Resistance against Herbicide Isoxaben and Cellulose Deficiency Caused by Distinct Mutations in Same Cellulose Synthase Isoform CESA6

Abstract

Isoxaben is a pre-emergence herbicide that inhibits cellulose biosynthesis in higher plants. Two loci identified by isoxabenresistant mutants (ixr1-1, ixr1-2, and ixr2-1) in Arabidopsis have been reported previously. IXR1 was recently shown to encode the cellulose synthase catalytic subunit CESA3 (W.-R. Scheible, R. Eshed, T. Richmond, D. Delmer, and C. Somerville [2001] Proc Natl Acad Sci USA 98: 10079-10084). Here, we report on the cloning of IXR2, and show that it encodes another cellulose synthase isoform, CESA6. ixr2-1 carries a mutation substituting an amino acid close to the C terminus of CESA6 that is highly conserved among CESA family members. Transformation of wild-type plants with the mutated gene and not with the wild-type gene conferred increased resistance against the herbicide. The simplest interpretation for the existence of these two isoxaben-resistant loci is that CESA3 and CESA6 have redundant functions. However, loss of function procuste1 alleles of CESA6 were previously shown to have a strong growth defect and reduced cellulose content in roots and dark-grown hypocotyls. This indicates that in these mutants, the presence of CESA3 does not compensate for the absence of CESA6 in roots and dark-grown hypocotyls, which argues against redundant functions for CESA3 and CESA6. Together, these observations are compatible with a model in which CESA6 and CESA3 are active as a protein complex.Cellulose synthesis remains a poorly understood process (for review, see Delmer, 1999). This linear 1,4-␤-linked glucan is synthesized in terrestrial plants by a hexameric protein complex referred to as the terminal complex or rosette embedded in the plasma membrane (Kimura et al., 1999). The composition of this complex is unknown. Each of the six particles in the complex is thought to contain several catalytic subunits with an unknown stoichiometry. The relationship between the rosette and the crystalline microfibril structure remains controversial. It is commonly held that one rosette produces one microfibril. This idea was based on calculations based upon the spacing of 1,4-␤-glucan chains in algal celluloses and it assumed the presence of 36 glucan chains in a single microfibril (Herth, 1983). Each rosette particle would then contribute six glucan chains to the microfibril. More recent studies using solidstate nuclear magnetic resonance lead to the conclusion that microfibrils in the primary wall consist of 2-nm crystalline units or elementary fibrils, each of which would not contain more than 10 to 15 glucan chains (Ha et al., 1998). The authors propose a scenario in which each of the six particles in the rosette produce the glucan chains of an elementary fibril, which in turn associate into a hexagonal 8-to 10-nm microfibril observed in most primary cell walls.Genes encoding the cellulose synthase catalytic subunits have been identified in bacteria and plants (Pear et al., 1996). The Arabidopsis genome encodes 10 isoforms of the cellulose synthase catalytic subunit, CESA (http://cellwall.stanford.edu). The ...

Two Leucine-Rich Repeat Receptor Kinases Mediate Signaling, Linking Cell Wall Biosynthesis and ACC Synthase in Arabidopsis

“…In longitudinally expanding cells, the cellulose microfibrils are deposited primarily in an orientation perpendicular to the axis of expansion, thus constricting radial expansion (Green, 1980;Taiz, 1984;Baskin, 2005). Consistent with this, disruption of cellulose biosynthesis by treatment with various chemical inhibitors results in a rapid loss of growth anisotropy (Scheible et al, 2001;Desprez et al, 2002).…”

The COBRA Family of Putative GPI-Anchored Proteins in Arabidopsis. A New Fellowship in Expansion

“…These families tend to be large. For example, the ␣-expansins consist of 26 family members (Lee et al, 2001) and the cellulose synthase family is represented by at least 10 CesA-type genes and 41 Csl (cellulose synthase-like) genes (Richmond and Somerville, 2000;Carpita et al, 2001;Desprez et al, 2002). Here, we report that COBRA is part of a new multigene family with the identification of 11 new family members.…”

Expression of a Mutant Form of Cellulose Synthase AtCesA7 Causes Dominant Negative Effect on Cellulose Biosynthesis

“…Because all other reported CesA mutants with defects in cellulose synthesis are recessive except two herbicide-resistant CesA mutants showing semidominant resistance to herbicides (Scheible et al, 2001;Desprez et al, 2002), it was intriguing to find that the fra5 mutation results in a semidominant effect on cellulose biosynthesis. There are at least two possibilities to explain the semidominant phenotype of the fra5 mutant.…”

“…Since then, several additional Arabidopsis mutants with mutations in CesA genes have been isolated. It has been shown that at least four CesA genes, namely AtCesA1 (RSW1), AtCesA2, AtCesA3 (IXR1/CEV1), and AtCesA6 (PRC1/IXR2), are involved in primary wall synthesis, and mutations or antisense repression of these genes cause a reduction in cellulose synthesis (Arioli et al, 1998;Fagard et al, 2000;Scheible et al, 2001;Burn et al, 2002;Desprez et al, 2002;Ellis et al, 2002). The reduced cellulose synthesis in primary walls is associated with a decrease in cell elongation, which was shown in some of these mutants to be correlated with an increased production of jasmonate and ethylene (Ellis et al, 2002 Article, publication date, and citation information can be found at www.plantphysiol.org/cgi…”

“…However, analyses of the irx1 and irx3 mutants showed that both mutants are recessive (Turner and Somerville, 1997). In fact, all reported CesA mutants with defects in cellulose synthesis have been shown to be recessive except two herbicide-resistant CesA mutants exhibiting semidominant resistance to herbicides (Scheible et al, 2001;Desprez et al, 2002).…”