Aditya Banerjee

WRKY Proteins: Signaling and Regulation of Expression during Abiotic Stress Responses

WRKY proteins are emerging players in plant signaling and have been thoroughly reported to play important roles in plants under biotic stress like pathogen attack. However, recent advances in this field do reveal the enormous significance of these proteins in eliciting responses induced by abiotic stresses. WRKY proteins act as major transcription factors, either as positive or negative regulators. Specific WRKY factors which help in the expression of a cluster of stress-responsive genes are being targeted and genetically modified to induce improved abiotic stress tolerance in plants. The knowledge regarding the signaling cascade leading to the activation of the WRKY proteins, their interaction with other proteins of the signaling pathway, and the downstream genes activated by them are altogether vital for justified targeting of the WRKY genes. WRKY proteins have also been considered to generate tolerance against multiple abiotic stresses with possible roles in mediating a cross talk between abiotic and biotic stress responses. In this review, we have reckoned the diverse signaling pattern and biological functions of WRKY proteins throughout the plant kingdom along with the growing prospects in this field of research.

Abscisic-acid-dependent basic leucine zipper (bZIP) transcription factors in plant abiotic stress

One of the major causes of significant crop loss throughout the world is the myriad of environmental stresses including drought, salinity, cold, heavy metal toxicity, and ultraviolet-B (UV-B) rays. Plants as sessile organisms have evolved various effective mechanism which enable them to withstand this plethora of stresses. Most of such regulatory mechanisms usually follow the abscisic-acid (ABA)-dependent pathway. In this review, we have primarily focussed on the basic leucine zipper (bZIP) transcription factors (TFs) activated by the ABA-mediated signalosome. Upon perception of ABA by specialized receptors, the signal is transduced via various groups of Ser/Thr kinases, which phosphorylate the bZIP TFs. Following such post-translational modification of TFs, they are activated so that they bind to specific cis-acting sequences called abscisic-acid-responsive elements (ABREs) or GC-rich coupling elements (CE), thereby influencing the expression of their target downstream genes. Several in silico techniques have been adopted so far to predict the structural features, recognize the regulatory modification sites, undergo phylogenetic analyses, and facilitate genome-wide survey of TF under multiple stresses. Current investigations on the epigenetic regulation that controls greater accessibility of the inducible regions of DNA of the target gene to the bZIP TFs exclusively under stress situations, along with the evolved stress memory responses via genomic imprinting mechanism, have been highlighted. The potentiality of overexpression of bZIP TFs, either in a homologous or in a heterologous background, in generating transgenic plants tolerant to various abiotic stressors have also been addressed by various groups. The present review will provide a coherent documentation on the functional characterization and regulation of bZIP TFs under multiple environmental stresses, with the major goal of generating multiple-stress-tolerant plant cultivars in near future.

Group II late embryogenesis abundant (LEA) proteins: structural and functional aspects in plant abiotic stress

AbstractGroup II late embryogenesis abundant (LEA) proteins are crucial phytomolecules which accumulate mainly in the late phases of seed development and also in the vegetative tissues in response to exogenous stress. In spite of considerable research, their mechanism of action to generate plant tolerance against abiotic stresses still remains obscure. The present review focuses on the varied structural aspects which ultimately dictate the multifarious functions of the Group II LEA proteins when the plants are exposed to intense desiccation. Currently, several reports have been documented regarding newer in silico approaches in predicting LEA protein structure and the corresponding cis-elements. Coupled to recent transgenic approaches, these reports need to be properly structured to further characterize the physico-chemical and functional importance of LEA proteins in regulating tolerance against multiple abiotic stresses.

