Functional analysis of tomato calmodulin gene family during fruit development and ripening

Calmodulin is a ubiquitous calcium sensor to recognize the different developmental and/or stimulus-triggered calcium changes and regulate plant growth and development. However, the function of calmodulin remains elusive for fleshy fruit development. We performed expression studies of a family of six calmodulin genes (SlCaMs) in tomato fruit. All calmodulins showed a double peak expression pattern. The first flat peak appeared at 10-30 days after anthesis, but their expression rapidly declined at mature green and breaker. Then a sharp and even higher peak came at turning/pink stages. Among six calmodulins, SlCaM1 had the highest expression during fruit enlargement, whereas SlCaM2 was the major calmodulin during fruit ripening. However, SlCaMs showed different patterns in three ripening mutants rin, Nor and Nr. In particular, at the stages corresponding to mature green and breaker, the expression levels of SlCaMs in those mutants were significantly higher than wild-type. Furthermore, SlCaMs, especially SlCaM2 were upregulated by ethylene. Transiently overexpressing SlCaM2 in mature green fruit delayed ripening, while reducing SlCaM2 expression accelerated ripening. Our results suggest that SlCaMs play double roles to regulate fruit ripening. Prior to the ethylene burst, the ethylene-independent repression of SlCaMs might be critical for fruit to initiate the ripening process. After the ethylene burst, SlCaMs could participate in the ethylene coordinated rapid ripening.