Mutation in Arabidopsis mitochondrial Pentatricopeptide repeat 40 gene affects tolerance to water deficit

MAIN CONCLUSION

The Arabidopsis Pentatricopeptide repeat 40 (PPR40) insertion mutants have increased tolerance to water deficit compared to wild-type plants. Tolerance is likely the consequence of ABA hypersensitivity of the mutants. Plant growth and development depend on multiple environmental factors whose alterations can disrupt plant homeostasis and trigger complex molecular and physiological responses. Water deficit is one of the factors which can seriously restrict plant growth and viability. Mitochondria play an important role in cellular metabolism, energy production, and redox homeostasis. During drought and salinity stress, mitochondrial dysfunction can lead to ROS overproduction and oxidative stress, affecting plant growth and survival. Alternative oxidases (AOXs) and stabilization of mitochondrial electron transport chain help mitigate ROS damage. The mitochondrial Pentatricopeptide repeat 40 (PPR40) protein was implicated in stress regulation as ppr40 mutants were found to be hypersensitive to ABA and high salinity during germination. This study investigated the tolerance of the knockout ppr40-1 and knockdown ppr40-2 mutants to water deprivation. Our results show that these mutants display an enhanced tolerance to water deficit. The mutants had higher relative water content, reduced level of oxidative damage, and better photosynthetic parameters in water-limited conditions compared to wild-type plants. ppr40 mutants had considerable differences in metabolic profiles and expression of a number of stress-related genes, suggesting important metabolic reprogramming. Tolerance to water deficit was also manifested in higher survival rates and alleviated growth reduction when watering was suspended. Enhanced sensitivity to ABA and fast stomata closure was suggested to lead to improved capacity for water conservation in such environment. Overall, this study highlights the importance of mitochondrial functions and in particular PPR40 in plant responses to abiotic stress, particularly drought.

The zinc finger protein 3 of Arabidopsis thaliana regulates vegetative growth and root hair development

Introduction

Zinc finger protein 3 (ZFP3) and closely related C2H2 zinc finger proteins have been identified as regulators of abscisic acid signals and photomorphogenic responses during germination. Whether ZFP3 and related ZFP factors regulate plant development is, however, not known.

Results

ZFP3 overexpression reduced plant growth, limited cell expansion in leaves, and compromised root hair development. The T-DNA insertion zfp3 mutant and transgenic lines with silenced ZFP1, ZFP3, ZFP4, and ZFP7 genes were similar to wild-type plants or had only minor differences in plant growth and morphology, probably due to functional redundancy. RNAseq transcript profiling identified ZFP3-controlled gene sets, including targets of ABA signaling with reduced transcript abundance. The largest gene set that was downregulated by ZFP3 encoded regulatory and structural proteins in cell wall biogenesis, cell differentiation, and root hair formation. Chromatin immunoprecipitation confirmed ZFP3 binding to several target promoters.

Discussion

Our results suggest that ZFP3 and related ZnF proteins can modulate cellular differentiation and plant vegetative development by regulating the expression of genes implicated in cell wall biogenesis.

Small paraquat resistance proteins modulate paraquat and ABA responses and confer drought tolerance to overexpressing Arabidopsis plants

Adaptation of higher plants to extreme environmental conditions is under complex regulation. Several small peptides have recently been described to modulate responses to stress conditions. The Small Paraquat resistance protein (SPQ) of Lepidium crassifolium has previously been identified due to its capacity to confer paraquat resistance to overexpressing transgenic Arabidopsis plants. Here, we show that overexpression of the closely related Arabidopsis SPQ can also enhance resistance to paraquat, while the Arabidopsis spq1 mutant is slightly hypersensitive to this herbicide. Besides being implicated in paraquat response, overexpression of SPQs enhanced sensitivity to abscisic acid (ABA), and the knockout spq1 mutant was less sensitive to ABA. Both Lepidium- and Arabidopsis-derived SPQs could improve drought tolerance by reducing water loss, stabilizing photosynthetic electron transport and enhancing plant viability and survival in a water-limited environment. Enhanced drought tolerance of SPQ-overexpressing plants could be confirmed by characterizing various parameters of growth, morphology and photosynthesis using an automatic plant phenotyping platform with RGB and chlorophyll fluorescence imaging. Our results suggest that SPQs can be regulatory small proteins connecting ROS and ABA regulation and through that influence responses to certain stresses.

Genome-wide identification of the glutathione transferase superfamily in the model organism Brachypodium distachyon

The detoxification of harmful metabolites can determine the effectiveness of plant stress responses. Scavenging some of these toxic stress by-products through the reduced form of glutathione is catalysed by members of the glutathione transferase (GST) enzyme superfamily. The involvement of these enzymes was studied in the model organism Brachypodium distachyon (L.)P.Beauv. Bd21 and in its derivative Bd21-3, a more drought tolerant line. Osmotic stress treatment resulted in a decrease in the water potential of both Brachypodium genotypes, the difference between the control and treated plant's ψw decreased by the last sampling day in Bd21-3, suggesting some degree of adaptation to the applied osmotic stress. Increased GST activity revealed a severe defence reaction against the harmful imbalance of the redox environment. Screening for the gene sequences led to the identification of 91 full-length or partial GST sequences. Although purple false brome has a relatively small genome, the number of identified GST genes was almost as high as the number predicted in wheat. The estimation of GST expression showed stress-induced differences: higher expression levels or the fast induction of BdGSTF8, BdGSTU35 and BdGSTU42 gene products presumably indicate a strong detoxification under osmotic stress.

