Symplastic intercellular transport in plants is achieved by plasmodesmata (PD). These cytoplasmic channels are well known to interconnect plant cells to facilitate intercellular movement of water, nutrients, and signaling molecules including hormones. However, it is not known whether Al may affect this cell-to-cell transport process, which is a critical feature for roots as organs of nutrient/water uptake. We have microinjected the dye lucifer yellow carbohydrazide into peripheral root cells of an Al-sensitive wheat (Triticum aestivum cv Scout 66) either before or after Al treatment and followed the cell-to-cell dye-coupling through PD. Here we show that the Al-induced root growth inhibition is closely associated with the Al-induced blockage of cell-to-cell dye coupling. Immunofluorescence combined with immuno-electron microscopic techniques using monoclonal antibodies against 133-␤-d-glucan (callose) revealed circumstantial evidence that Al-induced callose deposition at PD may responsible for this blockage of symplastic transport. Use of 2-deoxy-d-glucose, a callose synthesis inhibitor, allowed us to demonstrate that a reduction in callose particles correlated well with the improved dye-coupling and reduced root growth inhibition. While assessing the tissue specificity of this Al effect, comparable responses were obtained from the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cells. Analyses of the Al-induced expression of PD-associated proteins, such as calreticulin and unconventional myosin VIII, showed enhanced fluorescence and co-localizations with callose deposits. These results suggest that Al-signal mediated localized alterations to calcium homeostasis may drive callose formation and PD closure. Our data demonstrate that extracellular Al-induced callose deposition at PD could effectively block symplastic transport and communication in higher plants.

Cryptochromes in different evolutionary lineages act as either photoreceptors or light-independent transcription repressors. The flavin cofactor of both types of cryptochromes can be photoreduced in vitro by electron transportation via three evolutionarily conserved tryptophan residues known as the "Trp triad." It was hypothesized that Trp triad-dependent photoreduction leads directly to photoexcitation of cryptochrome photoreceptors. We tested this hypothesis by analyzing mutations of Arabidopsis cryptochrome 1 (CRY1) altered in each of the three Trp-triad tryptophan residues (W324, W377, and W400). Surprisingly, in contrast to a previous report all photoreduction-deficient Trp-triad mutations of CRY1 remained physiologically and biochemically active in Arabidopsis plants. ATP did not enhance rapid photoreduction of the wild-type CRY1, nor did it rescue the defective photoreduction of the CRY1 W324A and CRY1 W400F mutants that are photophysiologically active in vivo. The lack of correlation between rapid flavin photoreduction or the effect of ATP on the rapid flavin photoreduction and the in vivo photophysiological activities of plant cryptochromes argues that the Trp triad-dependent photoreduction is not required for the function of cryptochromes and that further efforts are needed to elucidate the photoexcitation mechanism of cryptochrome photoreceptors.cryptochrome | blue light | Arabidopsis | Trp-triad | photoreduction

Aluminum (Al) toxicity in acidic soils affects crop production worldwide. C2H2-type zinc finger transcription factor STOP1/ART1-mediated expression of Al tolerance genes has been shown to be important for Al resistance in Arabidopsis, rice and other crop plants. Here, we identified and characterized four STOP1-like proteins (SbSTOP1a, SbSTOP1b, SbSTOP1c, and SbSTOP1d) in sweet sorghum, a variant of grain sorghum (Sorghum bicolor L.). Al induced the transcription of the four SbSTOP1 genes in both time- and Al concentration-dependent manners. All SbSTOP1 proteins localized to the cell nucleus, and they showed transcriptional activity in a yeast expression system. In the HEK 293 coexpression system, SbSTOP1d showed transcriptional regulation of SbSTAR2 and SbMATE, indicating the possible existence of another SbSTOP1 and SbSTAR2-dependent Al tolerance mechanism in sorghum apart from the reported SbMATE-mediated Al exclusion mechanism. A transgenic complementation assay showed that SbSTOP1d significantly rescued the Al-sensitivity characteristic of the Atstop1 mutant. Additionally, yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assays showed that SbSTOP1d interacted with SbSTOP1b and SbSTOP1d itself, suggesting that SbSTOP1 may function as a homodimer and/or heterodimer. These results indicate that STOP1 plays an important role in Al tolerance in sweet sorghum and extend our understanding of the complex regulatory mechanisms of STOP1-like proteins in response to Al toxicity.

Abscisic acid (ABA) plays an important role in mediating some biotic and abiotic stresses. In the present study, to better understand the relationship of ABA production and Aluminum (Al)-resistance in plants, Al-resistance genotype (Jiyu70) of soybean was adopted to investigate the accumulation and transport of ABA in plants exposed to Al. Results showed that exogenous application of ABA and ABA synthesis inhibitor-fluridone respectively increased and reduced endogenous ABA content in root apices of soybean, and results in the corresponding reduction and aggravation of Al toxicity. Increasing of either Al concentration (0-50 μM) or treatment duration (0-12 h, 30 μM Al) cause a higher inhibition of root elongation and ABA accumulation in root apices of soybean. Al-induced enhancement of endogenous ABA production not only was in roots but also in leaves, whereas La 3+ (behaves similarly as Al 3+ at the level of cell surface) only increased ABA accumulation in roots. In split-root experiments, Al treatment in two parts of roots (Part A, + Al; Part B, + Al) both decreased root elongation and increased ABA accumulation in root apices of soybean. Whereas when only part A of roots was exposed to Al (Part A, + Al; Part B, -Al), endogenous ABA content in root apices increased in part A but inversely in part B, but root elongation inhibition only was found in part A. Using [ 3 H]-ABA radioisotope technique, it was found that [ 3 H]-ABA can transport at the rate of more than 3.2 cm·min −1 in the whole plants, and this can be accelerated by Al supply. In addition, [ 3 H]-ABA tended to distribute in the root part under Al stress. Together, these results suggest that ABA may play an important role in regulating Al resistance of soybean as an Al-stress signal.