Low Night Temperature Affects the Phloem Ultrastructure of Lateral Branches and Raffinose Family Oligosaccharide (RFO) Accumulation in RFO-Transporting Plant Melon (Cucumismelo L.) during Fruit Expansion.

Jinghong Hao,Liping Wang,Shaowei Lu,Cory J Xian,Weizhi Chen,Shuangxi Fan,Yifei Liu,Fengying Gu,Jie Zhu,Yunfei Li,Wagner L Araujo

doi:10.1371/journal.pone.0160909

Abstract

Due to the importance and complexity of photo assimilate transport in raffinose family oligosaccharide (RFO)-transporting plants such as melon, it is important to study the features of the transport structure (phloem) particularly of the lateral branches connecting the source leaves and the sink fruits, and its responses to environmental challenges. Currently, it is unclear to what extents the cold environmental temperature stress would alter the phloem ultrastructure and RFO accumulation in RFO-transporting plants. In this study, we firstly utilized electron microscopy to investigate the changes in the phloem ultrastructure of lateral branches and RFO accumulation in melons after being subjected to low night temperatures (12°C and 9°C). The results demonstrated that exposure to 9°C and 12°C altered the ultrastructure of the phloem, with the effect of 9°C being more obvious. The most obvious change was the appearance of plasma membrane invaginations in 99% companion cells and intermediary cells. In addition, phloem parenchyma cells contained chloroplasts with increased amounts of starch grains, sparse cytoplasm and reduced numbers of mitochondria. In the intermediary cells, the volume of cytoplasm was reduced by 50%, and the central vacuole was present. Moreover, the treatment at 9°C during the night led to RFO accumulation in the vascular bundles of the lateral branches and fruit carpopodiums. These ultrastructural changes of the transport structure (phloem) following the treatment at 9°C represented adaptive responses of melons to low temperature stresses. Future studies are required to examine whether these responses may affect phloem transport.

Highlights

Phloem is responsible for the transport of photoassimilates, such as carbohydrates, amino acids, and other nutrients from source tissues such as mature leaves to sink tissues such as fruits, shoots and root apical tissues, and developing organs [1,2,3]
The phloem in lateral branches of melons was mainly composed of sieve elements (SE), companion cells (CC), intermediary cells (IC) and phloem parenchyma cells (PP) (Fig 2A)
The raffinose family oligosaccharide (RFO) content in carpopodiums was not significantly affected by the low night temperature treatment at 12°C. These results indicated that low night temperature treatment at 9°C led to a large accumulation of RFO in carpopodiums, whereas the 12°C treatment had no such adverse effects

Summary

Introduction

Phloem is responsible for the transport of photoassimilates, such as carbohydrates, amino acids, and other nutrients from source tissues such as mature leaves to sink tissues such as fruits, shoots and root apical tissues, and developing organs [1,2,3]. Active retrieval and the release of solutes (lateral transport) occur through carriers of the sieve element (SE) plasma membrane along the transport phloem [6,7]. By this way, the transport phloem provides the surrounding tissues with assimilates for growth, and it is plausible that changes in the phloem ultrastructure of the lateral branches may bring about differences in the metabolic energy for the long-distance transport

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