A heat treatment induced the expression of a Na +/H + antiport gene ( cNHX1) in citrus fruit

Abstract

In previous studies, we showed that a short hot water treatment (at 62 °C for 20 s) increased chilling tolerance in grapefruit. In order to gain more insight into the molecular mechanisms involved in heat-induced chilling tolerance responses, we performed cDNA differential display analysis and isolated a 600-bp polymerase chain reaction (PCR) fragment whose expression was enhanced 24 h after the heat treatment, and that showed high sequence homology with the 5′ region of various NHX1 vacuolar Na +/H + antiporter genes. The full-length cDNA sequence of cNHX1 (for citrus NHX1) was further isolated by 3′-RACE; it had a total length of 2135 bp with an open reading frame of 1626 bp, and encoded a predicted polypetide of 542 amino acids with a molecular mass of 59.8 kDa. The cNHX1 protein shares 83, 78 and 56% identity with the other recently isolated Arabidopsis thaliana, Atriplex gmelini and Oryza sativa NHX1 proteins, respectively. It has been reported that the Arabidopsis AtNHX1 gene partially complemented a salt-sensitive yeast mutation and that overexpression of this gene significantly increased salt tolerance in transgenic plants. In the current study, we showed that the expression of the cNHX1 gene was temporarily induced by a heat treatment, and that its mRNA levels markedly increase during a 6-week cold storage period at 2 °C in heat-treated fruit but not in control untreated fruit, thus suggesting its possible involvement in heat-induced chilling tolerance responses. The expression of the cNHX1 gene was also markedly induced by salt stress, but not by other stresses, such as exposure to ethylene or UV irradiation.