Arabidopsis Photosynthetic and Heterotrophic Cell Suspension Cultures

Cell suspension cultures represent a widely used experimental tool suitable to perform a variety of structural and physiological studies in a more simplified system compared to the organism in toto. In this chapter we describe the methods routinely used in our laboratory to establish and maintain Arabidopsis photosynthetic and heterotrophic cell suspension cultures, containing either chloroplasts or amyloplasts, respectively. The use of these in vitro systems may allow to obtain insights into the unique features of chloroplasts versus non-green plastids, as well as their integration in the structural and metabolic compartmentalization of the plant cell.

Starch formation inside plastids of higher plants

Starch is a water-insoluble polyglucan synthesized inside the plastid stroma within plant cells, serving a crucial role in the carbon budget of the whole plant by acting as a short-term and long-term store of energy. The highly complex, hierarchical structure of the starch granule arises from the actions of a large suite of enzyme activities, in addition to physicochemical self-assembly mechanisms. This review outlines current knowledge of the starch biosynthetic pathway operating in plant cells in relation to the micro- and macro-structures of the starch granule. We highlight the gaps in our knowledge, in particular, the relationship between enzyme function and operation at the molecular level and the formation of the final, macroscopic architecture of the granule.

Down-regulation of PvTRX1h increases nodule number and affects auxin, starch, and metabolic fingerprints in the common bean (Phaseolus vulgaris L.)

The legume-rhizobium symbiotic relationship has been widely studied and characterized. However, little information is available about the role of histone lysine methyltransferases in the legume-rhizobium interaction and in the formation of nitrogen-fixing nodules in the common bean. Thus, this study aimed to gain a better understanding of the epigenetic control of nodulation in the common bean. Specifically, we studied the role of PvTRX1h, a histone lysine methyltransferase coding gene, in nodule development and auxin biosynthesis. Through a reverse genetics approach, we generated common bean composite plants to knock-down PvTRX1h expression. Here we found that the down-regulation of PvTRX1h increased the number of nodules per plant, but reduced the number of colony-forming units recovered from nodules. Genes coding for enzymes involved in the synthesis of the indole-3-acetic acid were up-regulated, as was the concentration of this hormone. In addition, PvTRX1h down-regulation altered starch accumulation as determined by the number of amyloplasts per nodule. Metabolic fingerprinting by direct liquid introduction-electrospray ionization-mass spectrometry (DLI-ESI-MS) revealed that the root nodules were globally affected by PvTRX1h down-regulation. Therefore, PvTRX1h likely acts through chromatin histone modifications that alter the auxin signaling network to determine bacterial colonization, nodule number, starch accumulation, hormone levels, and cell proliferation.

Purification of Nongreen Plastids (Proplastids and Amyloplasts) from Angiosperms, and Isolation of Their Envelope Membranes

Plastids, a wide family of plant specific organelles, exist in all plant cells in a number of different forms with different functions essential for plant life. Among them, chloroplasts are by far the more extensively studied owing to their central role in photosynthesis. However, other plastid family members, often referred to as nongreen plastids, play also major roles in the physiology of higher plants and could be better suited for studies of specific metabolic processes in heterotrophic plant cells. Unfortunately, serious technical problems are frequently encountered with separating intact, active nongreen plastids from contaminating membranes and mitochondria. Here, we provide detailed protocols suitable for the large scale preparation of intact and highly pure proplastids from cauliflower buds, as well as amyloplasts from sycamore cultured cells, and for the subsequent separation of their surrounding envelope membranes from the stroma and other plastid fractions. Both methods proved to be highly reliable and have been instrumental for in-depth investigations on biochemistry and physiology of nongreen plastids.

Exogenous hydrogen peroxide inhibits primary root gravitropism by regulating auxin distribution during Arabidopsis seed germination

Hydrogen peroxide (H₂O₂) is the key factor in many physiological and metabolic processes in plants. During seed germination, exogenous H₂O₂ application influences gravitropism and induces curvature of the primary root in grass pea and pea seedlings. However, it remains unclear whether and how this happens in the model plant Arabidopsis thaliana. In the present study, the effect of exogenous H₂O₂ on the gravitropic response of primary roots during Arabidopsis seed germination was studied using histology and molecular biology approaches. Appropriate H₂O₂ treatment not only restrained primary root growth, but also disrupted gravitropism and induced root curvature. Histological staining and molecular analysis demonstrated that exogenous H₂O₂ correlated with lack of starch-dense amyloplasts in root tip columella cells, which ultimately results in the lack of gravisensing. Detection of calcium ion (Ca²⁺) by a fluorescent probe showed that Ca²⁺ distribution changed and intracellular Ca²⁺ concentration increased in H₂O₂-treated primary root, which was consistent with alterations in auxin distribution and concentration triggered by H₂O₂ treatment. Furthermore, the normally polar localization of Pin-formed 1 (PIN1) and PIN2 became uniformly distributed on root tip cell membranes after treatment with H₂O₂. This leads to speculation that the IAA signaling pathway was affected by exogenous H₂O₂, causing asymmetrical distribution of IAA on both sides of the primary root, which would influence the gravitropic response.

