In mature leaves, cell-to-cell transport via plasmodesmata (PD) between mesophyll cells links assimilate production in photosynthesis with their export to sink organs. This study addresses the question how signals derived from chloroplasts and photosynthesis influence PD permeability. Cell-to-cell transport was analyzed in leaves of Arabidopsis chlorophyll b-less ch1-3 mutant, the same mutant complemented with a cyanobacterial CAO gene (PhCAO) over-accumulating chlorophyll b, the trxm3 mutant lacking plastidial thioredoxin m3, and the ntrc mutant lacking functional NADPH : thioredoxin reductase C. The regulation of PD permeability in these lines could not be traced back to the reduction state of the thioredoxin system or types and levels of reactive oxygen species produced in chloroplasts; however, it could be related to chloroplast ATP and NADPH production. The results suggest that light enables PD closure via an increase in ATP and NADPH levels produced in photosynthesis, providing a control mechanism for assimilate export based on the rate of photosynthate production in the Calvin-Benson cycle. The level of chlorophyll b influences PD permeability via not yet identified signals. The data also suggest a role of thioredoxin m3 in the regulation of cyclic electron flow around photosystem I.
In plants, organogenesis and specification of cell layers and tissues rely on precise symplastic delivery of regulatory molecules via plasmodesmata. Accordingly, abundance and aperture of plasmodesmata at individual cell boundaries should be controlled by the plant. Recently, studies in Arabidopsis established reactive oxygen species as major regulators of plasmodesmata formation and gating. We show that in a barley mutant deficient in the synthesis of chlorophyll b, the numbers of plasmodesmata in leaves and in the shoot apical meristem are significantly higher than in the corresponding wild type, probably due to redox imbalance in the mutant. The resulting disturbance of symplasmic transport is likely to be the reason for the observed delayed floral transition in these mutants. In plants, plasmodesmata (PD) are indispensable regulators of cell-to-cell communications. 1 The function of meristems as well as the specification of cell layers and tissues during organogenesis rely on the symplasmic transport of regulatory molecules such as miRNAs, transcription factors or their transcripts.2 For instance, PD serve for cell-to-cell transport of the homeodomain-containing meristem regulators WUS and KNOX in shoot apical meristems (SAMs), and of floral homeotic MADS-box proteins in inflorescence meristems.3 Leaves are determinate organs the main function of which lies in the production and release of photosynthates to other plant organs. However, leaves are also the source of many signals which spread over the symplasmic route. For instance, Flowering locus T (FT) is synthesized in phloem companion cells in leaves and moves systemically to SAMs where it initiates floral transition upon unloading via PD. 4 Most likely, movement and activity of non-cell-autonomous transcription factors are controlled by developmental changes of the numbers and functional state of PD. 5,3 At the same time, we know relatively little not only about the fine structure of PD and mechanisms of protein translocation via PD, 6,7 but also about how plants control numbers and states of PD at individual cell boundaries. One of the factors involved in these processes could be the plant hormone cytokinine.8 Recent studies provided a major breakthrough in the elucidation of mechanisms regulating PD formation in plant cells. They revealed that the production of reactive oxygen species (ROS) by cellular organelles can differentially influence the formation of PD as well as their aperture. 9,10,11,12 Mutations in a gene encoding the plastid-localized thioredoxin m led to an increase in ROS and a decrease in PD conductivity in Arabidopsis roots; the mutation could be mimicked by application of methyl viologen.9 An opposite effect, namely an increase in PD conductivity, was observed in the ise1 mutant which carries a defect in a mitochondrial RNA helicase; plants defective for ISE1 also exhibited increased ROS production. 13The formation of secondary plasmodesmata was enhanced in the ise1 mutant and also in a mutant lacking the plastid RNA helicase...
Crop yield strongly depends on time of the onset of flowering as well as of the initiation of senescence. These processes are under tight control of multiple gene complexes. Suboptimal environmental conditions, as well as mutations, may cause changes in the expression levels of these genes, which, in turn, can result in a delay of flowering and/or early senescence, and, ultimately, in a decrease of yield. Recently, crucial role in the regulation of plant development via retrograde signaling pathways has been revealed for chlorophyll b. Chlorophyll b is an obligate component of the photosynthetic apparatus of land plants, and the main regulator of the biosynthesis and degradation of photosynthetic antennae. It is becoming clear that the size and stability of photosynthetic antennae are not only important for photosynthesis but also represents a source of signaling beyond chloroplasts. The absence of chlorophyll b in mutants of Arabidopsis thaliana (ch1) and Hordeum vulgare (chlorina f2 3613) leads to a decrease in the growth rate, leaf size and biomass production. In addition, and independently of the downregulation of photosynthesis, the lack of chlorophyll b results in the delay of flowering and early onset of ontogenetic as well as induced senescence. This review addresses the role of chlorophyll b in energy balance, and discusses new data on the role of chlorophyll b in regulation of ontogenesis not related to photosynthesis. Mutants of economically important crops impaired in chlorophyll b biosynthesis represent promising models for physiological, biochemical and molecular studies of regulation of flowering and senescence, as the results can be directly applied to agricultural practice. Also, we review the novel data on the potential importance of plants with truncated photosynthetic antenna for increase in vegetative and grain biomass production. A decrease in chlorophyll b contents and the following down-regulation of antenna proteins were shown to influence the rate of electron transport within the photosystem II, as well as the rate of CO 2 assimilation relative to chlorophyll unit. Strikingly, these parameters in chlorina mutants are higher than in wild type plants by 15-20 %. Using plants with this type of photosynthetic apparatus can potentially bring about a considerable increase in yield. This suggestion has been recently supported by data on transgenic tobacco plants with truncated photosynthetic antenna (H. Kirst et al. 2017). At the same time, the consequences of the decrease in chlorophyll b levels for ontogenetic regulation and photoprotection typically negate the potential benefit of the acceleration of the limiting factor of photosynthesis, the photosystem II. This review discuss the possible ways to search for optimization of plant functions regulated by chlorophyll b, to provide new mechanisms of the increase in photosynthesis and crop production in agriculture.
The barley (Hordeum vulgare L.) chlorina f2 3613 mutant exhibits low photosynthesis and slow growth. This results from downregulation of the levels of photosynthetic antenna proteins caused by the absence of chl b, the major regulator of photosynthetic antennae in land plants. Here, we demonstrate that, when grown in the field in full sunlight, this mutant displays a changed pattern of stomatal responses compared with the parental wild-type cultivar Donaria. However, stomatal regulation of chlorina f2 3613 plants was restored when plants were placed under a shade cover for several days. The shade cover reduced incident PAR from 2000–2200 μmol m–2 s–1 to 800–880 μmol m–2 s–1 as measured at noon. Contents of ABA, the xanthophyll precursors of ABA biosynthesis and minor antenna proteins, as well as reactive oxygen species levels in stomata and the sensitivity of stomata to exogenously supplied ABA, were determined in leaves of wild-type Donaria and chlorina f2 3613 before and after shading. The results support the view that the restoration of stomatal control in barley chlorina f2 3613 is correlated with an increase in the levels of the minor antenna protein Lhcb6, which has recently been implicated in the enhancement of stomatal sensitivity to ABA in Arabidopsis thaliana (L.) Heynh.
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