Photosynthesis in higher plants relies upon collection of light by chlorophyll molecules associated with light harvesting chlorophyll a/b-binding (LHC) proteins. The two most abundant of these are Lhcb1 and Lhcb2, which make up light harvesting complex (LHC) II trimers. They are also involved in facilitating state transitions, a process during which energy balancing between photosystem (PS) II and I is achieved. Overexcitation of PSII reduces the plastoquinone pool which activates STN7, a kinase, that phosphorylates a threonine residue on Lhcb1 and Lhcb2. In order to studythe kinetics of this we developed antibodies capable of recognizingphosphorylated forms of each of these proteins. This showed that Lhcb2 is more rapidly phosphorylated than Lhcb1, that there are no differences in the migration of phosphorylated and non-phosphorylated forms of Lhcb1 and Lhcb2 and that the majority of phosphorylated LHCII (P-Lhcb1 and PLhcb2) are associated with super- and megacomplexes. Furthermore, a state 2-specific LHCII-PSI-LHCI band contains P-Lhcb2 but almost no P-Lhcb1, and a band corresponding to M trimers (band 4 from sucrose gradients, composed of LHCII, CP24 and CP29), contains only P-Lhcb1 but no P-Lhcb2. We also developed artificial microRNA lines specifically depleted in either Lhcb1 or Lhcb2, amiLhcb1 and amiLhcb2 respectively. We show that the roles of Lhcb1 and Lhcb2 in state transitions are complementary. Lhcb1 modulate the size of grana stacks. In the absence of Lhcb1 only a few LHC trimers are formed, while in the absence of Lhcb2, the antenna looks like in the wild type although the plants cannot perform state transitions normally. Trimers containing P-Lhcb2 functionally detach from PSII and connect to PSI to balance the relative excitation pressure. State transitions only occur when both Lhcb1 and Lhcb2 are present, presumably in a (Lhcb1)2 Lhcb2 heterotrimer. In absence of Lhcb2, the LHCII-PSI-LHCI supercomplex is not formed indicting that P-Lhcb2 mediates attachment of LHCII to PSI. We tried complementing amiLhcb2 with modified Lhcb2 genes coding for proteins with altered amino acids, Arg2 to Lys or the phosphorylatable Thr3 residue to Asn or Ser. Introduction of the additional gene often causes loss of amiRNA-inhibition, however we could confirm that substitution of the Thr3 with Asn led to the absence of Lhcb2 phosphorylation and thus no state transition. / Klorofyll a/b-bindande proteiner (s k light harvesting chlorophyll a/b-binding proteins eller LHC proteiner) är viktiga för högre växters fotosyntes, då deras klorofyllmolekyler skördar solljuset. Två av dessa proteiner, Lhcb1 och Lhcb2, bygger upp ”LHCII trimerer” och finns i större mängd än de andra och dessa är även viktiga för s k ”state transtions”, en process som ser till att fotosystem (PS) I och PSII exciteras lika mycket. Om PSII exciteras för mycket reduceras plastoquinon-poolen som i sin tur aktiverar ett proteinkinas, STN7, som fosforylerar en av Lhcb1/Lhcb2s treoniner. För att studera denna fosforylering har vi utvecklat antikroppar som är specifika för dessa fosforylerade former av proteinerna, och vi använde dem för att visa att Lhcb2 fosforyleras snabbare än Lhcb1, och att största delen av det fosforylerade proteinerna (P-Lhcb1 och P-Lhcb2) finns i s k super- eller megakomplex. Ett komplex som bara finns finns i ”state 2” består av LHCII, PSI och LHCI, och det innehåller endast P-Lhcb2 men nästan inget P-Lhcb1, och ett band som består av LHCII, CP24 och CP29 innehåller endast PLhcb1. Vi skapade artificiella mikro-RNA-linjer, amiLhcb1 och amiLhcb2, som saknade antingen Lhcb1 eller Lhcb2. Lhcb1 påverkar höjden av grana stackarna. Med hjälp av dessa visade vi att Lhcb1 och Lhcb2 har komplementära roller för state transitions, saknas Lhcb1 gör växten bara få LHCII trimerer, medan om Lhcb2 gör växten antennener som liknar vildtypens, men den kan inte utföra state transitions som den. Mängden Lhcb1 påverkar storleken av ”grana stacks”. Trimerer som innehåller PLhcb2 kopplas över från PSII till PSI för att balansera excitationstrycket. Både Lhcb1 och Lhcb2, antagligen i trimerer bestående av en Lhcb2 och the Lhcb1, behövs för state transitions. Saknas Lhcb2 bildas inga komplex bestående av LHCII, PSI och LHCI, vilket visar att P-Lhcb2 antagligen möjliggör LHCIIs bindning till PSI. Vi försökte komplementera amiLhcb2 med Lhcb2 gener där amino syror bytts ut, Arg2 till Lys eller den fosforylerbara Thr3 till Asn eller Ser. När denna gen introducerades försvann dock ofta amiRNA-inhiberingen, men vi kunde visa att om Thr3 ersattes med Asn skedde inga state transitions.