Wortmannin-induced vacuole fusion enhances amyloplast dynamics in Arabidopsis zigzag1 hypocotyls

Gravitropism in Arabidopsis shoots depends on the sedimentation of amyloplasts in the endodermis, and a complex interplay between the vacuole and F-actin. Gravity response is inhibited in zigzag-1 (zig-1), a mutant allele of VTI11, which encodes a SNARE protein involved in vacuole fusion. zig-1 seedlings have fragmented vacuoles that fuse after treatment with wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and underscore a role of phosphoinositides in vacuole fusion. Using live-cell imaging with a vertical stage microscope, we determined that young endodermal cells below the apical hook that are smaller than 70 μm in length are the graviperceptive cells in dark-grown hypocotyls. This result was confirmed by local wortmannin application to the top of zig-1 hypocotyls, which enhanced shoot gravitropism in zig-1 mutants. Live-cell imaging of zig-1 hypocotyl endodermal cells indicated that amyloplasts are trapped between juxtaposed vacuoles and their movement is severely restricted. Wortmannin-induced fusion of vacuoles in zig-1 seedlings increased the formation of transvacuolar strands, enhanced amyloplast sedimentation and partially suppressed the agravitropic phenotype of zig-1 seedlings. Hypergravity conditions at 10 g were not sufficient to displace amyloplasts in zig-1, suggesting the existence of a physical tether between the vacuole and amyloplasts. Our results overall suggest that vacuole membrane remodeling may be involved in regulating the association of vacuoles and amyloplasts during graviperception.

Plastids and Carotenoid Accumulation

Plastids are ubiquitously present in plants and are the organelles for carotenoid biosynthesis and storage. Based on their morphology and function, plastids are classified into various types, i.e. proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. All plastids, except proplastids, can synthesize carotenoids. However, plastid types have a profound effect on carotenoid accumulation and stability. In this chapter, we discuss carotenoid biosynthesis and regulation in various plastids with a focus on carotenoids in chromoplasts. Plastid transition related to carotenoid biosynthesis and the different capacity of various plastids to sequester carotenoids and the associated effect on carotenoid stability are described in light of carotenoid accumulation in plants.

Unidirectional transport of orthophosphate across the envelope of isolated cauliflower-bud amyloplasts

Isolated amyloplasts from cauliflower (Brassica oleracea L. var botrytis) buds are able to export orthophosphate unidirectionally into the incubation medium. This orthophosphate transport appears to be protein-mediated, as indicated by the following observations: (i) low temperature and the presence of inhibitors of protein-mediated transport reduced the rate of orthophosphate export, and (ii) the rate of orthophosphate export became saturated with rising internal substrate concentrations. Micromolar concentrations of 4,4'-diisothiocyano-2,2'-stilbene disulphonic acid inhibited the rate of unidirectional orthophosphate export, thus indicating the involvement of the amyloplastic glucose-6-phosphate (Glc6P)translocator in the unidirectional export of orthophosphate. The effect of rising concentrations of orthophosphate upon the activity of ADP glucose pyrophosphorylase in desalted extracts was determined. Orthophosphate given in concentrations similar to those measured in the amyloplastic stroma under conditions of steady-state rates of Glc6P-dependent starch synthesis inhibited the activity of ADP-glucose pyrophosphorylase significantly. However, even under strong limiting substrate conditions the residual activity was sufficient to catalyze the flux of carbon into starch. The maximal rates of orthophosphate transport (in the counter-exchange mode) by isolated spinach (Spinacia oleracea L.) chloroplasts and by isolated cauliflower-bud amyloplasts were also determined. These rates were compared with the maximal rates of undirectional orthophosphate export by these plastids. From these measurements we can conclude that, compared with spinach chloroplasts, isolated amyloplasts of cauliflower exhibit a fivefold greater ratio of unidirectional orthophosphate transport to maximal rate of orthophosphate transport in the counter-exchange mode compared to spinach chloroplasts. The determined rate of maximal unidirectional orthophosphate export is sufficient to catalyze the release of additional inorganic phosphate liberated in the amyloplastic stroma during the process of Glc6P-dependent starch synthesis.