Drying without senescence in resurrection plants

Trehalose Accumulation Triggers Autophagy during Plant Desiccation

Global climate change, increasingly erratic weather and a burgeoning global population are significant threats to the sustainability of future crop production. There is an urgent need for the development of robust measures that enable crops to withstand the uncertainty of climate change whilst still producing maximum yields. Resurrection plants possess the unique ability to withstand desiccation for prolonged periods, can be restored upon watering and represent great potential for the development of stress tolerant crops. Here, we describe the remarkable stress characteristics of Tripogon loliiformis, an uncharacterised resurrection grass and close relative of the economically important cereals, rice, sorghum, and maize. We show that T. loliiformis survives extreme environmental stress by implementing autophagy to prevent Programmed Cell Death. Notably, we identified a novel role for trehalose in the regulation of autophagy in T.loliiformis. Transcriptome, Gas Chromatography Mass Spectrometry, immunoblotting and confocal microscopy analyses directly linked the accumulation of trehalose with the onset of autophagy in dehydrating and desiccated T. loliiformis shoots. These results were supported in vitro with the observation of autophagosomes in trehalose treated T. loliiformis leaves; autophagosomes were not detected in untreated samples. Presumably, once induced, autophagy promotes desiccation tolerance in T.loliiformis, by removal of cellular toxins to suppress programmed cell death and the recycling of nutrients to delay the onset of senescence. These findings illustrate how resurrection plants manipulate sugar metabolism to promote desiccation tolerance and may provide candidate genes that are potentially useful for the development of stress tolerant crops.

Heat shock transcription factors involved in seed desiccation tolerance and longevity retard vegetative senescence in transgenic tobacco

MAIN CONCLUSION

Transcription factors normally expressed in sunflower seeds delayed vegetative senescence induced by severe stress in transgenic tobacco. This revealed a novel connection between seed heat shock factors, desiccation tolerance and vegetative longevity. HaHSFA9 and HaHSFA4a coactivate a genetic program that, in sunflower (Helianthus annuus L.), contributes to seed longevity and desiccation tolerance. We have shown that overexpression of HaHSFA9 in transgenic tobacco seedlings resulted in tolerance to drastic dehydration and oxidative stress. Overexpression of HaHSFA9 alone was linked to a remarkable protection of the photosynthetic apparatus. In addition, the combined overexpression of HaHSFA9 and HaHSFA4a enhanced all these stress-resistance phenotypes. Here, we find that HaHSFA9 confers protection against damage induced by different stress conditions that accelerate vegetative senescence during different stages of development. Seedlings and plants that overexpress HaHSFA9 survived lethal treatments of dark-induced senescence. HaHSFA9 overexpression induced resistance to effects of culture under darkness for several weeks. Only some homoiochlorophyllous resurrection plants are able to withstand this experimental severe stress condition. The combined overexpression of HaHSFA9 and HaHSFA4a did not result in further slowing of dark-induced seedling senescence. However, combined expression of the two transcription factors caused improved recovery of the photosynthetic organs of seedlings after lethal dark treatments. At later stages of vegetative development, HaHSFA9 delayed the appearance of senescence symptoms in leaves of plants grown under normal illumination. This delay was observed under either control or stress treatments. Thus, HaHSFA9 delayed both natural and stress-induced leaf senesce. These novel observations connect transcription factors involved in desiccation tolerance with leaf longevity.

Role of microRNAs in plant drought tolerance

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.

Photoprotection conferred by changes in photosynthetic protein levels and organization during dehydration of a homoiochlorophyllous resurrection plant

During desiccation, homoiochlorophyllous resurrection plants retain most of their photosynthetic apparatus, allowing them to resume photosynthetic activity quickly upon water availability. These plants rely on various mechanisms to prevent the formation of reactive oxygen species and/or protect their tissues from the damage they inflict. In this work, we addressed the issue of how homoiochlorophyllous resurrection plants deal with the problem of excessive excitation/electron pressures during dehydration using Craterostigma pumilum as a model plant. To investigate the alterations in the supramolecular organization of photosynthetic protein complexes, we examined cryoimmobilized, freeze-fractured leaf tissues using (cryo)scanning electron microscopy. These examinations revealed rearrangements of photosystem II (PSII) complexes, including a lowered density during moderate dehydration, consistent with a lower level of PSII proteins, as shown by biochemical analyses. The latter also showed a considerable decrease in the level of cytochrome f early during dehydration, suggesting that initial regulation of the inhibition of electron transport is achieved via the cytochrome b6f complex. Upon further dehydration, PSII complexes are observed to arrange into rows and semicrystalline arrays, which correlates with the significant accumulation of sucrose and the appearance of inverted hexagonal lipid phases within the membranes. As opposed to PSII and cytochrome f, the light-harvesting antenna complexes of PSII remain stable throughout the course of dehydration. Altogether, these results, along with photosynthetic activity measurements, suggest that the protection of retained photosynthetic components is achieved, at least in part, via the structural rearrangements of PSII and (likely) light-harvesting antenna complexes into a photochemically quenched state.

In vivo spectroscopy and NMR metabolite fingerprinting approaches to connect the dynamics of photosynthetic and metabolic phenotypes in resurrection plant Haberlea rhodopensis during desiccation and recovery

The resurrection plant Haberlea rhodopensis was used to study dynamics of drought response of photosynthetic machinery parallel with changes in primary metabolism. A relation between leaf water content and photosynthetic performance was established, enabling us to perform a non-destructive evaluation of the plant water status during stress. Spectroscopic analysis of photosynthesis indicated that, at variance with linear electron flow (LEF) involving photosystem (PS) I and II, cyclic electron flow around PSI remains active till almost full dry state at the expense of the LEF, due to the changed protein organization of photosynthetic apparatus. We suggest that, this activity could have a photoprotective role and prevent a complete drop in adenosine triphosphate (ATP), in the absence of LEF, to fuel specific energy-dependent processes necessary for the survival of the plant, during the late states of desiccation. The NMR fingerprint shows the significant metabolic changes in several pathways. Due to the declining of LEF accompanied by biosynthetic reactions during desiccation, a reduction of the ATP pool during drought was observed, which was fully and quickly recovered after plants rehydration. We found a decline of valine accompanied by lipid degradation during stress, likely to provide alternative carbon sources for sucrose accumulation at late stages of desiccation. This accumulation, as well as the increased levels of glycerophosphodiesters during drought stress could provide osmoprotection to the cells.