Phosphate Deprivation Can Impair Mechano-Stimulated Cytosolic Free Calcium Elevation in Arabidopsis Roots

The root tip responds to mechanical stimulation with a transient increase in cytosolic free calcium as a possible second messenger. Although the root tip will grow through a heterogeneous soil nutrient supply, little is known of the consequence of nutrient deprivation for such signalling. Here, the effect of inorganic phosphate deprivation on the root’s mechano-stimulated cytosolic free calcium increase is investigated. Arabidopsisthaliana (cytosolically expressing aequorin as a bioluminescent free calcium reporter) is grown in zero or full phosphate conditions, then roots or root tips are mechanically stimulated. Plants also are grown vertically on a solid medium so their root skewing angle (deviation from vertical) can be determined as an output of mechanical stimulation. Phosphate starvation results in significantly impaired cytosolic free calcium elevation in both root tips and whole excised roots. Phosphate-starved roots sustain a significantly lower root skewing angle than phosphate-replete roots. These results suggest that phosphate starvation causes a dampening of the root mechano-signalling system that could have consequences for growth in hardened, compacted soils.

A glutathione-dependent control of the indole butyric acid pathway supports Arabidopsis root system adaptation to phosphate deprivation

Root system architecture results from a highly plastic developmental process to adapt to environmental conditions. In particular, the development of lateral roots and root hair growth are constantly optimized to the rhizosphere properties, including biotic and abiotic constraints. The development of the root system is tightly controlled by auxin, the driving morphogenic hormone in plants. Glutathione, a major thiol redox regulator, is also critical for root development but its interplay with auxin is scarcely understood. Previous work showed that glutathione deficiency does not alter root responses to indole acetic acid (IAA), the main active auxin in plants. Because indole butyric acid (IBA), another endogenous auxinic compound, is an important source of IAA for the control of root development, we investigated the crosstalk between glutathione and IBA during root development. We show that glutathione deficiency alters lateral roots and root hair responses to exogenous IBA but not IAA. Detailed genetic analyses suggest that glutathione regulates IBA homeostasis or conversion to IAA in the root cap. Finally, we show that both glutathione and IBA are required to trigger the root hair response to phosphate deprivation, suggesting an important role for this glutathione-dependent regulation of the auxin pathway in plant developmental adaptation to its environment.

IBA endogenous auxin regulates Arabidopsis root system development in a glutathione-dependent way and is important for adaptation to phosphate deprivation

Root system architecture results from a highly plastic developmental process to perfectly adapt to environmental conditions. In particular, the development of lateral roots (LR) and root hair (RH) growth are constantly optimized to the rhizosphere properties, including biotic and abiotic constraints. Every step of root system development is tightly controlled by auxin, the driving morphogenic hormone in plants. Glutathione, a major thiol redox regulator, is also critical for root system development but its interplay with auxin is still scarcely understood. Indeed, previous works showed that glutathione deficiency does not alter root responses to exogenous indole acetic acid (IAA), the main active auxin in plants. Because indole butyric acid (IBA), another endogenous auxinic compound, is an important source of IAA for the control of root development, we investigated the crosstalk between glutathione and IBA during root development. We show that glutathione deficiency alters LR and RH responses to exogenous IBA but not IAA. Although many efforts have been deployed, we could not identify the precise mechanism responsible for this control. However, we could show that both glutathione and IBA are required for the proper responses of RH to phosphate deprivation, suggesting an important role for this glutathione-dependent regulation of auxin pathway in plant developmental adaptation to its environment.

Transcriptomic responses to thermal stress and varied phosphorus conditions in Symbiodinium kawagutii

Symbiodinium species are essential symbionts of tropical reef-building corals and the disruption of their symbiosis with corals as a consequence of seawater warming and other stress conditions leads to the globally widespread coral bleaching. As coral reefs live in the oligotrophic environment, Symbiodinium photosynthesis can also face nutrient stress. How metabolic pathways in Symbiodinium respond to thermal stress and phosphate depletion is poorly understood and underexplored for many species. Here we conducted RNA-seq analysis to investigate transcriptomic responses to thermal stress, phosphate deprivation and glycerol-3-phosphate (Gro3P) replacement in S. kawagutii. RNA-seq and bioinformatic analysis were conducted for the above-mentioned three treatments and a control. We identified 221 (2.04%) genes showing no significant differential expression among all conditions, and defined them as “core” genes of S. kawagutii, which mostly were in the Gene Ontology terms of catalytic activity and binding. Using algorithms edgeR and NOIseq in combination, we identified a set of differentially expressed genes (DEGs) for each treatment relative to the control. Under heat stress 357 (4.42%) DEGs were found, with predicted roles in active molecular (protein-protein/RNA/DNA) interaction, cell wall modulation and transport (including nutrients, iron, and oxygen). About as many DEGs (396, 4.73%) were identified under P deprivation while nearly double of that (671, 8.05%) were detected under Gro3P utilization; in both cases most of the DEGs were up-regulated and predicted to function in photosystem and defensome. Further KEGG pathway comparison revealed different molecular responses between phosphate deprivation and Gro3P utilization. Catalytic activity and binding seem to be two important core functions in S. kawagutii. The most significant transcriptional response in S. kawagutii to heat stress was regulation of molecular interaction, cell wall modulation, and transport of iron, oxygen, and major nutrients, suggesting that this species uses a unique mechanism to cope with heat stress, possibly conferring thermal tolerance. The greatest transcriptomic impact of phosphate deprivation and Gro3P replacement were the up-regulation of photosystem and defense. This study provides new clues about molecular mechanisms underpinning responses in Symbiodinium to temperature and nutrient stresses, which will generate new hypotheses and set a new framework for future investigations.