Phosphorolytic degradation of leaf starch via plastidic α-glucan phosphorylase leads to optimized plant growth and water use efficiency over the diel phases of Crassulacean acid metabolism.

Nathalie Ceusters,Susanna F Boxall,Jana Kneřová,Natalia Hurtado-Castano,James Hartwell,Johan Ceusters,Rebecca Rodick,Louisa V Dever,Jade L Waller,Wim Van Den Ende,Anne M Borland

doi:10.1093/jxb/erab132

Abstract

In plants with Crassulacean acid metabolism (CAM), it has been proposed that the requirement for nocturnal provision of phosphoenolpyruvate as a substrate for CO2 uptake has resulted in a re-routing of chloroplastic starch degradation from the amylolytic route to the phosphorolytic route. To test this hypothesis, we generated and characterized four independent RNAi lines of the obligate CAM species Kalanchoë fedtschenkoi with a >10-fold reduction in transcript abundance of plastidic α-glucan phosphorylase (PHS1). The rPHS1 lines showed diminished nocturnal starch degradation, reduced dark CO2 uptake, a reduction in diel water use efficiency (WUE), and an overall reduction in growth. A re-routing of starch degradation via the hydrolytic/amylolytic pathway was indicated by hyperaccumulation of maltose in all rPHS1 lines. Further examination indicated that whilst operation of the core circadian clock was not compromised, plasticity in modulating net dark CO2 uptake in response to changing photoperiods was curtailed. The data show that phosphorolytic starch degradation is critical for efficient operation of the CAM cycle and for optimizing WUE. This finding has clear relevance for ongoing efforts to engineer CAM into non-CAM species as a means of boosting crop WUE for a warmer, drier future.

Highlights

In plants with Crassulacean acid metabolism (CAM), the nocturnal production of phosphosphoenolpyruvate (PEP) is a key limiting factor for night-time CO2 uptake (Dodd et al., 2003; J. Ceusters et al, 2008; 2010)
All four independent rPHS1 lines showed diminished nocturnal starch degradation and higher basal levels of starch throughout the 24 h diel cycle compared to the wild type, indicating that enzyme activity encoded by KfPHS1 plays a central role in nocturnal starch degradation during CAM
Curtailment in nocturnal starch degradation was accompanied by reduced CAM activity in all lines, as indicated by diminished nocturnal acid accumulation and dark net CO2 uptake

Summary

Introduction

In plants with Crassulacean acid metabolism (CAM), the nocturnal production of phosphosphoenolpyruvate (PEP) is a key limiting factor for night-time CO2 uptake (Dodd et al., 2003; J. Ceusters et al, 2008; 2010). Phase III commences as malate exits the vacuole and decarboxylation releases CO2 which is refixed by Rubisco and processed to sucrose and/or starch via the photosynthetic carbon reduction cycle. During this day-time phase of CAM, the CO2 concentration inside the leaf rises in the light due to malate decarboxylation, stomata close and transpirational water loss is curtailed. Deeper understanding of the mechanisms that CAM plants use to optimise net carbon gain and water use efficiency (WUE) requires identification and functional characterization of the enzymes and transporters responsible for nocturnal starch degradation in plants with this photosynthetic specialization. Aspirations to engineer CAM into non-CAM species as a means of improving plant water use efficiency (Borland et al, 2014) demand better understanding of starch degradation in CAM to establish if the nocturnal generation of PEP will require a re-routing of starch degradation in C3 plants (Borland et al, 2016)

Methods

Results

Conclusion