577 Protein and Peptide Letters, Vol. 11, No. 6, pp. 577-582, 2004 Bentham Science Publishers Ltd. 0929-8665/04 $ 45.00 + .00 COMPARATIVE GENE EXPRESSION STUDY TO IDENTIFY GENES POSSIBLY RELATED TO STORAGE ROOT FORMATION IN CASSAVA *Cláudia Regina Batista de Souza 1,2 , Luiz Joaquim Castelo Branco Carvalho 1 and Júlio Cézar de Mattos Cascardo 3 1 EMBRAPA- Recursos Genéticos e Biotecnologia, Parque Estação Biológica- PqEB, Caixa postal 02372, Brasilia-DF, Brazil 70770-900. 2 Current Address: Universidade Federal do Pará (UFPA), Rua Augusto Correa n. 01, Centro de Ciências Biológicas, Departamento de Genética, Guamá, Belém-PA, Brazil, 66075- 110. 3 Universidade Estadual de Santa Cruz (UESC), Laboratório de Genomica e Expressão Gênica, Ilhéus-BA, Brazil, 45650-000. * Corresponding author: E-mail: bsouza@ufpa.br Abstract: Cassava storage roots result from swelling of adventitious roots by secondary growth. In the present study we aimed to gain insight into the molecular processes occurring during cassava storage root formation. We report a comparative gene expression study in adventitious and storage roots in order to identify genes possibly related to storage organ formation. Our results revealed five genes with higher expression levels in secondary xylem of storage roots than adventitious roots. Among them, the Mec1 gene coding for Pt2L4 glutamic acid-rich protein and a putative RING Zinc Finger and LEA protein genes were strongly induced in secondary xylem tissue. Keywords: cassava, comparative analysis, gene expression, Pt2L4 protein, secondary growth, storage root formation. INTRODUCTION Cassava (Manihot esculenta Crantz) is one of the most important food crops in the tropics and with respect to calories, ranks fourth, after rice, maize and sugarcane [1]. In developing countries cassava roots are very often the primary source of calories. Cassava roots contain about 85% starch and only about 1-2% of proteins [2]. The cassava storage root results from swelling of adventitious roots by secondary growth. Anatomical studies of the cassava storage root have been reported [3, 4] and we have suggested a model of storage root tissue organization, suitable for gene expression analyses [5]. This model describes three tissue systems: System I is composed of phellogen and phelloderm, System II of phloem and vascular cambium, and System III of secondary xylem with its highly specialized parenchymatic cells packed with starch granules [6]. Protein polymorphism of adventitious and storage roots of cassava have been studied in a 2-D gel system and the results showed over 260 proteins unique to the storage root and possibly related to secondary growth [7]. Studies to isolate and characterize cassava storage root protein genes were initiated by our group [6, 8, 9, 10]. In these studies we identified an 18 kDa protein with high identity to the small heat shock class of proteins and an alcohol-soluble protein similar to allergenic Hev b5 from the rubber tree, designated Pt1L4 and Pt2L4, respectively [6, 8, 9]. Expression analyses suggested that Pt2L4 is possibly related to the secondary growth pattern of cassava storage root since the level of Mec1 transcripts