Inositol signaling and plant growth

Living organisms have evolved to contain a wide variety of receptors and signaling pathways that are essential for their survival in a changing environment. Of these, the phosphoinositide pathway is one of the best conserved. The ability of the phosphoinositides to permeate both hydrophobic and hydrophilic environments, and their diverse functions within cells have contributed to their persistence in nature. In eukaryotes, phosphoinositides are essential metabolites as well as labile messengers that regulate cellular physiology while traveling within and between cells. The stereospecificity of the six hydroxyls on the inositol ring provides the basis for the functional diversity of the phosphorylated isomers that, in turn, generate a selective means of intracellular and intercellular communication for coordinating cell growth. Although such complexity presents a difficult challenge for bench scientists, it is ideal for the regulation of cellular functions in living organisms.     

Inositol phosphate signaling and gibberellic acid

To respond to physical signals and endogenous hormones, plants use specific signal transduction pathways. We and others have previously shown that second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] is used in abscisic acid (ABA) signaling, and that some mutants with altered Ins(1,4,5)P₃ have altered responses to ABA. Specifically, mutants defective in the myo-inositol polyphosphate 5-phosphatases (5PTases) 1 and 2 genes that hydrolyze 5-phosphates from Ins(1,4,5)P₃ and other PtdInsP and InsP substrates, have elevated Ins (1,4,5)P₃, and are ABA-hypersensitive. Given the antagonistic relationship between ABA and gibberellic acid (GA), we tested the response of these same mutants to a GA synthesis inhibitor, paclobutrazol (PAC). We report here that 5ptase1, 5ptase2 and 5ptase11 mutants are hypersensitive to PAC, suggesting a relationship between elevated Ins(1,4,5)P₃ and decreased GA signal transduction. These data provide insight into signaling cross-talk between ABA and GA pathways.Key words: inositol, phosphatidylinositol phosphate, paclobutrazol, gibberellic acid, inositol trisphosphate, paclobutrazol     

Tuning plant signaling and growth to survive salt

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Inositol diphosphate signaling regulates telomere length

Activation of phospholipase C-dependent inositol polyphosphate signaling pathways generates distinct messengers derived from inositol 1,4,5-trisphosphate that control gene expression and mRNA export. Here we report the regulation of telomere length by production of a diphosphorylinositol tetrakisphosphate, PP-IP4, synthesized by the KCS1 gene product. Loss of PP-IP4 production results in lengthening of telomeres, whereas overproduction leads to their shortening. This effect requires the presence of Tel1, the yeast homologue of ATM, the protein mutated in the human disease ataxia telangiectasia. Our data provide in vivo evidence of a regulatory link between inositol polyphosphate signaling and the checkpoint kinase family and describe a third nuclear process modulated by phospholipase C activation.     

Involvement of phospholipid signaling in plant growth and hormone effects

Biochemical and genetics studies demonstrated the critical roles of phospholipid signaling and relevant molecules in regulating multiple processes of plant growth and development, signal transduction, mediating hormone effects and cell responses to environmental stimuli, through modulating protein subcellular localization, cross-talking with other signaling or metabolic pathways, or interacting with signaling molecules. The updated achievements of physiological effects and functional mechanism of phospholipid signaling in higher plants were reviewed.     

VOCs-mediated hormonal signaling and crosstalk with plant growth promoting microbes

In the natural environment, plants communicate with various microorganisms (pathogenic or beneficial) and exhibit differential responses. In recent years, research on microbial volatile compounds (MVCs) has revealed them to be simple, effective and efficient groups of compounds that modulate plant growth and developmental processes. They also interfere with the signaling process. Different MVCs have been shown to promote plant growth via improved photosynthesis rates, increased plant resistance to pathogens, activated phytohormone signaling pathways, or, in some cases, inhibit plant growth, leading to death. Regardless of these exhibited roles, the molecules responsible, the underlying mechanisms, and induced specific metabolic/molecular changes are not fully understood. Here, we review current knowledge on the effects of MVCs on plants, with particular emphasis on their modulation of the salicylic acid, jasmonic acid/ethylene, and auxin signaling pathways. Additionally, opportunities for further research and potential practical applications presented.     

Molecular interactions between light and hormone signaling to control plant growth

As sessile organisms, plants modulate their growth rate and development according to the continuous variation in the conditions of their surrounding environment, an ability referred to as plasticity. This ability relies on a web of interactions between signaling pathways triggered by endogenous and environmental cues. How changes in environmental factors are interpreted by the plant in terms of developmental or growth cues or, in other words, how they contribute to plant plasticity is a current, major question in plant biology. Light stands out among the environmental factors that shape plant development. Plants have evolved systems that allow them to monitor both quantitative and qualitative differences in the light that they perceive, that render important changes in their growth habit. In this review we focus on recent findings about how information from this environmental cue is integrated during de-etiolation and in the shade-avoidance syndrome, and modulated by several hormone pathways—the endogenous cues. In some cases the interaction between a hormone and the light signaling pathways is reciprocal, as is the case of the gibberellin pathway, whereas in other cases hormone pathways act downstream of the environmental cue to regulate growth. Moreover, the circadian clock adds an additional layer of regulation, which has been proposed to integrate the information provided by light with that provided by hormone pathways, to regulate daily growth.     

Inositol phosphates in receptor-mediated cell signaling: metabolic origins and interrelationships

We have investigated the metabolic interrelationships of the major inositol phosphates in vasopressin-stimulated WRK 1 mammary tumor cells which were labeled to equilibrium with [14C]inositol and briefly, just prior to stimulation, with [3H]inositol. A comparison of the 3H/14C ratios of these compounds with those of the cellular inositol lipids suggests that most of the known inositol mono-, bis-, tris-, and tetrakis-phosphates are derived from precursors with turnover rates similar to those of these lipids. However, Ins(3,4,5,6)P4 (which is the major inositol tetrakisphosphate to accumulate in stimulated WRK 1 cells), Ins(1,3,4,5,6)P5, and InsP6 had 3H/14C ratios of 0 in this experiment, indicating that they must have a different metabolic origin.     

