomato fruits in comparison to the wild type. These results indicate that SlKUF7 may negatively regulate fruit chloroplast development in tomato by lowering SlGLK2 abundance and promoting SlERF.D3-mediated ABA accumulation. Our findings deepen the understanding of the molecular mechanisms underlying fruit chloroplast development in tomato.