Expression Profiles of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxylase Kinase Genes in Phalaenopsis, Implications for Regulating the Performance of Crassulacean Acid Metabolism.

Huey-Ling Lin,Teen-Chi Cheng,Fure-Chyi Chen,Yung-I Lee,Chia-Yun Ping,Tzong-Shyan Lin,Wen-Ju Yang

doi:10.3389/fpls.2018.01587

Abstract

Phalaenopsis is one of the most important potted plants in the ornamental market of the world. Previous reports implied that crassulacean acid metabolism (CAM) orchids at their young seedling stages might perform C3 or weak CAM photosynthetic pathways, but the detailed molecular evidence is still lacking. In this study, we used a key species in white Phalaenopsis breeding line, Phalaenopsis aphrodite subsp. formosana, to study the ontogenetical changes of CAM performance in Phalaenopsis. Based on the investigations of rhythms of day/night CO2 exchange, malate contents and phosphoenolpyruvate carboxylase (PEPC) activities, it is suggested that a progressive shift from C3 to CAM occurred as the protocorms differentiated the first leaf. To understand the role of phosphoenolpyruvate carboxylase kinase (PEPC kinase) in relation to its target PEPC in CAM performance in Phalaenopsis, the expression profiles of the genes encoding PEPC (PPC) and PEPC kinase (PPCK) were measured in different developmental stages. In Phalaenopsis, two PPC isogenes were constitutively expressed over a 24-h cycle similar to the housekeeping genes in all stages, whereas the significant day/night difference in PaPPCK expression corresponds to the day/night fluctuations in PEPC activity and malate level. These results suggest that the PaPPCK gene product is most likely involved in regulation of CAM performance in different developmental stages of Phalaenopsis seedlings.

Highlights

Phalaenopsis, an epiphytic orchid, is regarded as an obligate crassulacean acid metabolism (CAM) plant because of its succulent leaf with large and highly vacuolated mesophyll cells
To elucidate the progression from C3 to CAM photosynthetic pathway in Phalaenopsis, we investigated the expression of PPC and PEPC kinase (PPCK) genes, the profiles of day/night CO2 exchange, malate contents and the phosphoenolpyruvate carboxylase (PEPC) activity of different developmental stages
Differences between day/night malate and PEPC activity was first observed at stage 3 and the breadth of day/night fluctuation enlarged as the seedlings developed (Figure 3)

Summary

Introduction

Phalaenopsis, an epiphytic orchid, is regarded as an obligate crassulacean acid metabolism (CAM) plant because of its succulent leaf with large and highly vacuolated mesophyll cells. It has been reported that CAM orchids at their protocorm or young seedling stages might perform C3 or weak CAM pathways (Goh et al, 1984). The in vitro young seedlings of Phalaenopsis exhibited the carbon isotopic values (δ13C) of −21.4 to −19.5 , suggesting a weak CAM photosynthetic pathway (Lo, 2008). These findings imply that an alteration of the photosynthetic pathway from C3 to CAM occurred at the early developmental stage of CAM orchids; definite proof has not yet been illustrated

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Conclusion