Abstract
BackgroundPolyhydroxyalkanoates are linear biodegradable polyesters produced by bacteria as a carbon store and used to produce a range of bioplastics. Widespread polyhydroxyalkanoate production in C4 crops would decrease petroleum dependency by producing a renewable supply of biodegradable plastics along with residual biomass that could be converted into biofuels or energy. Increasing yields to commercial levels in biomass crops however remains a challenge. Previously, lower accumulation levels of the short side chain polyhydroxyalkanoate, polyhydroxybutyrate (PHB), were observed in the chloroplasts of mesophyll (M) cells compared to bundle sheath (BS) cells in transgenic maize (Zea mays), sugarcane (Saccharum sp.), and switchgrass (Panicum virgatum L.) leading to a significant decrease in the theoretical yield potential. Here we explore various factors which might affect polymer accumulation in mesophyll cells, including targeting of the PHB pathway enzymes to the mesophyll plastid and their access to substrate.ResultsThe small subunit of Rubisco from pea effectively targeted the PHB biosynthesis enzymes to both M and BS chloroplasts of sugarcane and switchgrass. PHB enzyme activity was retained following targeting to M plastids and was equivalent to that found in the BS plastids. Leaf total fatty acid content was not affected by PHB production. However, when fatty acid synthesis was chemically inhibited, polymer accumulated in M cells.ConclusionsIn this study, we provide evidence that access to substrate and neither poor targeting nor insufficient activity of the PHB biosynthetic enzymes may be the limiting factor for polymer production in mesophyll chloroplasts of C4 plants.
Highlights
Polyhydroxyalkanoates are linear biodegradable polyesters produced by bacteria as a carbon store and used to produce a range of bioplastics
Plant material Sugarcane lines identified in our previous reports are relabelled as follows with PHB values reported earlier [12,13,15] as the mean ± standard error: Wild-type Q117 (WT), TA4 (LP, 0.45 ± 0.02% dry weight (DW) PHB), 4 F1 (MP1, 1.21 ± 0.24% DW PHB), 8C8 (MP2, 1.30 ± 0.11% DW PHB) and 7C3 (HP, 3.11 ± 0.31% DW PHB)
Switchgrass lines identified as wild type (WT), Low producer (LP) (0.60 ± 0.08% DW PHB, MP1 (1.10 ± 0.12% DW PHB), MP2 (1.60 ± 0.01% DW PHB), high polymerproducing (HP) (2.44 ± 0.43% DW PHB) represent independent transformation events expressing the PHB biosynthesis genes phaA, phaB and phaC, under the control of the maize cab-m5 promoter [15]
Summary
Polyhydroxyalkanoates are linear biodegradable polyesters produced by bacteria as a carbon store and used to produce a range of bioplastics. C4 photosynthesis is more complex than the ancestral C3 pathway of carbon fixation and involves morphological and biochemical modifications leading to a significant reduction in photorespiration. This is achieved by compartmentalization of the processes of carbon dioxide (CO2) fixation and reduction between physiologically distinct mesophyll (M) and bundle sheath (BS) cells. Polyhydroxyalkanoates (PHAs), linear polyesters produced naturally in bacteria as a means of storing carbon [6,7], are an ideal molecule to produce in C4 plants. Largescale production of PHAs in high biomass yielding crops would provide a sustainable supply of bioplastics and energy from one plant feedstock that could be processed economically in a biorefinery [8]. Appropriate targeting of the PHB biosynthesis enzymes to subcellular compartments where sufficient acetyl-CoA and reducing power are available is necessary
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