Syed Sarfraz Hussain

Polyamines: Natural and engineered abiotic and biotic stress tolerance in plants

Polyamines (PAs) are ubiquitous biogenic amines that have been implicated in diverse cellular functions in widely distributed organisms. In plants, mutant and transgenic plants with altered activity pointed to their involvement with different abiotic and biotic stresses. Furthermore, microarray, transcriptomic and proteomic approaches have elucidated key functions of different PAs in signaling networks in plants subjected to abiotic and biotic stresses, however the exact molecular mechanism remains enigmatic. Here, we argue that PAs should not be taken only as a protective molecule but rather like a double-faced molecule that likely serves as a major area for further research efforts. This review summarizes recent advances in plant polyamine research ranging from transgenic and mutant characterization to potential mechanisms of action during environmental stresses and diseases.

Transcription factors as tools to engineer enhanced drought stress tolerance in plants

Plant growth and productivity are greatly affected by abiotic stresses such as drought, salinity, and temperature. Drought stress is one of the major limitations to crop productivity worldwide due to its multigene nature, making the production of transgenic crops a challenging prospect. To develop crop plant with enhanced tolerance of drought stress, a basic understanding of physiological, biochemical, and gene regulatory networks is essential. In the signal transduction network that leads from the perception of stress signals to the expression of stress‐responsive genes, transcription factors (TFs) play an essential role. Because TFs, as opposed to most structural genes, tend to control multiple pathways steps, they have emerged as powerful tools for the manipulation of complex metabolic pathways in plants. One such class of TFs is DREB/CBF that binds to drought responsive cis‐acting elements. Transgenic plants have been developed with enhanced stress tolerance by manipulating the expression of DREB/CBF. Recently the functions of an increasing number of plant TFs are being elucidated and increased understanding of these factors in controlling drought stress response has lead to practical approaches for engineering stress tolerance in plants. The utility of the various TFs in plant stress research we review is illustrated by several published examples. The manipulation of native plant regularity networks therefore represents a new era for genetically modified crops. This review focuses on the recent understanding, latest advancements related to TFs and present status of their deployment in developing stress tolerant transgenic plants.

Transcriptomes of the desiccation-tolerant resurrection plant Craterostigma plantagineum.

Studies of the resurrection plant Craterostigma plantagineum have revealed some of the mechanisms which these desiccation-tolerant plants use to survive environments with extreme dehydration and restricted seasonal water. Most resurrection plants are polyploid with large genomes, which has hindered efforts to obtain whole genome sequences and perform mutational analysis. However, the application of deep sequencing technologies to transcriptomics now permits large-scale analyses of gene expression patterns despite the lack of a reference genome. Here we use pyro-sequencing to characterize the transcriptomes of C. plantagineum leaves at four stages of dehydration and rehydration. This reveals that genes involved in several pathways, such as those required for vitamin K and thiamin biosynthesis, are tightly regulated at the level of gene expression. Our analysis also provides a comprehensive picture of the array of cellular responses controlled by gene expression that allow resurrection plants to survive desiccation.

Role of microRNAs in plant drought tolerance

Summary Drought is a normal and recurring climate feature in most parts of the world and plays a major role in limiting crop productivity. However, plants have their own defence systems to cope with adverse climatic conditions. One of these defence mechanisms is the reprogramming of gene expression by microRNAs (miRNAs). miRNAs are small noncoding RNAs of approximately 22 nucleotides length, which have emerged as important regulators of genes at post‐transcriptional levels in a range of organisms. Some miRNAs are functionally conserved across plant species and are regulated by drought stress. These properties suggest that miRNA‐based genetic modifications have the potential to enhance drought tolerance in cereal crops. This review summarizes the current understanding of the regulatory mechanisms of plant miRNAs, involvement of plant miRNAs in drought stress responses in barley (Hordeum vulgare L.), wheat (Triticum spp.) and other plant species, and the involvement of miRNAs in plant‐adaptive mechanisms under drought stress. Potential strategies and directions for future miRNA research and the utilization of miRNAs in the improvement of cereal crops for drought tolerance are also discussed.

Beyond osmolytes and transcription factors: drought tolerance in plants via protective proteins and aquaporins

Mechanisms of drought tolerance have been studied by numerous groups, and a broad range of molecules have been identified to play important roles. A noteworthy response of stressed plants is the accumulation of novel protective proteins, including heat-shock proteins (HSPs) and late embryogenesis abundant (LEA) proteins. Identification of gene regulatory networks of these protective proteins in plants will allow a wide application of biotechnology for enhancement of drought tolerance and adaptation. Similarly, aquaporins are involved in the regulation of water transport, particularly under abiotic stresses. The molecular and functional characterization of protective proteins and aquaporins has revealed the significance of their regulation in response to abiotic stresses. Herein, we highlight new findings regarding the action mechanisms of these proteins. Finally, this review also surveys the current advances in engineering drought tolerant plants, particularly the engineering of protective proteins (sHSPs and LEA) and aquaporins for imparting drought stress tolerance in plants.

