Abstract
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.
Highlights
Acid soils are widespread and limit crop production all over the world
All SbSTOP1s contain four putative Cys2His2 zinc finger domains that resemble those of AtSTOP1, OsART1 and other homologs in different species (Figure 1A)
SbSTOP1d shows the highest similarity to AtSTOP1 and OsART1, with identities of 54.9 and 48.5%, respectively, while SbSTOP1a, SbSTOP1b, and SbSTOP1c share relatively lower identities with AtSTOP1 and OsART1 (Figure 1A)
Summary
Acid soils are widespread and limit crop production all over the world. Aluminum (Al) toxicity is a primary limiting factor in acid soils. At pH below 5, Al (the most abundant form, Al3+) inhibits root elongation within a few minutes, which leads to subsequent water and nutrient deficiency (Kochian et al, 2004; Ma, 2007; Bojórquez-Quintal et al, 2017). To cope with Al stress, plants develop a series of strategies that have been categorized into two main types of Al resistance mechanisms. Al exclusion mechanisms, which are external strategies, aim at preventing toxic Al from entering root cells by exuding organic compounds (e.g., organic acids or phenolics) into the rhizosphere to chelate Al. Al tolerance mechanisms, which are internal strategies, sequester and detoxify Al that enters the plant
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