Emerging techniques to decipher microRNAs (miRNAs) and their regulatory role in conferring abiotic stress tolerance of plants

MicroRNAs (miRNAs) are a distinct class of non-coding, small regulatory RNAs which have evolved significantly in generating abiotic stress tolerance across a variety of model plants and crop species. These miRNAs, while undergoing post-transcriptional modifications, have often been found to be linked with epigenetic regulations of stress-responsive gene expression. The discovery of isomers of miRNAs (isomiRs) is also a remarkable event, as some schools of scientists believe them to be regulatory molecules distinct from the conventional miRNAs. The link between isomiRs and abiotic stress responses in plants is now a field of intense research. In this review, we have highlighted the mechanism of various tools and techniques which are essential to visualize high-throughput data analysis. Such data are required for generating large-scale libraries of small RNAs, from which stress-responsive miRNAs are conventionally screened. The concluding part of the review especially contains an exhaustive discussion on the recent developments of miRNA-mediated tolerance towards multiple stresses, such as nutrient deficiency, salinity, drought, oxidative stress, hypoxia, temperature stress, radiation, and heavy metal toxicity. Both transgenic as well as miRNAome approaches have been focussed in this section of the review.

Epigenetic Control of Plant Cold Responses

Higher plants are sedentary organisms which inevitably endure a variety of environmental stresses throughout the life cycle. Abiotic stresses can be atmospheric like cold, heat and UV irradiation; or can also be edaphic like salinity, drought, and heavy metal toxicity (Wani and Gosal, 2011; Surekha et al., 2015). Of all these, cold stress is regarded as a major environmental factor which limits agricultural expansion and crop productivity in hilly terrains (Sanghera et al., 2011). Non-freezing low temperatures deteriorate plant growth physiology by inducing chilling injuries like photosynthesis-associated damages, chlorosis, unregulated apoptosis, loss of membrane fluidity and ultimately wilting (Wani et al., 2016). Depending on the extent of sensitivity among plants, cold stress has been sub-divided into two types. Chilling stress is characterized by 0–15C, whereas temperatures below 0C cause freezing stress (Wani et al., 2013). By virtue of cold acclimation and associated alterations at the molecular and biochemical levels, temperate climatic plants exhibit greater ranges of cold tolerance compared to their tropical and sub-tropical counterparts (Yamaguchi-Shinozaki and Shinozaki, 2006). Deciphering the epigenomic landscape in plants exposed to cold conditions is a rapidly developing field (Hu et al., 2011). Intricate research focussing on epigenetic processes during cold stress has led to the identification of molecular targets which can be genetically manipulated to generate cold tolerant lines. Vernalization is a floral regulatory process preventing precocious flowering during autumn or winter. It gradually promotes flowering competence after prolonged exposures to cold conditions in a species-dependent manner (Kim et al., 2009). Physiologically, vernalization is a “memory response” which is correlated with epigenetic regulation, as observed in themodel plantArabidopsis thaliana (Song et al., 2012).

The gymnastics of epigenomics in rice

Epigenomics is represented by the high-throughput investigations of genome-wide epigenetic alterations, which ultimately dictate genomic, transcriptomic, proteomic and metabolomic dynamism. Rice has been accepted as the global staple crop. As a result, this model crop deserves significant importance in the rapidly emerging field of plant epigenomics. A large number of recently available data reveal the immense flexibility and potential of variable epigenomic landscapes. Such epigenomic impacts and variability are determined by a number of epigenetic regulators and several crucial inheritable epialleles, respectively. This article highlights the correlation of the epigenomic landscape with growth, flowering, reproduction, non-coding RNA-mediated post-transcriptional regulation, transposon mobility and even heterosis in rice. We have also discussed the drastic epigenetic alterations which are reported in rice plants grown from seeds exposed to the extraterrestrial environment. Such abiotic conditions impose stress on the plants leading to epigenomic modifications in a genotype-specific manner. Some significant bioinformatic databases and in silico approaches have also been explained in this article. These softwares provide important interfaces for comparative epigenomics. The discussion concludes with a unified goal of developing epigenome editing to promote biological hacking of the rice epigenome. Such a cutting-edge technology if properly standardized, can integrate genomics and epigenomics together with the generation of high-yielding trait in several cultivars of rice.

Interactions of Brassinosteroids with Major Phytohormones: Antagonistic Effects

Brassinosteroids (BRs) constitute an important class of signaling molecules capable of executing diverse functions ranging from plant growth, development, reproduction, and even stress tolerance. The recent literature on BRs has discussed these wide ranging roles and potentials of BRs. However, the maintenance of metabolic equivalents in the global context of other phytohormones is largely unknown. In this article, we have highlighted such interactive antagonistic cross-talks between BRs and other phytohormones which are crucial in growth regulation and abiotic stress tolerance. Such competitive interactions with BRs have been observed in the cases of abscisic acid, ethylene, auxin, gibberellins, salicylic acid, and even polyamines during physiological growth or abiotic stresses. The discussion largely presents the unique characters of plant molecular physiology and development regarding BR- and other phytohormone-mediated interactive antagonism.

