Effects of Chronic Stress of Cadmium and Lead on Anatomical Structure of Tobacco Roots

Cadmium （Cd2＋） is one of the major widespread environmental pollutants, and can cause serious problems to all organisms. Lead （Pb2＋） is another wide spread dangerous heavy metal. Tobacco is a popular growing economic crop in China. Most tobacco growing region soils contain excessive Cd2＋ and Pb2＋. To assess anatomic changes of tobacco roots under Cd2＋, Pb2＋, and Cd2＋＋pb2＋ chronic stress, a pot experiment was carried out in field. The tobacco seedlings with 6 leaves were transplanted to pots in which soil was placed. The amounts of Cd2＋ added to soil were 0, 3, 6, 10, 30, 60, and 100 mg kg-1 dry soil. The amounts of Pb2＋ added to soil were 0, 150, 300, 450, 600, 750, and 1 000 mg kg-1 dry soil. The amounts of Cd2＋＋Pb2＋ added to soil were 0＋0, 3＋150, 6＋300, 10＋450, 30＋600, 60＋750, and 100＋1000 mg kg-1 dry soil. The contents of Cd2＋ and Pb2＋ in root systems were determined by inductively coupled plasma, and the anatomical structure was studied by method of paraffin sectioning. The results revealed that the amounts of exchangeable Cd2＋ and Pb2＋ and carbonate bound Cd2＋ and Pbz＋ in soil increased with the amounts of Cd2＋ and Pb2＋ added to soil, and the contents of both Cd2＋ and Pb1＋ in roots were significantly increased along with stress time and the amounts of Cd2＋ and Pb2＋ added to soil. The growing of tobacco in Cd2＋ and Cd2＋＋Pb2＋ polluted soil for 50, 100, and 150 d resulted in some abnormal extemal morphological and anatomical changes in ripe region of lateral roots. All the abnormal roots had abnormal vascular cylinders, and the ratio of abnormal extemal morphological and anatomical changes of roots positively correlated with the Cd2＋ contents in roots and stress time. While, there were no abnormal external morphological and anatomical changes of roots under Pb2＋ stress. It was suggested that Cd2＋ stress could cause abnormal anatomic changes of roots, but Pb2＋ stress could not.

Identification and functional characterization of the MdHB-1 gene promoter sequence from Malus×domestica

Homeobox 1 in Malusxdomestica （MdHB-1） is a transcription factor that belongs to homeodomain-leucine zipper I （HD-Zip I） protein subfamily. According to previous reports, MdHB-1 could regulate ethylene synthesis by binding with the MdAC01 promoter, but other functions of MdHB-1 are still unknown. To reveal more clues concerning the characters of the MdHB-1 gene promoter and the functions of MdHB-1, the promoter region of MdHB-1 was cloned from the Royal Gala apple genome and recombined with the 13-glucuronidase （GUS） gene in this study. This research was conducted in Nicotiana tabacum and supported by Agrobacterium-mediated transient transformation and bioinformatics analysis. Deletion analysis of the MdHB-1 promoter showed that the GUS gene could be activated by serially deleted promoters, and the activity promoted by 680 nucleotides （nt） was the lowest. The region, which is 266 nt upstream of the initiation code （ATG）, was effective for GUS expression. Meanwhile, the activity of the MdHB-1 promoter （-1 057 nt）, which was stronger than MdHB-1 promoter （-1 057 to -266 nt） and lack the 5＂-untranslated region （5＂-UTR）, showed that 5＂-UTR may have a positive effect on gene transcription. After the sequence analysis, the cis-acting elements that respond to hormones and environmental stresses were identified in the promoter region. The MdHB-1 promoter （1 057 nt） activity in Nicotiana tabacum was positively induced by ethrel and darkness, and it was suppressed by gibberellic acid （GA）, whereas abscisic acid （ABA）, salicylic acid （SA）, wounding, and Pseudomonas syringae pv. tomato （DC3000） treatments revealed a slight auxo-action. These results reveal that the MdHB-1 promoter receive internal or external signals, and MdHB-1 may refer to many biological activities in apple, such as its stress response, development, and ripening.