GENE-EXPRESSION PATTERNS ASSOCIATED WITH IN-VITRO TRACHEARY ELEMENT FORMATION IN ISOLATED SINGLE MESOPHYLL-CELLS OF ZINNIA-ELEGANS

Tracheary element formation from isolated Zinnia leaf mesophyll cells is an excellent system for the dissection of patterned secondary cell wall thickening and lignification. We used mRNAs from cells cultured f or 48 h in the induction medium to isolate differentially regulated ge nes. Thirteen unique cDNA clones were isolated using a subtractive hyb ridization method. These clones can be divided into three distinct gro ups according to their characteristic gene expression in different med ia. The first group includes those genes whose expression is induced i n the basal medium without 1-naphthaleneacetic acid (NAA) and benzylad enine; this indicates that the expression of these genes is regulated by chemical and physical factors other than these hormones. Three of t hese clones, p48h-229, p48h-114, and p48h-102, show significant homolo gy to a pathogenesis-related protein II, a serine proteinase inhibitor , and a sunflower anther-specific proline-rich protein, respectively. The second group includes those genes whose expression is mainly NAA i nduced. One of these clones, p48h-10, shows high protein sequence homo logy to a barley aleurone-specific cDNA, B11E. The p48h-10-encoded pro tein shares some common characteristics of plant nonspecific lipid tra nsfer proteins (low molecular weight, the secretion signal peptide, ei ght conserved cysteine residues, and a basic protein), although no sig nificant protein sequence homology is found between p48h-10 and other plant nonspecific lipid transfer proteins. The third group includes th ose genes whose expression is induced primarily in the induction mediu m; this indicates that the expression of these genes is closely associ ated with the process of tracheary element formation. Two of these clo nes, p48h-107 and p48h-17, show high homology to adenylate kinase and papaya proteinase I, respectively. The possible roles of these differe ntiation-specific genes during tracheary element formation are discuss ed.