JIA-2018-09

1947 XU Li-ming et al. Journal of Integrative Agriculture 2018, 17(9): 1946–1958 And soluble Al, which exists in Al 3+ form, is readily absorbed by plant roots even it is at micromolar concentration, then root growth is inhibited, subsequently nutrient and water uptake are decreased, thus crop yield is reduced (Ryan et al . 2001). Now, the mechanisms of root growth inhibition are not well understood. But, the root apex, where cell division and cell elongation are abundant, is the most sensitive part to Al 3+ on root (Degenhardt et al . 1998). The rapid response to Al stress indicates that Al can interact with multiple structures in the apoplasm and symplasm of root cells quickly. In the root tissue, as much as 50–90% of the total absorbed Al was bound in the apoplasm rapidly and localized in the extracellular compartments (Taylor et al . 2000). Al also accumulated in the symplasm of root rapidly. Plants exposure to Al would produce many reactions, including disorder of reactive oxygen species (ROS), alterations of the membrane structure, disruption of cytoskeletal dynamics, changes in Ca 2+ homeostasis and signaling, and mitochondrial dysfunction (Yamamoto et al . 2002). Further, Al 3+ interacting with the nuclei may inhibit mitotic activity via alterations in DNA composition, chromatin structure or template activity (Frantzios et al . 2001). In summary, these researches demonstrate that Al has deleterious effects on various cellular components, thus root growth is inhibited. Usually, an Al-stress environment would evolve plants either to reduce Al accumulation in the root by excluding Al from the root apex or to neutralize toxic Al absorbed in the symplasm (Ezaki et al . 2001; Kaczorek et al . 2002). For example, Al can be excluded via rhizosphere Al-organic acid anion complex formation, which is the most popular physiological mechanism of Al tolerance in crops (Delhaize et al . 1993; Piñeros et al . 2010). And in the plants with the mechanism, root can exude the key organic acid anions citrate, including malate, and oxalate (Emmanuel et al . 2007). Other proposed Al exclusion mechanisms also involve secretion of proteins and phenolic compounds (Basu et al . 1994), increased root-mediated of the rhizosphere pH (Degenhardt et al . 1998), changed selective permeability of the plasma membrane, and masking Al-binding sites at the cell wall (Yang et al . 2008). Additionally, tolerance mechanisms are active after Al 3+ enters into symplasm of root cell, such as Al can be quelled in the cytosol or vacuole, or bound with proteins directly (Delhaize and Ryan 1995; Knaggs and Andrew 2003). The identification of genes response to Al-stress will improve our understanding of stress mechanisms and provide effective strategies for improving stress adaptation. And many modern technologies, such as differential display reverse transcription-PCR (DDRT-PCR), suppression subtractive hybridization (SSH), cDNA-AFLP, expressed sequence tag (EST) libraries, cDNA libraries, etc., have been used to identify genes or transcripts related to Al- stress responses (Vij and Tyagi 2007). And a number of genes were identified in different plant species (Chandran et al . 2008; Kumari et al . 2008). TheAl-induced genes were categorized into many functional groups, which included stress and defense response, membrane transporter, organic acid metabolism, polysaccharide and cell wall metabolism, protein metabolism, signaling, hormones, transcription factors (TFs), cell structure and cell growth, vesicular transport, nucleotide processing and modification, transcription regulation, and translation regulation (Chandran et al . 2008; Kumari et al . 2008; Duressa et al . 2011). Maize ( Zea mays L.) is considered as a pioneering crop and widely grown in acid soils of USA, Brazil, and China (Horst et al . 2007). Citrate exudation from the root tips plays an important role for Al stress in maize (Horst et al . 2007). Piñeros et al . (2010) observed that the Al-activated root citrate release was not well correlated withAl tolerance, suggesting that other mechanisms of Al toxicity and resistance may also operate in maize roots. The research indicated that Al tolerance of maize may be a multi-genetic trait. Thus, an integrative molecular study appears to be necessary to further classify the mechanisms of Al toxicity and tolerance in maize. Up to now, the transcriptional expression with microarray techniques has been used to study mechanisms of Al toxicity and tolerance in Arabidopsis (Kumari et al . 2008), wheat (Guo et al . 2007), Medicago truncatula (Chandran et al . 2008), common bean (Eticha et al . 2010), and maize (Lyza et al . 2008), and a series of Al-responsive genes were identified. Further, in the present study, oligonucleotide microarrays was used to study the transcriptional profile of maize and to discover putative candidate genes related to mechanisms of Al toxicity and tolerance in maize. 2. Materials and methods 2.1. Plant materials and treatments Seedlings cultivation Maize inbred line 178 was provided by Sichuan Agricultural University, China. A total of 100 seeds were surface-sterilized in 1.0% (v/v) sodium hypochlorite for 5 min, washed 3–4 times with tap water, embedded in water for 5 h, and germinated at 28°C in dark for 3 d. Then, the seedlings were transferred to a growth chamber and cultured in Hoagland nutrient solution at 28°C/24°C (16 h light/8 h dark) for 3 d (Piñeros et al . 2010). The 60 uniform seedlings were selected and cultured in 2 L of 200 μmol L –1 CaCl 2 solution (pH=4.0) for a 24-h adaptation period. AlCl 3 concentration determination The seedlings were treated with different AlCl 3 concentrations to decide the