The tip of the iceberg: emerging roles of TORC1, and its regulatory functions in plant cells.

Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in coordinating cell growth with light availability, the diurnal cycle, energy availability, and hormonal pathways. TOR Complex 1 (TORC1) controls cell proliferation, growth, metabolism, and defense in plants. Sugar availability is the main signal for activation of TOR in plants, as it also is in mammals and yeast. Specific regulators of the TOR kinase pathway in plants are inorganic compounds in the form of major nutrients in the soils, and light inputs via their impact on autotrophic metabolism. The lack of TOR is embryo-lethal in plants, whilst dysregulation of TOR signaling causes major alterations in growth and development. TOR exerts control as a regulator of protein translation via the action of proteins such as S6K, RPS6, and TAP46. Phytohormones are central players in the downstream systemic physiological TOR effects. TOR has recently been attributed to have roles in the control of DNA methylation, in the abundance of mRNA splicing variants, and in the variety of regulatory lncRNAs and miRNAs. In this review, we summarize recent discoveries in the plant TOR signaling pathway in the context of our current knowledge of mammalian and yeast cells, and highlight the most important gaps in our understanding of plants that need to be addressed in the future.

Target of rapamycin signaling is tightly and differently regulated in the plant response under distinct abiotic stresses.

MAIN CONCLUSION: TOR signaling is finely regulated under diverse abiotic stresses and may be required for the plant response with a different time-course depending on the duration and nature of the stress. Target of rapamycin (TOR) signaling is a central regulator of growth and development in eukaryotic organisms. However, its regulation under stress conditions has not yet been elucidated. In Arabidopsis, we show that TOR transcripts and activity in planta are finely regulated within hours after the onset of salt, osmotic, cold and oxidative stress. The expression of genes encoding the partner proteins of the TOR complex, RAPTOR3G and LST8-1, is also regulated. Besides, the data indicate that TOR activity increases at some time during the adverse condition. Interestingly, in oxidative stress, the major TOR activity increment occurred transiently at the early phase of treatment, while in salt, osmotic and cold stress, it was around 1 day after the unfavorable condition was applied. Those results suggest that the TOR signaling has an important role in the plant response to an exposure to stress. Moreover, basal ROS (H2O2) levels and their modification under abiotic stresses were altered in TOR complex mutants. On the other hand, the root phenotypic analysis of the effects caused by the diverse abiotic stresses on TOR complex mutants revealed that they were differently affected, being in some cases less sensitive, than wild-type plants to long-term unfavorable conditions. Therefore, in this work, we demonstrated that TOR signaling is tightly regulated under abiotic stresses, at transcript and activity level, with different and specific time-course patterns according to the type of abiotic stress in Arabidopsis. Taking our results together, we propose that TOR signaling should be necessary during the plant stress response.

TOR signaling downregulation increases resistance to the cereal killer Fusarium graminearum.

TOR is the master regulator of growth and development that senses energy availability. Biotic stress perturbs metabolic and energy homeostasis, making TOR a good candidate to participate in the plant response. Fusarium graminearum (Fusarium) produces important losses in many crops all over the world. To date, the role of TOR in Fusarium infection has remained unexplored. Here, we show that the resistance to the pathogen increases in different Arabidopsis mutants impaired in TOR complex or in wild-type plants treated with a TOR inhibitor. We conclude that TOR signaling is involved in plant defense against Fusarium.

The riddle of mitochondrial alkaline/neutral invertases: A novel Arabidopsis isoform mainly present in reproductive tissues and involved in root ROS production.

Alkaline/neutral invertases (A/N-Inv), glucosidases that irreversibly hydrolyze sucrose into glucose and fructose, play significant roles in plant growth, development, and stress adaptation. They occur as multiple isoforms located in the cytosol or organelles. In Arabidopsis thaliana, two mitochondrial A/N-Inv genes (A/N-InvA and A/N-InvC) have already been investigated. In this study, we functionally characterized A/N-InvH, a third Arabidopsis gene coding for a mitochondrial-targeted protein. The phenotypic analysis of knockout mutant plants (invh) showed a severely reduced shoot growth, while root development was not affected. The emergence of the first floral bud and the opening of the first flower were the most affected stages, presenting a significant delay. A/N-InvH transcription is markedly active in reproductive tissues. It is also expressed in the elongation and apical meristem root zones. Our results show that A/N-InvH expression is not evident in photosynthetic tissues, despite being of relevance in developmental processes and mitochondrial functional status. NaCl and mannitol treatments increased A/N-InvH expression twofold in the columella root cap. Moreover, the absence of A/N-InvH prevented ROS formation, not only in invh roots of salt- and ABA-treated seedlings but also in invh control roots. We hypothesize that this isoform may take part in the ROS/sugar (sucrose or its hydrolysis products) signaling pathway network, involved in reproductive tissue development, cell elongation, and abiotic stress responses.

Ancient signal for nitrogen status sensing in the green lineage: Functional evidence of CDPK repertoire in Ostreococcus tauri.

