Oral Presentation 1-03

 

Xylem-specific and Tension Stress Responsive Expression of Cellulose Synthase Genes from Aspen Trees

 

Chandrashekhar P. Joshi

 

Plant Biotechnology Research Center, School of Forestry and Wood Products,

Michigan Technological University, Houghton, MI 49931, USA

 

Telephone:  (906) 487-3480; Fax:  (906) 487-2915; E-mail:  cpjoshi@mtu.edu

 

Cellulose is a major component of wood, and genetic improvement of cellulose content and quality in trees is highly desirable for cellulose-based forest product industries.  However, we know very little about the genes involved in this process.  The enzyme cellulose synthase (CesA) plays a major role in the process of cellulose biosynthesis and a big gene family exists in all plants examined.  Angiosperm trees offer a unique opportunity to decipher the roles of various CesA genes in cellulose production.  Under tension stress conditions, trees like aspen accumulate an elevated amount of highly crystalline cellulose with a concomitant decrease in lignin in secondary cell walls of tension-stressed woody tissues.  To further investigate the molecular basis of this stress response in trees, we recently cloned a full-length cellulose synthase (PtCesA1) cDNA from aspen (Populus tremuloides) that showed a high degree of similarity to known secondary cell wall synthesis related CesA cDNAs (Plant J 22:495-502, 2000). Various analytical techniques conclusively demonstrated that PtCesA expression is confined to developing xylem cells during normal stem growth.  During tension stress conditions, however, PtCesA promoter regulated GUS expression continued in xylem on the tension side and GUS expression was turned off in tissues undergoing compression on the opposite side of the bend.  Our results suggested a unique role for PtCesA1 in cellulose biosynthesis in both tension-stressed and normal tissues in aspen. Recently, we isolated two more full-length cellulose synthase genes from aspen xylem (PtCesA2 and PtCesA3) that showed remarkable similarity to two other distinct classes of secondary cellulose synthases.  We are, therefore, investigating whether expression of PtCesA2 and PtCesA3 genes in aspen is also xylem-specific and tension stress responsive similar to PtCesA1.  We hypothesize that secondary CesAs from aspen, PtCesA1, PtCesA2 and PtCesA3 coordinately express in normal and tension stressed xylem tissues and work cooperatively to produce cellulose with high degree of polymerization and crystallinity.