Mutation of the chitinase-like protein-encoding AtCTL2 gene enhances lignin accumulation in dark-grown Arabidopsis seedlings.

Several genes that encode a chitinase-like protein (called the CTL group) have been identified in Arabidopsis, rice, pea, and cotton. Members of the CTL group have attracted much attention because of their possible role in the biosynthesis of the cell wall in plants. The hot2 mutation in the CTL1 (AtCTL1) gene of Arabidopsis thaliana causes multiple defects in growth and development. The Arabidopsis genome possesses the AtCTL2 gene, which exhibits 70% similarity to AtCTL1 at the amino acid level. We showed that the AtCTL2 gene was predominantly expressed in stems, which was in contrast to the presence of AtCTL1 transcripts in most organs of Arabidopsis. In addition, beta-glucuronidase (GUS) staining was detectable in all tissues of the stem in transgenic plants expressing the AtCTL1::GUS construct, while GUS activity under control of the AtCTL2 promoter was significantly restricted to the xylem and to interfascicular fibers in stems. The phenotypes of atctl2 single mutant and of hot2, atctl2 double mutant plants were significantly similar to those of wild-type and of hot2 single mutant plants, respectively. The expression levels of CESA1 and CESA4 transcripts were not affected in the two single mutants or corresponding double mutant plants, compared with the levels in wild-type plants. The accumulation of lignin in etiolated hypocotyls, however, was increased by mutation of AtCTL2. These findings suggest that AtCTL2 is required for proper cell wall biosynthesis in etiolated seedlings of Arabidopsis.

Disruption of glycosylphosphatidylinositol-anchored lipid transfer protein gene altered cuticular lipid composition, increased plastoglobules, and enhanced susceptibility to infection by the fungal pathogen Alternaria brassicicola.

All aerial parts of vascular plants are covered with cuticular waxes, which are synthesized by extensive export of intracellular lipids from epidermal cells to the surface. Although it has been suggested that plant lipid transfer proteins (LTPs) are involved in cuticular lipid transport, the in planta evidence is still not clear. In this study, a glycosylphosphatidylinositol-anchored LTP (LTPG1) showing higher expression in epidermal peels of stems than in stems was identified from an Arabidopsis (Arabidopsis thaliana) genome-wide microarray analysis. The expression of LTPG1 was observed in various tissues, including the epidermis, stem cortex, vascular bundles, mesophyll cells, root tips, pollen, and early-developing seeds. LTPG1 was found to be localized in the plasma membrane. Disruption of the LTPG1 gene caused alterations of cuticular lipid composition, but no significant changes on total wax and cutin monomer loads were seen. The largest reduction (10 mass %) in the ltpg1 mutant was observed in the C29 alkane, which is the major component of cuticular waxes in the stems and siliques. The reduced content was overcome by increases of the C29 secondary alcohols and C29 ketone wax loads. The ultrastructure analysis of ltpg1 showed a more diffuse cuticular layer structure, protrusions of the cytoplasm into the vacuole in the epidermis, and an increase of plastoglobules in the stem cortex and leaf mesophyll cells. Furthermore, the ltpg1 mutant was more susceptible to infection by the fungus Alternaria brassicicola than the wild type. Taken together, these results indicated that LTPG1 contributed either directly or indirectly to cuticular lipid accumulation.