Strigolactones: multi-level regulation of biosynthesis and diverse responses in plant abiotic stresses

Strigolactones (SLs) are a small class of diverse metabolites derived from the carotenoid pathway. These active biomolecules are a recent inclusion to the list of non-traditional phytohormones or plant growth regulators. Previous reports and articles have discussed their pro-regulatory roles in plant growth, development, signaling and delay of senescence. However, the multi-level control of SL biosynthesis is less known. The anabolic genes are strictly regulated through synchronized co-operation between crucial phytohormones. Epigenetic and microRNA-mediated post-transcriptional regulation fine tunes the cellular accumulation of these putative phytohormones. The question now arises that why such multi-level intricate regulation at all is required for SLs, which were originally detected as under-rated germination and rhizosphere stimulants. This review answers the question in the backdrop of the positive roles of SLs in promoting abiotic stress resilience across diverse plant species. SLs reportedly accumulate in the plant tissues in response to environmental sub-optimal conditions like drought, salinity, temperature, nutrient deprivation and oxidative stresses. Fluctuations in the light quality and intensity also trigger variable accumulation of SLs, indicating their potential in regulating light stress as well. Though the exact roles of SLs have not yet been characterized, it is predicted that they possibly induce the expression of downstream osmolytes to maintain metabolic homeostasis in the stressed cells. Thus, exogenous treatments or transgenic approaches for higher SL bioaccumulation can be potential strategies for developing multiple abiotic stress tolerance in crops and plants.

Plant Responses to Light Stress: Oxidative Damages, Photoprotection, and Role of Phytohormones

Light stress is the most uncharacterized and less studied among the various types of abiotic stresses experienced by the plant systems. Plants, being sessile organisms, cannot escape from such stresses, and one of the mechanisms of adaptation under such hostile circumstances is mediated through the altered regulation of phytohormones. In this book chapter, we have presented an exhaustive literature-based study on the different kinds of light stress encompassing light quality and type, the basic mechanism of perception of UV-B (the most harmful) rays by the plant system, the general metabolites which get upregulated under stress, and then a detailed excerpt on the role of phytohormones like auxin, gibberellic acid, cytokinins, ethylene, and abscisic acid under such conditions. Based on this account, our chapter also aims at integrating the perception of light stress-signaling pathway with the phytohormone-signaling networks, thus providing the idea of a universal cross talk occurring in plant cells, exposed to a variety of light stresses.

Hydrogen sulphide trapeze: Environmental stress amelioration and phytohormone crosstalk

Hydrogen sulphide (H2S) is recognized as the third endogenous gasotransmitter in plants after nitric oxide (NO) and carbon monoxide (CO). Though initially visualized as a toxic gaseous molecule, recent studies have illustrated its diverse role in regulating plant growth and developmental physiology. H2S is also a potent inducer of osmolytes and cellular antioxidants of enzymatic and non-enzymatic origins. It interacts with the Ca2+ and NO signaling pathways. Exogenous fumigation of H2S or application of the H2S donor, sodium hydrosulphide (NaHS) has been found to be beneficial in the amelioration of multiple abiotic stresses like salinity, drought, temperature, hypoxia and heavy metal toxicity. H2S also protects stress-sensitive proteins via persulphidation of cysteine residues, prone to reactive oxygen species (ROS)-mediated oxidation. It is well established that plants are highly dependent on phytohormone signaling during any physiological process. By virtue of the diversity of the H2S-mediated signaling network, interactions and crosstalks of this gasotransmitter with the plant hormones are evident. This article presents a detailed summary regarding the role of H2S in oxidative and environmental stress tolerance; and furthermore illustrates the reported interactions with crucial hormones like abscisic acid, auxins, gibberellic acid, ethylene and salicylic acid under physiologically differing circumstances.