Proline Accumulates in Response to Higher Temperatures during Dehardening in Peach Shoot Tissues

Abstract

Changes in cold hardiness, proline (Pro) content, and related gene expression were confirmed in the shoots of two peach cultivars (Prunus persica ‘Janghowon Hwangdo’ and ‘Odoroki’) during natural cold hardening (CH) and dehardening (DH), and 10 supplemental peach cultivars during experimental DH. Specifically, transcript accumulations of P5CS (Δ¹-pyrroline-5-carboxylate synthase) and P5CR (Δ¹-pyrroline-5-carboxylate reductase) were examined using quantitative real-time RT-PCR. The cold hardiness of the shoots of the two cultivars differed significantly during the entire experimental period. The cold hardiness of both cultivars increased gradually until December 2012, and then decreased until April 2013, whereas their Pro contents decreased from the beginning of the experiment until February 2013 and then increased in the spring. While the cold hardiness of the 10 peach cultivars decreased, their Pro contents increased during DH. Interestingly, the expression of the P5CS gene encoding an enzyme that catalyzes the conversion from glutamate (Glu) to glutamic-γ-semialdehyde (GSA) in the first step of the Pro pathway showed patterns contrasting with the Pro contents in both experiments. In contrast, the expression of the P5CR gene encoding an enzyme that catalyzes the conversion from Δ¹-pyrroline-5-carboxylate (P5C) to Pro in the final step of the Pro pathway showed patterns similar to the Pro contents in both experiments. Our results demonstrate that Pro accumulation responds positively to higher temperatures in the shoots of different peach cultivars, including the 10 supplemental peach cultivars, and the expression of both P5CS and P5CR genes showed contrasting patterns. Our results suggest that identification of the expression of both P5CS and P5CR genes is required for the accurate analysis of Pro biosynthesis because Pro accumulation is more affected by the expression of P5CR genes. Furthermore, notably, during the experimental DH, the expression of OAT (ornithine-δ-aminotransferase) increased. Our results suggest that the ornithine pathway could serve as an alternative pathway in the Pro synthesis process during DH in peach.