The Ralstonia solanacearum Type III Effector RipAW Targets the Immune Receptor Complex to Suppress PAMP-Triggered Immunity.

Bacterial wilt, caused by Ralstonia solanacearum, one of the most destructive phytopathogens, leads to significant annual crop yield losses. Type III effectors (T3Es) mainly contribute to the virulence of R. solanacearum, usually by targeting immune-related proteins. Here, we clarified the effect of a novel E3 ubiquitin ligase (NEL) T3E, RipAW, from R. solanacearum on pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and further explored its action mechanism. In the susceptible host Arabidopsis thaliana, we monitored the expression of PTI marker genes, flg22-induced ROS burst, and callose deposition in RipAW- and RipAWC177A-transgenic plants. Our results demonstrated that RipAW suppressed host PTI in an NEL-dependent manner. By Split-Luciferase Complementation, Bimolecular Fluorescent Complimentary, and Co-Immunoprecipitation assays, we further showed that RipAW associated with three crucial components of the immune receptor complex, namely FLS2, XLG2, and BIK1. Furthermore, RipAW elevated the ubiquitination levels of FLS2, XLG2, and BIK1, accelerating their degradation via the 26S proteasome pathway. Additionally, co-expression of FLS2, XLG2, or BIK1 with RipAW partially but significantly restored the RipAW-suppressed ROS burst, confirming the involvement of the immune receptor complex in RipAW-regulated PTI. Overall, our results indicate that RipAW impairs host PTI by disrupting the immune receptor complex. Our findings provide new insights into the virulence mechanism of R. solanacearum.

COP1 Mediates Dark-Induced Stomatal Closure by Suppressing FT, TSF and SOC1 Expression to Promote NO Accumulation in Arabidopsis Guard Cells.

RING-finger-type ubiquitin E3 ligase Constitutively Photomorphogenic 1 (COP1) and floral integrators such as FLOWERING LOCUS T (FT), TWIN SISTER OF FT (TSF) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) have been identified as regulators of stomatal movement. However, little is known about their roles and relationship in dark-induced stomatal closure. Here, we demonstrated that COP1 is required for dark-induced stomatal closure using cop1 mutant. The cop1 mutant closed stomata in response to exogenous nitric oxide (NO) but not hydrogen peroxide (H2O2), and H2O2 but not NO accumulated in cop1 in darkness, further indicating that COP1 acts downstream of H2O2 and upstream of NO in dark-induced stomatal closure. Expression of FT, TSF and SOC1 in wild-type (WT) plants decreased significantly with dark duration time, but this process was blocked in cop1. Furthermore, ft, tsf, and soc1 mutants accumulated NO and closed stomata faster than WT plants in response to darkness. Altogether, our results indicate that COP1 transduces H2O2 signaling, promotes NO accumulation in guard cells by suppressing FT, TSF and SOC1 expression, and consequently leads to stomatal closure in darkness. These findings add new insights into the mechanisms of dark-induced stomatal closure.

Ethylene Acts as a Local and Systemic Signal to Mediate UV-B-Induced Nitrate Reallocation to Arabidopsis Leaves and Roots via Regulating the ERFs-NRT1.8 Signaling Module.

Nitrate is the preferred nitrogen source for plants and plays an important role in plant growth and development. Under various soil stresses, plants reallocate nitrate to roots to promote stress tolerance through the ethylene-ethylene response factors (ERFs)-nitrate transporter (NRT) signaling module. As a light signal, ultraviolet B (UV-B) also stimulates the production of ethylene. However, whether UV-B regulates nitrate reallocation in plants via ethylene remains unknown. Here, we found that UV-B-induced expression of ERF1B, ORA59, ERF104, and NRT1.8 in both Arabidopsis shoots and roots as well as nitrate reallocation from hypocotyls to leaves and roots were impaired in ethylene signaling mutants for Ethylene Insensitive2 (EIN2) and EIN3. UV-B-induced NRT1.8 expression and nitrate reallocation to leaves and roots were also inhibited in the triple mutants for ERF1B, ORA59, and ERF104. Deletion of NRT1.8 impaired UV-B-induced nitrate reallocation to both leaves and roots. Furthermore, UV-B promoted ethylene release in both shoots and roots by enhancing the gene expression and enzymatic activities of ethylene biosynthetic enzymes only in shoots. These results show that ethylene acts as a local and systemic signal to mediate UV-B-induced nitrate reallocation from Arabidopsis hypocotyls to both leaves and roots via regulating the gene expression of the ERFs-NRT1.8 signaling module.