Calcium-dependent protein kinases (CDPKs) regulate plant development and many stress signalling pathways through the complex cytosolic [Ca2+] signalling. The genome of Ostreococcus tauri (Ot), a model prasinophyte organism that is on the base of the green lineage, harbours three sequences homologous to those encoding plant CDPKs with the three characteristic conserved domains (protein kinase, autoregulatory/autoinhibitory, and regulatory domain). Phylogenetic and structural analyses revealed that putative OtCDPK proteins are closely related to CDPKs from other Chlorophytes. We functionally characterised the first marine picophytoeukaryote CDPK gene (OtCDPK1) and showed that the expression of the three OtCDPK genes is up-regulated by nitrogen depletion. We conclude that CDPK signalling pathway might have originated early in the green lineage and may play a key role in prasinophytes by sensing macronutrient changes in the marine environment.

Sunflower: a potential fructan-bearing crop?

Grain filling in sunflower (Helianthus annuus L.) mainly depends on actual photosynthesis, being the contribution of stored reserves in stems (sucrose, hexoses, and starch) rather low. Drought periods during grain filling often reduce yield. Increasing the capacity of stem to store reserves could help to increase grain filling and yield stability in dry years. Fructans improve water uptake in soils at low water potential, and allow the storage of large amount of assimilates per unit tissue volume that can be readily remobilized to grains. Sunflower is a close relative to Jerusalem artichoke (H. tuberosus L.), which accumulates large amounts of fructan (inulin) in tubers and true stems. The reason why sunflower does not accumulate fructans is obscure. Through a bioinformatics analysis of a sunflower transcriptome database, we found sequences that are homologous to dicotyledon and monocotyledon fructan synthesis genes. A HPLC analysis of stem sugar composition revealed the presence of low amounts of 1-kestose, while a drastic enhancement of endogenous sucrose levels by capitulum removal did not promote 1-kestose accumulation. This suggests that the regulation of fructan synthesis in this species may differ from the currently best known model, mainly derived from research on Poaceae, where sucrose acts as both a signaling molecule and substrate, in the induction of fructan synthesis. Thus, sunflower might potentially constitute a fructan-bearing species, which could result in an improvement of its performance as a grain crop. However, a large effort is needed to elucidate how this up to now unsuspected potential could be effectively expressed.

First evidence of sucrose biosynthesis by single cyanobacterial bimodular proteins.

The net synthesis of sucrose (Suc) is catalysed by the sequential action of Suc-phosphate synthase (SPS) and Suc-phosphate phosphatase (SPP). SPS and SPP from Anabaena sp. PCC 7120 (7120-SPS and 7120-SPP) define minimal catalytic units. Bidomainal SPSs, where both units are fused, occur in plants and cyanobacteria, but they display only SPS activity. Using recombinant proteins that have fused 7120-SPS and 7120-SPP, we demonstrated that they are bifunctional chimeras and that the arrangement 7120-SPS/SPP is the most efficient to catalyse the sequential reactions to yield Suc. Moreover, we present the first evidence of a bidomainal SPS present in the cyanobacterium Synechococcus elongatus PCC 7942 with both, SPS and SPP activity.

Sucrose signaling in plants: a world yet to be explored.

The role of sucrose as a signaling molecule in plants was originally proposed several decades ago. However, recognition of sucrose as a true signal has been largely debated and only recently this role has been fully accepted. The best-studied cases of sucrose signaling involve metabolic processes, such as the induction of fructan or anthocyanin synthesis, but a large volume of scattered information suggests that sucrose signals may control a vast array of developmental processes along the whole life cycle of the plant. Also, wide gaps exist in our current understanding of the intracellular steps that mediate sucrose action. Sucrose concentration in plant tissues tends to be directly related to light intensity, and inversely related to temperature, and accordingly, exogenous sucrose supply often mimics the effect of high light and cold. However, many exceptions to this rule seem to occur due to interactions with other signaling pathways. In conclusion, the sucrose role as a signal molecule in plants is starting to be unveiled and much research is still needed to have a complete map of its significance in plant function.

Protein phosphatase activity and sucrose-mediated induction of fructan synthesis in wheat.

In this work, we analyze protein phosphatase (PP) involvement in the sucrose-mediated induction of fructan metabolism in wheat (Triticum aestivum). The addition of okadaic acid (OA), a PP-inhibitor, to sucrose-fed leaves reduced fructosylsucrose-synthesizing activity (FSS) induction in a dose-dependent manner. The expression of the two enzymes that contribute to FSS activity, 1-SST (1-sucrose:sucrose fructosyltransferase, E.C. 2.4.1.99) and 6-SFT (6-sucrose:fructan fructosyltransferase, E.C. 2.4.1.10), was blocked by 1 microM OA. These results suggest the involvement of a PP type 2A in sucrose signaling leading to fructan synthesis. OA addition to the feeding medium impaired both sucrose accumulation in leaves and the expression of sucrose-H+ symporter (SUT1). It is known that sucrose concentration must exceed a threshold for the induction of fructan metabolism; hence PP2A inhibition may result in lower sucrose levels than required for this induction. OA also induced the vacuolar acid invertase (acid INV) transcript levels suggesting that PP activity might play a role in carbon partitioning. Total extractable PP2A activity decreased during 24 h of treatment with sucrose, in parallel with declining sugar uptake into leaf tissues. In conclusion, our results suggest that PP2A is involved in sucrose-induction of fructan metabolism and may play a role in regulating sucrose uptake, but do not rule out that further steps in sucrose signaling pathway may be affected.