Controlling hormone signaling is a plant and pathogen challenge for growth and survival

Plants and pathogens have continuously confronted each other during evolution in a battle for growth and survival. New advances in the field have provided fascinating insights into the mechanisms that have co-evolved to gain a competitive advantage in this battle. When plants encounter an invading pathogen, not only responses signaled by defense hormones are activated to restrict pathogen invasion, but also the modulation of additional hormone pathways is required to serve other purposes, which are equally important for plant survival, such as re-allocation of resources, control of cell death, regulation of water stress, and modification of plant architecture. Notably, pathogens can counteract both types of responses as a strategy to enhance virulence. Pathogens regulate production and signaling responses of plant hormones during infection, and also produce phytohormones themselves to modulate plant responses. These results indicate that hormone signaling is a relevant component in plant-pathogen interactions, and that the ability to dictate hormonal directionality is critical to the outcome of an interaction.     

Inositol trisphosphate-induced Ca2+ signaling modulates auxin transport and PIN polarity

The phytohormone auxin is an important determinant of plant development. Directional auxin flow within tissues depends on polar localization of PIN auxin transporters. To explore regulation of PIN-mediated auxin transport, we screened for suppressors of PIN1 overexpression (supo) and identified an inositol polyphosphate 1-phosphatase mutant (supo1), with elevated inositol trisphosphate (InsP(3)) and cytosolic Ca(2+) levels. Pharmacological and genetic increases in InsP(3) or Ca(2+) levels also suppressed the PIN1 gain-of-function phenotypes and caused defects in basal PIN localization, auxin transport and auxin-mediated development. In contrast, the reductions in InsP(3) levels and Ca(2+) signaling antagonized the effects of the supo1 mutation and disrupted preferentially apical PIN localization. InsP(3) and Ca(2+) are evolutionarily conserved second messengers involved in various cellular functions, particularly stress responses. Our findings implicate them as modifiers of cell polarity and polar auxin transport, and highlight a potential integration point through which Ca(2+) signaling-related stimuli could influence auxin-mediated development.     

Light signaling and UV‐B‐mediated plant growth regulation

Light plays an important role in plants’ growth and development throughout their life cycle. Plants alter their morphological features in response to light cues of varying intensity and quality. Dedicated photoreceptors help plants to perceive light signals of different wavelengths. Activated photoreceptors stimulate the downstream signaling cascades that lead to extensive gene expression changes responsible for physiological and developmental responses. Proteins such as ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) act as important factors which modulate light‐regulated gene expression, especially during seedling development. These factors function as central regulatory intermediates not only in red, far‐red, and blue light pathways but also in the UV‐B signaling pathway. UV‐B radiation makes up only a minor fraction of sunlight, yet it imparts many positive and negative effects on plant growth. Studies on UV‐B perception, signaling, and response in plants has considerably surged in recent times. Plants have developed different strategies to use UV‐B as a developmental cue as well as to withstand high doses of UV‐B radiation. Plants’ responses to UV‐B are an integration of its cross‐talks with both environmental factors and phytohormones. This review outlines the current developments in light signaling with a major focus on UV‐B‐mediated plant growth regulation.     

Inositol is a required nutrient for keratinocyte growth

Bovine hypothalamus is known to contain a growth-promoting activity for human epidermal keratinocytes. By sequential purification, the substance was isolated and found to be myo-inositol. The identity of the substance as myo-inositol was confirmed by ion modified partition, gas liquid, thin layer chromatography, by mass spectrometry, and quantitative bioassay. The inositol content of the crude hypothalamic extract and of an active acetone precipitate (the first step in the purification) was determined to be sufficient to account for their observed bioactivity. At an optimal concentration of 55 microM (10 micrograms/ml), myo-inositol approximately tripled keratinocyte yield compared to paired cultures in basal medium containing 0.3 microM, although this yield was only half that produced by a crude saline extract of hypothalamus, suggesting that there are additional growth-promoting activities in the tissue extract. No other skin-derived cell type tested was stimulated by supplemental inositol. These results establish that the inositol requirement for cultured human keratinocytes is markedly higher than for any other normal or malignant cell type investigated to date, and expand the list of brain-derived phospholipid precursors known to stimulate epithelial proliferation in vitro. These data suggest that inositol may subserve quantitatively or qualitatively different functions in the keratinocyte than in other cell types.     

Inositol 1,4,5-trisphosphate signaling regulates mating behavior in Caenorhabditis elegans males

Complex behavior requires the coordinated action of the nervous system and nonneuronal targets. Male mating in Caenorhabditis elegans consists of a series of defined behavioral steps that lead to the physiological outcomes required for successful impregnation. We demonstrate that signaling mediated by inositol 1,4,5-trisphosphate (IP(3)) is required at several points during mating. Disruption of IP(3) receptor (itr-1) function results in dramatic loss of male fertility, due to defects in turning behavior (during vulva location), spicule insertion and sperm transfer. To elucidate the signaling pathways responsible, we knocked down the six C. elegans genes encoding phospholipase C (PLC) family members. egl-8, which encodes PLC-beta, functions in spicule insertion and sperm transfer. itr-1 and egl-8 are widely expressed in the male reproductive system. An itr-1 gain-of-function mutation rescues infertility caused by egl-8 RNA interference, indicating that egl-8 and itr-1 function together as central components of the signaling events controlling sperm transfer.