Physiological and molecular responses to heavy metal stresses suggest different detoxification mechanism of Populus deltoides and P. x canadensis

Plants have divergent defense mechanisms against the harmful effects of heavy metals present in excess in soils and groundwaters. Poplars (Populus spp.) are widely cultivated because of their rapid growth and high biomass production, and members of the genus are increasingly used as experimental model organisms of trees and for phytoremediation purposes. Our aim was to investigate the copper and zinc stress responses of three outstanding biomass producer bred poplar lines to identify such transcripts of genes involved in the detoxification mechanisms, which can play an important role in the protection against heavy metals. Poplar cuttings were grown hydroponically and subjected to short-term (one week) mild and sublethal copper and zinc stresses. We evaluated the effects of the applied heavy metals and the responses of plants by detecting the changes of multiple physiological and biochemical parameters. The most severe cellular oxidative damage was caused by 30μM copper treatment, while zinc was less harmful. Analysis of stress-related transcripts revealed genotype-specific differences that are likely related to alterations in heavy metal tolerance. P. deltoides clones B-229 and PE 19/66 clones were clearly more effective at inducing the expression of various genes implicated in the detoxification process, such as the glutathione transferases, metallothioneins, ABC transporters, (namely PtGSTU51, PxMT1, PdABCC2,3), while the P. canadensis line M-1 accumulated more metal, resulting in greater cellular oxidative damage. Our results show that all three poplar clones are efficient in stress acclimatization, but with different molecular bases.

Isohydric and anisohydric strategies of wheat genotypes under osmotic stress: biosynthesis and function of ABA in stress responses

Changes in water potential (ψw), stomatal conductance, abscisic acid (ABA) accumulation, expression of the major genes involved in ABA biosynthesis, activities of abscisic aldehyde oxidase (AO, EC 1.2.3.1) and antioxidant enzymes were studied in two wheat cultivars with contrasting acclimation strategies subjected to medium strength osmotic stress (-0.976MPa) induced by polyethylene glycol (PEG 6000). Because the biosynthetic pathway of ABA involves multiple gene products, the aim of this study was to unravel how these genes are regulated in isohydric and anisohydric wheat genotypes. In the root tissues of the isohydric cultivar, Triticum aestivum cv. Kobomugi, osmotic stress increased the transcript levels of 9-cis-epoxycarotenoid dioxygenase (NCED) gene, controlling the rate limiting step of ABA biosynthesis. Moreover, this cultivar exhibited a higher basal activity and a higher induction of aldehyde oxidase isoenzymes (AAO2-AAO3), responsible for converting ABAldehyde to ABA. It was found that the fast activation of the ABA biosynthesis in the roots generated an enhanced ABA pool in the shoot, which brought about a faster closure of the stomata upon increasing osmotic stress and, as a result, the plants could maintain ψw in the tissues close to the control level. In contrast, the anisohydric genotype, cv. GK Öthalom, exhibited a moderate induction of ABA biosynthesis in the roots, leading to the maintenance but no increase in the concentration of ABA on the basis of tissue water content in the leaves. Due to the slower response of their stomata to water deficit, the tissues of cv. GK Öthalom have to acclimate to much more negative water potentials during increasing osmotic stress. A decreased activity of superoxide dismutase (SOD) was found in the leaves and roots of both cultivars exposed to osmotic stress, but in the roots elevated activities of catalase (CAT), peroxidase (POX), glutathione reductase (GR) and glutathione transferase (GST) were detected in the isohydric cultivar, suggesting that this genotype was more successful in the elimination of reactive oxygen species caused by the stress conditions.

The roles of ABA, reactive oxygen species and nitric oxide in root growth during osmotic stress in wheat: comparison of a tolerant and a sensitive variety

The effects of PEG 6000-induced osmotic stress (-0.976 MPa) on the root growth of young plants, and the changes in abscisic acid (ABA), reactive oxygen species (ROS) and NO contents were investigated in the root tips of a drought-tolerant and a drought-sensitive wheat cultivar (Triticum aestivum L. cvs. MV Emese and GK Élet, respectively). The root length of cv. MV Emese was more effectively reduced than that of GK Élet by osmotic stress. Concomitantly, the ABA content of the 15-mm apical zone of the roots remained at the control level in GK Élet cultivar, but in MV Emese it decreased significantly after the early phase of the experiment, indicating that the accumulation of ABA is necessary for the maintenance of root growth under osmotic stress. The extent of ROS accumulation relative to the respective control was more pronounced in the elongation zone of roots in MV Emese in the later stages of the experiment, while NO concentrations increased significantly early after PEG exposure, suggesting that high concentrations of ROS and NO were unfavourable for root expansion. In contrast, in cv. Élet, the high NO content in the elongation zone declined to the control level under osmotic stress within 4 days. The changes in root growth due to osmotic stress did not exhibit a correlation with the drought tolerance of the genotypes defined on the basis of the crop yield.