Aquaporins involvement in the regulation of melon (Cucumis melo L.) fruit cracking under different nutrient (Ca, B and Zn) treatments

Melon cracking is a physiopathy that is associated with both internal and external changes by the alteration of the water balance and nutrient homeostasis in the fruit. Aquaporins are channels for water and other small solutes, and they are potentially involved in the regulation of melon cracking. In this work, we studied the mineral concentration and expression of all aquaporins in non-cracked and cracked melon pulp in control and after the application of foliar mineral treatments (Ca+B+Zn and B+Zn) in field conditions. Also, we measured the mineral transport of some aquaporins to connect it to cracking. The results showed that both treatments could ameliorate the incidence of cracking. Mineral elements determination showed increases in B, Ca, Si and Zn content in non-cracked Ca+B+Zn treated melons. In control conditions, only NIP2;2 and NIP5;1 had a significant increase in expression in cracked melons compared with non-cracked ones pointing to their involvement in cracking. Furthermore, we were able to verify that the high expression of PIP1;1, PIP1;2 and TIP1;1, which are efficient water channels, was involved in the changes observed in cracking incidence. Finally, transport assays in oocytes were performed with selected isoforms, highlighting the water channel activity of NIP2;2 and NIP5;1, the B channel activity of TIP1;3, TIP1;2, NIP1;1 and NIP2;2, and the Si channel activity of NIP2;2. In conclusion, both effective foliar treatments for avoiding cracking point to the PIP1;1, PIP1;2, TIP1;1 and NIP5;1 and NIP2;2 aquaporins as possible molecular markers.