JIA-2018-09

1949 XU Li-ming et al. Journal of Integrative Agriculture 2018, 17(9): 1946–1958 to 25 μmol L –1 Al for 12 h (Chandran et al . 2008), soybean with 30 μmol L –1 Al treatment for 4 h (You et al . 2011), and Al-sensitive maize genotype exposure to 39 μmol L –1 Al for 24 h (Lyza et al . 2008). The different Al treatments all induced about 40–50% root length inhibition, respectively. Thus the corresponding concentration was selected to conduct a effective microarray analysis. Inhibition of root elongation is the primary and the most dramatic effect of Al toxicity (Sivaguru and Horst 1998; Kollmeier et al . 2000). Most studies showed root growth of plants such as wheat (Guo et al . 2007) and maize (Jorge and Arruda 1997; Piñeros et al . 2010) is almost entirely abolished after Al-treated for 24–96 h. Further physiological and metabolic parameters measured under Al treatment for 24-96 h would probably be distorted. Additionally, previous results in maize (Lyza et al . 2008) and soybean (You et al . 2011) showed Al 3+ stress could induce numbers of genes with differential expression for short stage treatment of 2, 4, or 6 h. Thus, 1 and 6 h were selected as the Al treatment duration in the present study. 3.2. Portrait of the transcriptome response to Al stress Kumari et al . (2008) has used microarray to investigate the profile of transcriptome changes in plant under Al stress. In the current study, microarrays were also used to measure the transcriptome changes in maize roots. A global gene expression was depicted under Al stress at each time point (Fig. 2-A, Appendix B). The result showed 34 715 distinct genes were represented by the probes spotted on the microarrays. When the log 2 Ratio value of one gene was approximately equal to 1, its expression would be considered non-significant, or it would be considered expressing significantly ( P <0.01). The majority of gene expression have no significant change under Al stress for 1 and 6 h. The genes with transcripts having more than 2.0-fold difference ( P <0.01) between the control and Al- treated maize seedlings were selected. Thus, a total of 60 genes were identified under Al treatment for 1 h, and abundance transcripts of 832 differently expressed genes were detected under Al treatment for 6 h (Fig. 2-B). Nearly the same number of Al-induced genes were detected in other plant species, such as M. truncatula (Chandran et al . 2008), maize, and soybean (You et al . 2011) using a microarray technology by similar selected criteria at different time points and Al concentration. Among the 60 genes detected at 1 h, 59 genes were up-regulated and only one were down-regulated, while 689 genes were up-regulated and 143 were down-regulated for the 832 genes detected at 6 h. And the majority of up-regulated genes detected in this study were nearly consistent with that in Al-tolerant maize genotype other than that in Al-sensitive maize genotype (Lyza et al . 2008), positively suggesting differential genes underlying different tolerance regulation in early stage of Al treatment. Some genes’ expression had more than 20-fold transcriptional changes, but most of those ranged from 2- to 4-fold (Fig. 2-C). Considering the fact that only one gene was down-regulated after 1 h Al treatment, the number was significantly lower than that of up-regulated genes, suggesting the positive regulation mechanism was most powerful for maize to cope with the early stage Al stress. However, negative regulation was also acted synchronously, which included members missed from the detection due to the weak signal or without the probe, as had been found by Lyza et al . (2008). Some genes were unique for both time points, but some genes could be detected at one time point only. Surprisingly, the 32 genes detected at the both time points for the two groups of differently regulated genes, indicating that genes regulated by the two treatments had a great common feature. Similar result had also been found in Al-sensitive maize genotype (Lyza et al . 2008) following the great overlap of differential genes in short stage of Al stress. To evaluate the biological functions of Al-responsive genes, the genes identified in maize roots after 6 h of Al treatment were assigned for functional enrichment analysis by GO terms. Nearly 61% of the total differential genes’ functions were known, among them approximately 22% of the up-regulated genes and 32% of the down-regulated genes were not classified into any functional classes (Fig. 3). Eleven and nine functional categories were identified for the transcript abundance increased and decreased genes, respectively. Interestingly, the same functional categories Fig. 1 The effect of different aluminum (Al) concentration on the net growth length (NGL). Bars mean SD. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 20 40 60 100 NGL of root (cm) Al concentration (μmol L –1 ) 1 h 6 h