Diversity in root apoplastic barrier deposition in salt treated wild and domesticated barley seedlings

Salt stress causes changes in root apoplastic barriers, such as the endodermis and the exodermis, and these changes are associated with variation in abiotic stress tolerance. We explored variation in root apoplastic barrier traits, O consumption and root and shoot Na and K content in a diverse collection of commercial and wild barley accessions subjected to non-saline (control) and saline treatments. Lignin and suberin deposition in endo- and exo-dermal cell walls varied between the accessions and in response to salt treatments. Twenty-two wild barley accessions formed an exodermis in response to salt treatments, whereas the commercial barley cultivar Barke did not develop an obvious exodermis. Accessions with pronounced root barrier deposition tended to have lower O consumption relative to the accessions with less obvious barriers. Treatment with abscisic acid enhanced suberisation and lead to a pronounced formation of an exodermis in wild barley accessions, whereas treatment with an ethylene precursor had no obvious effect on suberisation. Principal component analysis revealed associations between suberin deposition, root and shoot Na and K, and root respiration. The variation in root apoplastic barrier traits within the barley accessions represents a useful resource for future crop breeding to improve environmental stress tolerance.

Overexpression of OsCASP1 Improves Calcium Tolerance in Rice

The Casparian strip domain protein 1 (OsCASP1) is necessary for the formation of the Casparian strip (CS) in the rice endodermis. It also controls Ca2+ transport to the stele. Here, we demonstrated that OsCASP1 overexpression enhanced Ca tolerance in rice. Under normal conditions, OsCASP1-overexpressed lines showed similar concentrations of essential metals in the roots and shoots compared to the wild type, while under high Ca conditions, Ca in the roots, shoots, and xylem sap of the OsCASP1-overexpressed lines was significantly decreased. This did not apply to other essential metals. Ca-inhibited growth was significantly alleviated in the OsCASP1-overexpressed lines. Furthermore, OsCASP1 overexpression resulted in earlier formation of both the CS and functional apoplastic barrier in the endodermis but did not induce ectopic CS formation in non-endodermal cell layers and affect suberin accumulation in the endodermis. These results indicate that the overexpression of OsCASP1 promotes CS formation in endodermal cells and inhibits Ca2+ transport by the apoplastic pathway, restricting Ca accumulation in the roots and shoots under high Ca conditions. Taken together, the results suggest that OsCASP1 overexpression is an effective way to improve rice adaptation to high Ca environments.

CHANGE IN CONTRIBUTION OF AQUAPORINS TO THE HYDRAULIC CONDUCTIVITY OF PLANTS DURING THE FORMATION OF APOPLASTIC BARRIERS UNDER SALINITY

The study of plant adaptation mechanisms during the salt stress is required to provide an increase in plant productivity under such conditions. Along with a decrease in the availability of water for plants, the NaCL-induced inhibition of plant growth is associated with the toxic effect of sodium ions. The formation of apoplastic barriers due to the deposition of suberin and lignin restricts passive ion diffusion. However, the formation of such barriers reduces the capacity of the apoplastic pathway for water movement. In these conditions the role of transmembrane water transport is increased. This process is provided by aquaporin water channels. Thus the purpose of this work was to determine the contribution of aquaporins to hydraulic conductivity of peas plants under salinity-induced apoplastic barrier formation. An only slight decrease in plants transpiration caused by mercury chloride in the absence of salinization was in accordance with the ideas the apoplast is the dominant pathway when the Casparian bands is not formed yet. Salt stress in our experiments accelerated the development of the Casparian bands formation which could be visualized as an appearance of suberin strips in root endodermis which in turn was accompanied by a decrease in hydraulic conductivity. The decrease in hydraulic conductivity in 2 times during the mercury chloride treatment under salinity confirmed that contribution of aquaporins to the total hydraulic conductivity was increased under conditions when Casparian bands have had formed.

CsPrx25, a class III peroxidase in Citrus sinensis, confers resistance to citrus bacterial canker through the maintenance of ROS homeostasis and cell wall lignification

