Annamaria Genga

Plant MYB Transcription Factors: Their Role in Drought Response Mechanisms.

Water scarcity is one of the major causes of poor plant performance and limited crop yields worldwide and it is the single most common cause of severe food shortage in developing countries. Several molecular networks involved in stress perception, signal transduction and stress responses in plants have been elucidated so far. Transcription factors are major players in water stress signaling. In recent years, different MYB transcription factors, mainly in Arabidopsis thaliana (L.) Heynh. but also in some crops, have been characterized for their involvement in drought response. For some of them there is evidence supporting a specific role in response to water stress, such as the regulation of stomatal movement, the control of suberin and cuticular waxes synthesis and the regulation of flower development. Moreover, some of these genes have also been characterized for their involvement in other abiotic or biotic stresses, an important feature considering that in nature, plants are often simultaneously subjected to multiple rather than single environmental perturbations. This review summarizes recent studies highlighting the role of the MYB family of transcription factors in the adaptive responses to drought stress. The practical application value of MYBs in crop improvement, such as stress tolerance engineering, is also discussed.

Transcriptional Basis of Drought-Induced Susceptibility to the Rice Blast Fungus Magnaporthe oryzae

Plants are often facing several stresses simultaneously. Understanding how they react and the way pathogens adapt to such combinational stresses is poorly documented. Here, we developed an experimental system mimicking field intermittent drought on rice followed by inoculation by the pathogenic fungus Magnaporthe oryzae. This experimental system triggers an enhancement of susceptibility that could be correlated with the dampening of several aspects of plant immunity, namely the oxidative burst and the transcription of several pathogenesis-related genes. Quite strikingly, the analysis of fungal transcription by RNASeq analysis under drought reveals that the fungus is greatly modifying its virulence program: genes coding for small secreted proteins were massively repressed in droughted plants compared to unstressed ones whereas genes coding for enzymes involved in degradation of cell-wall were induced. We also show that drought can lead to the partial breakdown of several major resistance genes by affecting R plant gene and/or pathogen effector expression. We propose a model where a yet unknown plant signal can trigger a change in the virulence program of the pathogen to adapt to a plant host that was affected by drought prior to infection.

OsMADS26 negatively regulates resistance to pathogens and drought tolerance in rice.

A MADS-box transcription factor represses resistance to pathogenic microorganisms and water deficit, and its down-regulation results in improved biotic and abiotic stress tolerance of rice. Functional analyses of MADS-box transcription factors in plants have unraveled their role in major developmental programs (e.g. flowering and floral organ identity) as well as stress-related developmental processes, such as abscission, fruit ripening, and senescence. Overexpression of the rice (Oryza sativa) MADS26 gene in rice has revealed a possible function related to stress response. Here, we show that OsMADS26-down-regulated plants exhibit enhanced resistance against two major rice pathogens: Magnaporthe oryzae and Xanthomonas oryzae. Despite this enhanced resistance to biotic stresses, OsMADS26-down-regulated plants also displayed enhanced tolerance to water deficit. These phenotypes were observed in both controlled and field conditions. Interestingly, alteration of OsMADS26 expression does not have a strong impact on plant development. Gene expression profiling revealed that a majority of genes misregulated in overexpresser and down-regulated OsMADS26 lines compared with control plants are associated to biotic or abiotic stress response. Altogether, our data indicate that OsMADS26 acts as an upstream regulator of stress-associated genes and thereby, a hub to modulate the response to various stresses in the rice plant.

Plant Metabolomics: A Characterisation of Plant Responses to Abiotic Stresses

As with all organisms, plants thrive within a range of environmental conditions that are optimal for their growth and development. They must, however, respond and adapt to conditions that deviate from the optimal, such as low/high temperature, dehydration, high salinity, oxidative stress, heavy metals and nutrient deficiency; these deviations are often responsible for losses in productivity and for spatial (geographical) and temporal (growing season) limitations in the cultivation of crops. Although plants and animals share some responsive mechanisms to unfavourable environmental conditions, plants, as sessile organisms, have developed highly sophisticated and efficient strategies of response. Because of the great interest for both basic and applied research, many scientific endeavours have long addressed the understanding of the mechanisms underlying the stress response and the identification of the specific genes/metabolites that are responsible for tolerance phenotypes. In recent years, the “omics” approaches have allowed high-throughput analyses of the changes that are induced by environmental stresses, confirming data previously obtained with targeted analysis and extending the scope of investigation. It is noteworthy that the metabolomic changes that have been observed in plants subjected to stress conditions depend on different causes; therefore, they have different significance and are expected to differently correlate with tolerance/sensitivity phenotypes. Namely, changes in the metabolome composition due to adverse environmental conditions may depend on i) the stability and catalytic activity of enzymes involved in the production/degradation of specific metabolites, ii) the production of abnormal compounds (or abnormal concentrations of normal compounds) as a result of cell damage, iii) the adjustment of concentration of some metabolites to restore homeostasis and normal metabolic fluxes and iv) the synthesis and/or accumulation of compounds involved in mediating tolerance mechanisms. The main goal of studying metabolic changes during stress responses is to identify metabolites belonging to the (iii) and (iv) groups that are responsible for stress tolerance. Upon exposure to osmotic stress as a result of low temperature, drought and high salinity, plants accumulate a range of osmolytes with the primary function of turgor maintenance.

