Genetic and molecular analysis of xylem development in Arabidopsis thaliana.
Plant cell walls play a central role in cell growth and morphogenesis. All plant cells
have a primary wall. The formation of a secondary cell wall is restricted to particular cell
types, such as the xylem cells, highly lignified cells that provide support and transport
functions to the plant. The mechanisms regulating secondary cell wall biogenesis remain
To identify genes involved in such mechanisms, a genetic screen for mutants with
altered xylem development in the primary root of Arabidopsis thaliana has been conducted.
Three different classes of mutants were identified. They are characterised by increased
number of xylem strands (m"), altered timing of protoxylem differentiation (tpx) and
ectopic lignification (eh). Initial characterisation of the mutant phenotypes, establishment
of different complementation groups and their map position in the Arabidopsis genome
has been determined.
Mutations in the EL [I locus have been characterised in further detail. The eli l
mutants exhibited ectopic lignification of cells throughout the plant that never normally
lignify. Xylem cells in elil were misshapen and failed to differentiate into continuous
strands, causing a disorganised xylem. elil mutants also exhibited altered cell expansion
resulting in a stunted phenotype. Abnormal distribution of cellulose and lignin was
observed in elil cell walls. Ultrastructural analysis of elil cell walls using an anti-lignin
antibody has revealed that that the ectopic deposition of lignin-like compounds occurs
within an altered secondary wall.
Furthermore, other previously described cell expansion mutants, such as lit, rswl
(at the conditional temperature) and det3, exhibited lignification patterns reminiscent to
that of elil mutants. Analysis of the genetic interactions of elil with the lit mutant revealed
that ELlI and LIT genes act in independent pathways to control cell expansion. These
results, together with the double mutant analysis of eli l with other cell expansion mutants
suggested a link between cell growth and differentiation of secondary thickened walls.
Map-based cloning placed the ELJ1 gene in a 140-Kb interval on the top arm of
Arabidopsis chromosome V. A candidate gene approach was used that identified a gene
encoding a cellulose synthase catalytic subunit (CesA), AthCesA-3 as a candidate. Sequence
analysis revealed that the AthCesA-3 gene is mutated in two elil alleles sequenced, both
mutations leading to amino acid substitutions. Initial complementation experiments of
elil plants with the wild type AthCesA-3 gene appeared to restore the wild type phenotype,
suggesting that mutations in the AthCesA-3 gene gave rise to the elil phenotypes.
These studies represent an important contribution to our understanding of the
molecular mechanism of cellulose deposition during cell expansion and secondary cell
wall deposition during plant morphogenesis.