Hydrogen peroxide as a mediator of 5-aminolevulinic acid-induced Na+ retention in roots for improving salt tolerance of strawberries.

To explore the mechanisms of 5-aminolevulinic acid (ALA)-improved plant salt tolerance, strawberries (Fragaria × ananassa Duch. cv. 'Benihoppe') were treated with 10 mg l-1 ALA under 100 mmol l-1 NaCl stress. We found that the amount of Na+ increased in the roots but decreased in the leaves. Laser scanning confocal microscopy (LSCM) observations showed that ALA-induced roots had more Na+ accumulation than NaCl alone. Measurement of the xylem sap revealed that ALA repressed Na+ concentrations to a large extent. The electron microprobe X-ray assay also confirmed ALA-induced Na+ retention in roots. qRT-PCR showed that ALA upregulated the gene expressions of SOS1 (encoding a plasma membrane Na+ /H+ antiporter), NHX1 (encoding a vacuolar Na+ /H+ antiporter) and HKT1 (encoding a protein of high-affinity K+ uptake), which are associated with Na+ exclusion in the roots, Na+ sequestration in vacuoles and Na+ unloading from the xylem vessels to the parenchyma cells, respectively. Furthermore, we found that ALA treatment reduced the H2 O2 content in the leaves but increased it in the roots. The exogenous H2 O2 promoted plant growth, increased root Na+ retention and stimulated the gene expressions of NHX1, SOS1 and HKT1. Diphenyleneiodonium (DPI), an inhibitor of H2 O2 generation, suppressed the effects of ALA or H2 O2 on Na+ retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H2 O2 accumulation in roots, which mediates Na+ transporter gene expression and more Na+ retention in roots, thereby improving plant salt tolerance.

[Staged strategy of plants in response to drought stress].

Drought is one of the most important abiotic factors limiting plant growth, development and survival. Due to global warming, the drought episodes in the future will become more frequent and severe. Therefore, the plant responses to drought stress, especially the drought-resistant mechanisms, have been a focus of the studies. This paper summarized the plant growth and physiological responses to drought stress, and proposed a staged response strategy of plants for the adaptation to natural drought stress. Overall, from drought initiation to drought-induced death, plants experienced five stress stages with increasing drought intensity, i. e., drought initiation-mild drought-moderate drought-severe drought-extreme drought, and the plant responsive and adaptive process in this whole event could be acute response-active adaptation to mild and severe drought stresses-passive adaptation to extreme drought stress. The essential function of the plant drought-resistant mechanisms varied with drought stress stages. Some key scientific problems and research directions about the staged response characteristics of plants to drought stress needed to be solved urgently in the future were pointed out.

[Effects of drying and re-watering on the photosynthesis and active oxygen metabolism of Periploca sepium seedlings].

Taking two-year-old Periploca sepium seedlings as test materials, an experiment with controlled soil water contents was conducted to study the effects of repeated drying and re-watering on the leaf photosynthetic characteristics and the lipid peroxidation and antioxidant system in young leaves, mature leaves, old leaves, new stems, and fine roots. The seedlings were subjected to three cycles of drying and re-watering, with regular irrigation to maintain the soil water content at around 80% of field capacity as the control (CK). Under drying, the leaf relative water content (RWC) and net photosynthesis rate (Pn) decreased significantly, while the leaf photosynthetic pigments content increased. When the seedlings were re-watered, their leaf RWC recovered to the CK level, showing a strong repair capacity after drying. Both the leaf chlorophyll content and the Pn after repeated drying and re-watering presented a higher level than those of the CK, indicating a compensatory effect appeared and an appropriate drought stress being able to induce the adaptability of P. sepium to drought stress. Stomatal closure was the main factor limiting P. sepium photosynthesis under drought stress, while non-stomatal limitation only worked at noon. Under drying, the superoxide anion radical (O2-*) production rate in young leaves, new stems, and fine roots increased while the malondialdehyde (MDA) contents decreased, suggesting that these young tissues were not suffered from the oxidative stress. The activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in different organs had different variation trends, with those in fine roots changed actively, suggesting the important role of fine roots in the acclimation of P. sepium to drought environment. It was the cooperation and coordination among plant organs that made P. sepium more adaptive to the repeated drying and wetting conditions in drought-prone regions.