Aquaporins Respond to Chilling in the Phloem by Altering Protein and mRNA Expression

Abstract

Previous experiments using heat exchangers (liquid cooled blocks) to chill a portion of plant stem have shown a transient stoppage in phloem translocation and an increase in measured phloem pressure. Although a chilled-induced stoppage of phloem transport has been known for over 100 years, the mechanism of this phenomenon is still poorly understood. Recently, work has highlighted that aquaporins occur within the plasma membrane of the sieve tubes along the entire source-to-sink pathway, and that isoforms of these water channel proteins may change dynamically. Aquaporins show regulatory roles in controlling tissue and cellular water status in response to environmental hardships. Thus, we tested if protein localization and mRNA transcript abundance changes occur in response to chilling in balsam poplar (Populus balsamifera) using immunohistochemistry and qrtPCR. The results of the immunolocalization experiments show that the labeling intensity of the sieve elements treated for only 2 min of chill time significantly increased for PIP2. After 10 min of chilling, this signal declined significantly to lower than that of the pre-chilled sieve elements. Overall, the abundance of mRNA transcript increased for the tested PIP2s following cold application. We discuss the implication that aquaporins are responsible for the alleviation of sieve tube pressure and the resumption of flow following a cold-induced blockage event.

Exploring aquaporin functions during changes in leaf water potential

“…Aquaporin protein abundance does not always scale with mRNA levels ( Buccitelli and Selbach, 2020 ). The regulation of aquaporin mRNA transcript levels and the abundance of the proteins they encode has sometimes been observed to be linked, but they have also been observed to be regulated independently ( Marulanda et al., 2010 ; Muries et al., 2011 ; Stanfield and Laur, 2019 ; Jiménez et al., 2022 ). This means there is not necessarily a consistent relationship between transcript level changes and cell membrane aquaporin protein abundance changes ( Fox et al., 2017 ).…”