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

Ryan Stanfield,Joan Laur

doi:10.3390/cells8030202

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.

Highlights

The phloem vascular tissue is the predominant passageway for photosynthetically derived nutrients to be propagated around the body of the plant
We found there to be an increased signal of PIP2 aquaporins in the sieve tube membranes after 2 min of chilling
The results of this study provide a mechanism for pressure release following cold application

Summary

Introduction

The phloem vascular tissue is the predominant passageway for photosynthetically derived nutrients to be propagated around the body of the plant. Within the phloem lies the sieve tube conduit responsible for the transport of sugars, and for the transmission of signals in the form of mRNA [1], amino acids [2] or electrical action potentials [3]. Previous research on sugar translocation is often focused on loading near source tissue (e.g., leaves) [7,8,9]. The intervening transport phloem which connects source and sink is often neglected due to the difficulty of accessing this deeply embedded tissue. The importance of the transport phloem for influencing long-distance translocation cannot be understated, as it may act as an exchange point for water and nutrients [11], which has been modeled to significantly influence pressure profiles [12]

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