Molecular genetic analysis of cellulose biosynthesis in poplars

Molecular understanding of cellulose biosynthetic process is of considerable importance for improving cellulose production in economically important forest trees. In order to support this long-term goal, we used a combination of tension wood (TW) and opposite wood (OW) system. TW is a special type of wood with drastically different anatomical, chemical and mechanical properties as compared to vertical wood (VW). TW fibers typically contain an innermost thickened cell wall layer (known as gelatinous or G layer) consisting of almost entirely of cellulose (98.5%). The wood formed on the lower side of the bent tree is called opposite wood (OW). OW is also a special type of wood with properties intermediate between TW and VW. Thus comparative studies of TW, OW, VW systems using Affymetrix GeneChipRTM poplar whole genome arrays offer a unique opportunity for deciphering the molecular mechanism of cellulose biosynthesis during TW formation. Our lab has so far isolated and studied seven distinct cellulose synthase (CesA) genes from aspen trees and also recent release of the Populus trichocarpa genome sequence has opened many new avenues for studying the other poplar CesAs. There are 17 CesAs in poplar genome comprising nine different types; all CesAs are duplicated except for PoptrCesA3. To determine the expression levels of five secondary wall associated CesAs, a real-time quantitative RT-PCR was carried out. Although all five secondary CesA genes were upregulated during xylem development, the level of upregulation of each of these genes was distinctly different. These observations suggest that appropriate quantities of each of these CesA transcripts are pivotal for proper wood formation. To test this hypothesis further, we have overexpressed one of the CesAs, PtrCesA1 in transgenic aspen that resulted in silencing of the endogenous gene as well as transgene. The resulting transgenic aspen plants were dwarf, weak, and had fragile stem. The histological analysis of transgenic stem revealed high perturbation of secondary cell wall formation showing typical 'irregular xylem' phenotype with collapsed vessels. Transgenic wood samples contained ∼70% less cellulose as compared to the control wood samples. This massive cellulose reduction was accompanied by the proportional increase in lignins and xylan amounts. These novel results signify the structural importance of cellulose during cell wall formation that in turn controls overall growth and development of trees.