The Mechanism Of Aluminum-induced Callose Accumulation In Root Apices Of Sweet Sorghum[Sorghum Bicolor(L.)Moench]

Aluminium (Al) is the most abundant metal element after oxygen and silicon inthe earth’s crust. It is recognized as non-essential nutrient on plant growth, lowconcentration could stimulate plant growth. By contrast, high concentration trivalentcation Al3+in soil solution bring about Al toxicity by inhibiting plant growth and cropproduction in acidic soils (pH<5.5) worldwide. Callose deposition, one of the mostimportant responses to Al toxicity, has been proposed as a sensitive indicator. Despitethis knowledge, the exact mechanism of Al-induced callose accumulation in plants isnot clearly understood.Sweet sorghum [Sorghum bicolor (L.) Moench] is a variant of grain sorghum,which origins in Africa. Due to its high sugar and tolerance, it has been considered asa potentially useful energy crop and received more attention. However, less study onsweet sorghum has been performed in physiology and molecular by Al stress. In thisstudy, we acquired an Al-tolerant genotype ROMA and an Al-sensitive genotypePOTCHETSTRM by screening of71sweet sorghum genotypes. Comparing withmaize and soybean, sweet sorghum showed significantly lower tolerance and morecallose accumulation by Al. Taking advantage of the difference in Al tolerancebetween ROMA and POTCHETSTRM, we identified a series of meaningfulrelationships between enzyme activities and their transcription levels in terms of theAl-induced callose accumulation. Meanwhile, heterologous expression ofβ-1,3-glucanase gene SbGlu1response to Al in Arabidopsis to further study on itsfunction. Additionally, comparison of SbGlu1sequence, copy number and upstreampromoter sequence between the two cultivars may explain the difference of SbGlu1expression.Relative root growth (RRG) was used as a parameter to compare the Al tolerancein sweet sorghum. ROMA as an Al-tolerant genotype and POTCHETSTRM as anAl-sensitive genotype were chosen for further research. Comparison of Al toleranceamong sweet sorghum, soybean and maize, there were lower RRGs, higher Al content and callose accumulation in sweet sorghum than in soybean and maize, showing thatsweet sorghum is lower tolerance than soybean or maize to Al stress.With5μM、10μM、15μM Al treated up to24h, callose content obviouslyincreased in both two cultivars with the increasing of the Al treatment concentrationand time. Callose content in ROMA is lower than in POTACHETSTRM. In somedegree of Al stress, callose deposition showed the stablized level in both cultivars.Morover, callose deposition has a negative correlation with the RRG, and a positivecorrelation with the Al content. These results indicated that callose accumulation,which is a sensitive marker, was positively correlated with Al toxicity in sweetsorghum.Callose deposition is controlled by the callose synthase in callose metabolism.Callose synthase activity was either stimulated or inhibited by Al. The callosesynthase activity increased in0h-12h and then sharply decreased at24h with the10μM Al treatment. The activity in POTCHETSTRM was higher than in ROMA. Theresults could explain the more callose accumulation in POTCHETSTRM. Theexpression levels of six callose synthase-like genes were very low exposure of10μMAl upon to24h in ROMA, but POTCHETSTRM exhibited the highest expressionlevel only at24h. Therefore, callose synthase-like genes maybe regulate callosedeposition in the later stage of Al stress in sweet sorghum.Moreover, Callose deposition is controlled by β-1,3-glucanase yet. In contrast,β-1,3-glucanase activity were inhibited by Al. The β-1,3-glucanase activity wasobviously higher in ROMA than in POTCHETSTRM by Al stress. These resultsillustrated that the decrease of β-1,3-glucanase activity by Al could lead to calloseaccumulation. In POTCHETSTRM, five β-1,3-glucanase genes expression wereup-regulated, and a gene expression was down-regulated. In ROMA, only oneβ-1,3-glucanase gene， SbGlu1(Sb03g045630.1) expressed response to Al, and theexpression was higher in ROMA than in POTCHETSTRM. In addition, the SbGlu1expression positively correlated with callose content in both cultivars. These resultsindicated that SbGlu1expression maybe involve in callose degradation in sweetsorghum by Al stress.The full-length cDNAs of SbGlu1were cloned from the root tips of both ROMAand POTCHETSTRM, respectively. The SbGlu1were transient expressed in onionepidermal cells for subcellular localization, showed that SbGLU1is soluble with no specificity localization. The increased β-1,3-glucanase activity, alleviated theAl-induced root growth inhibition and reduced Al and callose accumulation inArabidopsis thaliana heterologously expressed SbGlu1, suggested a role of SbGlu1inenhanced Al tolerance.The SNPs identified the ORF of SbGlu1from the two different Al-tolerantcultivars, did not alter the Al tolerance of the different transgenic lines. These resultssuggest that SNPs cannot explain the differences in SbGlu1expression. Here, thesame gene copy number in both cultivars also did not lead to the SbGlu1expressiondifferences. However, thirteen core promoter element of transcription start TATA-boxand3common cis-acting element enhancer regions CAAT-box of SbGlu1upstream2kb region in ROMA were more than in POTCHETSTRM. Thus, the differences ofTATA-box and CAAT-box elements in SbGlu1upstream region maybe generate thedifferences of SbGlu1expression in two sweet sorghum cultivars.