<p>The Arbuscular mycorrhiza (AM) is an endosymbiont of higher plant roots. Most land plants and cultivated crops are concerned to AM symbiosis. This endosymbiosis is based on the mutual exchange of nutrients between plant and fungus. Therefore, AM symbiosis leads to an increased demand for photosynthetic products. The aim of this study was to investigate the pathway used by plants during AM symbiosis to increase photosynthetic performance. Therefore, we have carried out a systematic characterization of photosynthesis in Medicago truncatula (M. truncatula), which is a model legume. We observed colonization by the fungus in roots and that AM symbiosis increases the fresh and dry plant biomass. This could be attributed to an increase in both photosynthetic efficiency and capacity in AM plants. Consistent with these observations, AM symbiosis enhanced phosphorus uptake from the soil into roots, stems and leaves, as based on analyses of phosphorus content. Based on equal chl loading, no differences were found regarding D1, Lhcb1 and Lhcb2 protein content in four plant groups. This indicates similar ratio between chl and PSII proteins. Furthermore, AM symbiosis increases the amount of chlorophyll, steady state oxygen evolution activities, maximum quantum yield (Fv/Fm), and photosynthetic electron transport rate (about 5 fold). Nevertheless, photoprotection was not affected by AM symbiosis. We observed an increase in weight of seed/fruit and weight of seed/plant in AM plants (about 2 fold). Based on these results, we propose that AM symbiosis increases both the efficiency and the capacity of photosynthetic apparatus in the M. truncatula.</p>

The Arbuscular mycorrhiza (AM) is an endosymbiont of higher plant roots. Most land plants and cultivated crops are concerned to AM symbiosis. This endosymbiosis is based on the mutual exchange of nutrients between plant and fungus. Therefore, AM symbiosis leads to an increased demand for photosynthetic products. The aim of this study was to investigate the pathway used by plants during AM symbiosis to increase photosynthetic performance. Therefore, we have carried out a systematic characterization of photosynthesis in Medicago truncatula (M. truncatula), which is a model legume. We observed colonization by the fungus in roots and that AM symbiosis increases the fresh and dry plant biomass. This could be attributed to an increase in both photosynthetic efficiency and capacity in AM plants. Consistent with these observations, AM symbiosis enhanced phosphorus uptake from the soil into roots, stems and leaves, as based on analyses of phosphorus content. Based on equal chl loading, no differences were found regarding D1, Lhcb1 and Lhcb2 protein content in four plant groups. This indicates similar ratio between chl and PSII proteins. Furthermore, AM symbiosis increases the amount of chlorophyll, steady state oxygen evolution activities, maximum quantum yield (Fv/Fm), and photosynthetic electron transport rate (about 5 fold). Nevertheless, photoprotection was not affected by AM symbiosis. We observed an increase in weight of seed/fruit and weight of seed/plant in AM plants (about 2 fold). Based on these results, we propose that AM symbiosis increases both the efficiency and the capacity of photosynthetic apparatus in the M. truncatula.