Resurrection Plants: Physiology and Molecular Biology

The ability to survive desiccation is commonly found in seeds or pollen and it is widespread among bryophytes, but it is rarely present in vegetative tissues. A small group of vascular plants also known as resurrection plants exhibit tolerance to nearly complete desiccation of vegetative tissues. To date more than 300 angiosperm species have been identified that possess this kind of desiccation tolerance. The different problems associated with desiccation require specific mechanisms to be in place. Conservation of structure of membranes and macromolecules is correlated with the synthesis of large amounts of desiccation-induced protective proteins such as late embryogenesis abundant (LEA) proteins, sugars, and reactive oxygen scavengers. The desiccation-induced molecules may have different roles in cellular protection: (1) proteins may conserve structures of macromolecules and membranes, (2) the sugars may be effective in osmotic adjustment and they may stabilize membrane structures and proteins, (3) mechanical damage due to vacuole shrinkage in dehydrating cells is probably avoided by cell wall folding or by replacing the water in vacuoles with non-aqueous substances, and (4) oxidative stress, due to enhanced production of reactive oxygen species (ROS) especially by chloroplasts, is minimized through diverse ROS scavenging molecules. Photochemical activities of resurrection plants are inhibited with loss of water similar to those of non-tolerant plants; however, photosynthesis is rapidly reactivated during rehydration even when plants lost more than 95% water. In this chapter, we describe the molecular and biochemical mechanisms associated with desiccation tolerance as well as morphological and physiological adaptations of resurrection plants. Acquisition of desiccation tolerance requires the coordinated expression of diverse genes, which is achieved through a complex regulatory network. Many of the signalling pathways are mediated via the plant hormone abscisic acid (ABA), a key molecule in the process. Possible evolutionary mechanisms selecting for desiccation-tolerant plants are discussed.

Cotton somatic embryo morphology affects its conversion to plant

Somatic embryos differentiated from hypocotyl explant in cotton (Gossypium hirsutum L.) exhibited very divergent morphologies. Six different types of somatic embryos based on cotyledon development were observed. The growth hormones (2,4-dichlorophenoxyacetic acid and kinetin) used in induction and maintenance media did not affect embryo rooting and germination. The 95 % conversion of normal embryos (with two cotyledons) was achieved, while an overall conversion was only 38 %. Horn shaped embryos failed to exhibit shoot growth. Poorly developed apical meristems were responsible for lower conversion percentages in some of embryo classes. However, regenerated plants phenotypically resembled to seed grown control plants regardless of somatic embryo morphology.

Transgenic plants for abiotic stress tolerance: current status

Abiotic stress is one of the primary causes of crop losses worldwide (Bray et al. 2000. Responses to abiotic stresses. In: Buchanana BB, Gruissem W, Jones RL, editors. Biochemistry and Molecular Biology of Plants. Rockville (MD): American Society of Plant Physiologists. p. 1158–1249). To cope with the detrimental effects of stress, plants have evolved many biochemical and molecular mechanisms. One of the well-documented stress responses in plants is accumulation of osmolytes during stress. Although their actual roles in plant-stress tolerance remain controversial, these molecules are thought to have positive effects on enzyme and membrane integrity, along with adaptive roles in mediating osmotic adjustment in plants grown under drought conditions. Recent studies have demonstrated that the manipulation of genes involved in the biosynthesis of these osmolytes have improved plant tolerance to drought and salinity in a number of crops. There is renewed hope of understanding the molecular basis of osmolyte accumulation under stress and manipulating these processes via genetic engineering. For future work on generating transgenic plants with still higher levels of tolerance, the new knowledge may be used via guided genetic engineering of multiple genes to create crop plants with significantly increased productivity under drought stress. This review surveys the current advances in engineering abiotic stress-tolerant plants, particularly the genetic engineering of osmolyte genes (osmoprotectants) for imparting drought stress tolerance in plants.

Role of miRNAs in Abiotic and Biotic Stresses in Plants

Plants are exposed to a wide array of biotic and abiotic stresses. To cope with these stresses, plants exhibit an integrated molecular response that may eventually result in their adaptation and/or acclimation to the stresses. In the past two decades, miRNAs have emerged as a key player in translational, transcriptional, and post-transcriptional regulation of plant genes important for plant responses to different stresses. However, the role of miRNAs in stress-responsive pathways is not yet known and, furthermore, how these signals and responses are integrated in nature is also unclear. In this chapter, we present current knowledge on miRNA-mediated responses to stresses and emphasize the possibilities of using miRNA-mediated methods for generating stress-tolerant crop plants.

The wheat TabZIP2 transcription factor is activated by the nutrient starvation-responsive SnRK3/CIPK protein kinase

Key messageThe understanding of roles of bZIP factors in biological processes during plant development and under abiotic stresses requires the detailed mechanistic knowledge of behaviour of TFs.AbstractBasic leucine zipper (bZIP) transcription factors (TFs) play key roles in the regulation of grain development and plant responses to abiotic stresses. We investigated the role and molecular mechanisms of function of the TabZIP2 gene isolated from drought-stressed wheat plants. Molecular characterisation of TabZIP2 and derived protein included analyses of gene expression and its target promoter, and the influence of interacting partners on the target promoter activation. Two interacting partners of TabZIP2, the 14-3-3 protein, TaWIN1 and the bZIP transcription factor TaABI5L, were identified in a Y2H screen. We established that under elevated ABA levels the activity of TabZIP2 was negatively regulated by the TaWIN1 protein and positively regulated by the SnRK3/CIPK protein kinase WPK4, reported previously to be responsive to nutrient starvation. The physical interaction between the TaWIN1 and the WPK4 was detected. We also compared the influence of homo- and hetero-dimerisation of TabZIP2 and TaABI5L on DNA binding. TabZIP2 gene functional analyses were performed using drought-inducible overexpression of TabZIP2 in transgenic wheat. Transgenic plants grown under moderate drought during flowering, were smaller than control plants, and had fewer spikes and seeds per plant. However, a single seed weight was increased compared to single seed weights of control plants in three of four evaluated transgenic lines. The observed phenotypes of transgenic plants and the regulation of TabZIP2 activity by nutrient starvation-responsive WPK4, suggest that the TabZIP2 could be the part of a signalling pathway, which controls the rearrangement of carbohydrate and nutrient flows in plant organs in response to drought.