The rice Mybleu transcription factor increases tolerance to oxygen deprivation in Arabidopsis plants

Mybleu is a natural incomplete transcription factor of rice (Oryza sativa), consisting of a partial Myb repeat followed by a short leucine zipper. We previously showed its localization to the apical region of rice roots and coleoptiles. Specifically, in coleoptiles, Mybleu is expressed under both aerobic and anaerobic conditions, whereas in roots, it is expressed only under aerobic conditions. Mybleu is able to dimerize with canonical leucine zippers and to activate transcription selectively. To investigate Mybleu function in vivo, we transformed Arabidopsis thaliana and evaluated several morphological, physiological and biochemical parameters. In agreement with a hypothesized role of Mybleu in cell elongation in the differentiation zone, we found that the constitutive expression of this transcription factor in Arabidopsis induced elongation in the primary roots and in the internodal region of the floral stem; we also observed a modification of the root apex morphology in transformed lines. Based on the high expression of Mybleu in anaerobic rice coleoptiles, we studied the role of this transcription factor in transgenic plants grown under low-oxygen conditions. We found that overexpression of this transcription factor increased tolerance to oxygen deficit. In transgenic plants, this effect may depend both on the maintenance of a higher metabolism during stress and on the higher expression levels of certain genes involved in the anaerobic response.

Hd3a, RFT1 and Ehd1 integrate photoperiodic and drought stress signals to delay the floral transition in rice

Plants show a high degree of developmental plasticity in response to external cues, including day length and environmental stress. Water scarcity in particular can interfere with photoperiodic flowering, resulting in the acceleration of the switch to reproductive growth in several species, a process called drought escape. However, other strategies are possible and drought stress can also delay flowering, albeit the underlying mechanisms have never been addressed at the molecular level. We investigated these interactions in rice, a short day species in which drought stress delays flowering. A protocol that allows the synchronization of drought with the floral transition was set up to profile the transcriptome of leaves subjected to stress under distinct photoperiods. We identified clusters of genes that responded to drought differently depending on day length. Exposure to drought stress under floral-inductive photoperiods strongly reduced transcription of EARLY HEADING DATE 1 (Ehd1), HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1), primary integrators of day length signals, providing a molecular connection between stress and the photoperiodic pathway. However, phenotypic and transcriptional analyses suggested that OsGIGANTEA (OsGI) does not integrate drought and photoperiodic signals as in Arabidopsis, highlighting molecular differences between long and short day model species.

Analysis of transcript and metabolite levels in Italian rice (Oryza sativa L.) cultivars subjected to osmotic stress or benzothiadiazole treatment

One of the major objectives of rice (Oryza sativa L.) breeding programs is the development of new varieties with higher tolerance/resistance to both abiotic and biotic stresses. In this study, Italian rice cultivars were subjected to osmotic stress or benzothiadiazole (BTH) treatments. An analysis of the expression of selected genes known to be involved in the stress response and (1)H nuclear magnetic resonance ((1)H NMR) metabolic profiling were combined with multivariate statistical analyses to elucidate potential correlations between gene expression or metabolite content and observed tolerant/resistant phenotypes. We observed that the expression of three chosen genes (two WRKY genes and one peroxidase encoding gene) differed between susceptible and resistant cultivars in response to BTH treatments. Moreover, the analysis of metabolite content, in particular in the osmotic stress experiment, enabled discrimination between selected cultivars based on differences in the accumulation of some primary metabolites, primarily sugars. This research highlights the potential usefulness of this approach to characterise rice varieties based on transcriptional or metabolic changes due to adverse environmental conditions.

Ectopic expression of a rice transcription factor, Mybleu, enhances tolerance of transgenic plants of Carrizo citrange to low oxygen stress

Oxygen deficit, which occurs in flooded or poorly drained soils, can limit plant growth and development. Low-oxygen environmental conditions also limit the distribution of many woody plants, such as citrus trees, which are considered flood-sensitive crops, although tolerance to this stress varies among genotypes and rootstocks. In this study, the rice transcription factor Mybleu was inserted into the pGA470 plant cloning vector and transferred into the epicotyl explants of the Carrizo citrange rootstock (Citrus sinensis × Poncirus trifoliata) using Agrobacterium tumefaciens-mediated transformation. The transgenic lines were confirmed for the presence and expression of the transgene, and physiological, biochemical and molecular parameters were evaluated for adaptation to hypoxic and anoxic stress conditions. The ectopic expression of Mybleu increased tolerance to oxygen deprivation in the transgenic lines, contributing to increased viability under this stress condition. This improved tolerance correlates with, and may depend on, the induction of genes and the activation of enzymes from various fermentation and carbohydrate metabolic pathways, antioxidant systems and nonsymbiotic haemoglobin-nitric oxide homeostasis mechanisms. Together, our data suggest a key role for Mybleu in coordinating the multifaceted plant response to low oxygen stress and the conservation of Mybleu-regulated pathways among species.