19-18. Taiz. Plants differ in their ability to fix ambient CO2Photorespiration: light-dependent O2uptake and CO2releasePHOTORESPIRATION: A process where O2is consumed and CO2is given off in light by photosynthetic tissues (leaf). Why? O2consumption by RuBP Case/oxygenaseC4 METABOLISM: A mechanism to concentrate CO2in the chloroplasts to reduce RUBP oxygenase activity. C4 PLANTS: e.g. Corn, sugar cane, many weeds 1. Carboxylation: in MESOPHYLL CELLCO2+ PEP (3c)---> OAA (c4) ---> MAL (c4) Mal is transported to BUNDLE SHEATH cells2. Decarboxylation : MAL (c4) ---> CO2+ PVA3. C3 reduction: 3CO2--> G3PTransport of PVA back to MESOPHYLL4. Regeneration of PEP: pva ---> pepRubP carboxylase/ oxygenase2RuBP + 2O2-->2PGA + 2 p-Glycolate -->2PGA + 1PGA + CO28-9 taiz. Photorespiration: net loss of CC4-Sorghum & Atriplex8-9 TaizAnatomy of C4 leafC4 Plants:corn, sugar cane, sorghum, weeds: crabgrass8-9A. Sugar cane- C4 monocot212-45 Buchanan. C4 plant maize shows Krantz anatomy.8-10 TaizC4 Carbon cycleHatch & Slack 1966-19711. Carboxylation: in MESOPHYLL CELLCO2+ PEP (3C)---> OAA (c4) ---> MAL (c4) 2. Mal is transported to BUNDLE SHEATH cellsDecarboxylation : MAL (c4) ---> CO2+ PVAC3 reduction: 3CO2--> G3PCarboxylation- rubiscoreductionregeneration3. Transport of PVA back to MESOPHYLLRegeneration of PEP: pva ---> pep8-11Ecological significance of C4 plants1. C4 plants have low CO2 compensation conc.2. C3 plants have high transpiration ratio : water loss/CO2uptake3. Quantum yield: CO2fixed/mol quantumis higher in C4 plants at high temp and high lightConclusion:C4 plants are not always more competitive than C3 plants.C4 have an advantage at high temp, high light, and low water conditions.9-23 Taiz. CO2absorbed per quantum is higher at high temperature in C4 plant.3AgaveVanillaCAM plants 9-19 Taiz: CAM plants take up CO2at nightWhat? How?Why?CAM (Crassulacean Acid Metabolism) plants fix CO2at nightNIGHT: PEP Case1. Carboxylation: PEP + CO2---> OAA --> MAL2. Mal is stored in the vacuole DAY3. Mal is transported back to the cytoplasm4. Decarboxylation:MAL + NADP+ -(NADP MALIC ENZYME) ->PVA + CO2+ NADPH5. Carbon Reduction Cycle: CO2-> PGA -> HEXOSE6. Regeneration of PEP:PVA + ATP + Pi-(PYRUVATE DIKINASE)-> PEP +AMP + PPi8-12 Taiz. CAM pathwayHow do plants regulate this?Prevent futile cycling of malate.8-15 Taiz. CAM PEP Case is active at nightPEP Case is activated by phosphorylation.P- enzyme is active.4Regulation of CAM1. PEP Case: is active at night and shuts down in the day.2. Mal product inhibits PEP Case in the day3. Malic enzyme is active in the day (Decarboxylation).SummaryCAM reduce water loss by separating reactions in time.CAM plants are suited to dry habitats.Sugar cane from web site Yama……..Sucrose and starch synthesisRice from website of Yama…….Synthesis, Export and Storage of Photosynthetic Products(or increasing starch in potato chips)Synthesis:a. Starch synthesis in the chloroplast: G3P--> starchb. Sucrose synthesis in the cytoplasm of mesophyll cellsG3P moves into the cytoplasm. G3P--> --> SUCc. Cellulose synthesisTransportSuc is exported to sinks via the phloem. Fate of sucrose in sinks :a. -Suc. --> Hexose --> Respiration & Synthesis b. -Suc. --> Hexose --> Starch for storage Regulation of sucrose or starch synthesis depends on how G3P is distributed.Photosynthate partitioning determines the harvest index.8-14 Taiz. Synthesis of starch in the chloroplast & synthesis of sucrose in the cytosolC metabolism and transport in the day and in the night5Starch degradation at night2 triose-p condense to make a fructose-1,6-P and produce hexose-p poolhexose-P poolFructose-1,6-PSucrose synthesis: adding glucose (in UDPG) to fructose-6-P 8-15. Taiz. BS cell from maizeStarch grainChloroplastReview13-8 Buchanan. Hexose-P pool contributes intermediates to glycolysis and biosynthesis6Genetic engineering:Increase starch content in potato chipsHow?Increase enzyme forming ADPGOverexpress enzyme in potatoJ PreissMethod1. Plasmid containing gene ADPG Ppase2. Introduce into Agrobacterium tumefaciens3. Transform plants with bacteria carrying gene4. Gene integrates into plant chromosome.21-4 Taiz. Tumour induction by cytokinin.Agrobacterium-mediated transformation- introducing new gene into plantsBox 21-1 Taiz. Regenerating whole plants from transgenic cells or tissues.Concerns of GM-modified crops•Antibiotic resistance markers used for selection --> lead to antibiotic-resistance bacteria•Allergens or toxins introducedFigure 1. Photosynthesis gene expression peak near the middle of the subjective day.Harmer SL et al 2000. Science. 0 h = 6 a.m.LHCA LHCBPSI and PSIIDiurnal regulation of PS protein gene expressionGenes encoding starch-mobilizing enzymes peak during the subjective night. (A) Cycling genes encode a putative starch kinase (accession number AAD31337) that is related to potato R1 protein (38) (dark blue); a -amylase (AJ250341) (gold); putative fructose-bisphosphate aldolase,plastidic form (AAD14543), and putative fructose-bisphosphate aldolase, predicted to beplastidic (AAD23681) (red); a putative sugar transporter (AAD03450) (light blue); and a sucrose-phosphate synthase homolog (T04062) (green). (B) Model for the enzymatic functions of these gene products in the mobilization of starch. Colored arrows indicate the function of the corresponding gene indicated in (A). The chloroplast is bounded by a green boxand the cytoplasm by a black box. Harmer et al 2000 ScienceStarchDiurnal regulation of enzymes7Is maltose exported?Not known until recently.A mutant with high maltoseWT mutantmaltosestarchmutantmutantWtNiittyla et al 2004. ScienceMex1 transports maltose out of chloroplastMEX1-YFP localized to chloroplast(I.E)Double mutant of mex1/dpe gluc exporterWt??E coli mutant grown on maltose +/- Mex1Maltose and glucose are