Exogenous abscisic acid alleviates Cd toxicity in Arabidopsis thaliana by inhibiting Cd uptake, translocation and accumulation, and promoting Cd chelation and efflux.

Exogenous abscisic acid (ABA) has been implicated in plant response to cadmium (Cd) stress, but the underlying mechanism remains unclear. In the present study, we found that exogenous ABA application decreased Cd fixation in wild type (WT) root cell wall through reducing the hemicelluloses content, in parallel with the decreased expression of IRT1, ZIP1, ZIP4, HMA2 and HMA4, which are related to Cd uptake and translocation, and the increased expression of PDF2.6, PDR8 and AIT1, which are related to Cd chelation, efflux, and accumulation inhibition. These changes might be associated with the reduced Cd accumulation in roots and shoots and the alleviated Cd toxicity. In contrast, the mutation of ABI4, a transcription factor in ABA signaling pathway, significantly increased the expression of IRT1, ZIP1, ZIP4, HMA2 and HMA4, while decreased the expression of AIT1, PDF2.6 and PDR8, enhancing Cd accumulation in roots and shoots of abi4. The enhanced Cd-sensitivity in abi4 mutant could not be rescued by exogenous ABA addition compared with WT. In a word, we conclude that exogenous ABA mitigates Cd toxicity in Arabidopsis thaliana via inhibiting Cd uptake, translocation and accumulation, promoting Cd chelation and efflux, a pathway that might be regulated by ABI4.

Salicylic Acid Acts Upstream of Auxin and Nitric Oxide (NO) in Cell Wall Phosphorus Remobilization in Phosphorus Deficient Rice.

Salicylic acid (SA) is thought to be involved in phosphorus (P) stress response in plants, but the underlying molecular mechanisms are poorly understood. Here, we showed that P deficiency significantly increased the endogenous SA content by inducing the SA synthesis pathway, especially for up-regulating the expression of PAL3. Furthermore, rice SA synthetic mutants pal3 exhibited the decreased root and shoot soluble P content, indicating that SA is involved in P homeostasis in plants. Subsequently, application of exogenous SA could increase the root and shoot soluble P content through regulating the root and shoot cell wall P reutilization. In addition, - P + SA treatment highly upregulated the expression of P transporters such as OsPT2 and OsPT6, together with the increased xylem P content, suggesting that SA also participates in the translocation of the P from the root to the shoot. Moreover, both signal molecular nitric oxide (NO) and auxin (IAA) production were enhanced when SA is applied while the addition of respective inhibitor c-PTIO (NO scavenger) and NPA (IAA transport inhibitor) significantly decreased the root and shoot cell wall P remobilization in response to P starvation. Taken together, here SA-IAA-NO-cell wall P reutilization pathway has been discovered in P-starved rice.

The Role of Nitric Oxide Signaling in Plant Responses to Cadmium Stress.

Nitric oxide (NO) is a widely distributed gaseous signaling molecule in plants that can be synthesized through enzymatic and non-enzymatic pathways and plays an important role in plant growth and development, signal transduction, and response to biotic and abiotic stresses. Cadmium (Cd) is a heavy metal pollutant widely found in the environment, which not only inhibits plant growth but also enters humans through the food chain and endangers human health. To reduce or avoid the adverse effects of Cd stress, plants have evolved a range of coping mechanisms. Many studies have shown that NO is also involved in the plant response to Cd stress and plays an important role in regulating the resistance of plants to Cd stress. However, until now, the mechanisms by which Cd stress regulates the level of endogenous NO accumulation in plant cells remained unclear, and the role of exogenous NO in plant responses to Cd stress is controversial. This review describes the pathways of NO production in plants, the changes in endogenous NO levels in plants under Cd stress, and the effects of exogenous NO on regulating plant resistance to Cd stress.