Citrus bacterial canker (CBC) results from Xanthomonas citri subsp. citri (Xcc) infection and poses a grave threat to citrus production. Class III peroxidases (CIII Prxs) are key proteins to the environmental adaptation of citrus plants to a range of exogenous pathogens, but the role of CIII Prxs during plant resistance to CBC is poorly defined. Herein, we explored the role of CsPrx25 and its contribution to plant defenses in molecular detail. Based on the expression analysis, CsPrx25 was identified as an apoplast-localized protein that is differentially regulated by Xcc infection, salicylic acid, and methyl jasmone acid in the CBC-susceptible variety Wanjincheng (C. sinensis) and the CBC-resistant variety Calamondin (C. madurensis). Transgenic Wanjincheng plants overexpressing CsPrx25 were generated, and these transgenic plants exhibited significantly increased CBC resistance compared with the WT plants. In addition, the CsPrx25-overexpressing plants displayed altered reactive oxygen species (ROS) homeostasis accompanied by enhanced H2O2 levels, which led to stronger hypersensitivity responses during Xcc infection. Moreover, the overexpression of CsPrx25 enhanced lignification as an apoplastic barrier for Xcc infection. Taken together, the results highlight how CsPrx25-mediated ROS homeostasis reconstruction and cell wall lignification can enhance the resistance of sweet orange to CBC.

Regulation of CYP94B1 by WRKY33 controls apoplastic barrier formation in the roots leading to salt tolerance

Salinity is an environmental stress that causes decline in crop yield. Avicennia officinalis and other mangroves have adaptations such as ultrafiltration at the roots aided by apoplastic cell-wall barriers to thrive in saline conditions. We studied a Cytochrome P450 gene, AoCYP94B1 from A. officinalis and its Arabidopsis ortholog AtCYP94B1 that are involved in apoplastic barrier formation, and are induced by 30 minutes of salt treatment in the roots. Heterologous expression of AoCYP94B1 in atcyp94b1 Arabidopsis mutant and wild-type rice conferred increased NaCl tolerance to seedlings by enhancing root suberin deposition. Histochemical staining and GC-MS/MS quantification of suberin precursors confirmed the role of CYP94B1 in suberin biosynthesis. Using chromatin immunoprecipitation, yeast one-hybrid and luciferase assays, we identified AtWRKY33 as the upstream regulator of AtCYP94B1 in Arabidopsis. In addition, atwrky33 mutants exhibited reduced suberin and salt sensitive phenotypes, which were rescued by expressing 35S::AtCYP94B1 in atwrky33 mutant. This further confirms that the regulation of AtCYP94B1 by AtWRKY33 is part of the salt tolerance mechanism, and our findings can help in generating salt tolerant crops.One sentence summaryAtWRKY33 transcription factor regulates AtCYP94B1 to increase plant salt tolerance by enhanced suberin deposition in the endodermal cells of Arabidopsis roots

Exodermis and Endodermis Respond to Nutrient Deficiency in Nutrient-Specific and Localized Manner

The exodermis is a common apoplastic barrier of the outer root cortex, with high environmentally-driven plasticity and a protective function. This study focused on the trade-off between the protective advantages provided by the exodermis and its disadvantageous reduction of cortical membrane surface area accessible by apoplastic route, thus limiting nutrient acquisition from the rhizosphere. We analysed the effect of nutrient deficiency (N, P, K, Mg, Ca, K, Fe) on exodermal and endodermal differentiation in maize. To differentiate systemic and localized effects, nutrient deficiencies were applied in three different approaches: to the root system as a whole, locally to discrete parts, or on one side of a single root. Our study showed that the establishment of the exodermis was enhanced in low–N and low–P plants, but delayed in low-K plants. The split-root cultivation proved that the effect is non-systemic, but locally coordinated for individual roots. Within a single root, localized deficiencies didn’t result in an evenly differentiated exodermis, in contrast to other stress factors. The maturation of the endodermis responded in a similar way. In conclusion, N, P, and K deficiencies strongly modulated exodermal differentiation. The response was nutrient specific and integrated local signals of current nutrient availability from the rhizosphere.

Effect of 24-epibrassinolide on localization of ABA, AZP, and dehydrins in wheat plant roots during dehydration

We studied the immunohistochemical localization of abscisic acid (ABA), wheat germ agglutinin (WGA) and dehydrins in the roots of wheat seedlings (Triticum aestivum L.) during 24-epibrassinolide-pretreatment (EB-pretreatment) and PEG-induced dehydration. It was found coimmunolocalization of ABA, WGA and dehydrins in the cells of central cylinder of basal part untreated and EB-pretreated roots of wheat seedlings under normal conditions and under osmotic stress. Such mutual localization ABA and protective proteins, WGA and dehydrins, indicates the possible effect of their distribution in the tissues of EB-pretreated wheat roots during dehydration on the apoplastic barrier functioning, which apparently contributes to decrease the water loss under dehydration. Perhaps, the significant localization of ABA and wheat lectin in the metaxylem region enhances EB-induced transport of ABA and WGA from roots to shoots under stress. It can be assumed that brassinosteroids can serve as intermediates in the realization of the protective effect of WGA and wheat dehydrins during water deficit.