Upon fertilization, the ovary increases in size and undergoes a complex developmental process to become a fruit. We show that cytokinins (CKs), which are required to determine ovary size before fertilization, have to be degraded to facilitate fruit growth. The expression of CKX7, which encodes a cytosolic CK-degrading enzyme, is directly positively regulated post-fertilization by the MADS-box transcription factor STK. Similar to stk, two ckx7 mutants possess shorter fruits than wild type. Quantification of CKs reveals that stk and ckx7 mutants have high CK levels, which negatively control cell expansion during fruit development, compromising fruit growth. Overexpression of CKX7 partially complements the stk fruit phenotype, confirming a role for CK degradation in fruit development. Finally, we show that STK is required for the expression of FUL, which is essential for valve elongation. Overall, we provide insights into the link between CKs and molecular pathways that control fruit growth.

Dipartimento di BioScienze Università degli Studi di Milano Milan Milan 20133 Italy

Dipartimento di BioScienze Università degli Studi di Milano Milan Milan 20133 Italy; Instituto de Biologia Molecular y Celular de Plantas Consejo Superior de Investigacione Cientificas Universidad Politecnica de Valencia Valencia Valencia 46022 Spain

Laboratory of Growth Regulators Faculty of Science of Palacký University and Institute of Experimental Botany of the Czech Academy of Sciences Olomouc Olomouc 78371 Czech Republic

Unidad de Genómica Avanzada Irapuato Guanajuato 36824 México

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Impairment of root auxin-cytokinins homeostasis induces collapse of incompatible melon grafts during fruit ripening