New Role for ACC in Cell Wall Biosynthesis

Plant cell walls need to be functional in supporting the cell even as the cell grows and deposits new wall components. The regulation of cell wall biosynthesis is thus vital for plant cell growth. Receptor-like Ser/Thr protein kinases (RLKs) are ubiquitous signaling modules, and a number of plasma membrane–bound RLKs are involved in regulating cell expansion in Arabidopsis (reviewed in Hématy and Hofte, 2008).

Xu et al. (pages 3065–3079) describe two Arabidopsis leucine-rich repeat RLKs, FEI1 and FEI2 (named for the Chinese word for fat), whose behavior suggests a link between cell wall biosynthesis and the ethylene biosynthetic pathway. Both proteins appear to be localized to the plasma membrane and show a broad pattern of expression that is particularly high in the root meristem and elongation zone. fei1 fei2 double mutants have short, swollen roots on high sucrose or elevated NaCl media. The roots have normal cell numbers but exhibit radial swelling especially of the epidermal cells (see figure), a phenotype that is indicative of a defect in cellulose synthesis. Accordingly, Xu et al. found that cellulose biosynthesis is decreased in the presence of high sucrose in the double mutant and that the mutant is hypersensitive to a cellulose synthase inhibitor, even in low sucrose.

Figure 1.

Radial swelling in fei1 fei2 double mutant roots. Wild type and mutant roots shown after growth on high sucrose media. Bar = 1 mm.

The authors then examined genetic interactions with other genes known to affect cellulose synthesis. These results suggest that FEI1 and FEI2 act in separate pathways from COBRA (COB), known to be involved in microfibril orientation, and PRC1, which encodes a subunit of cellulose synthase. However, FEI1 and FEI2 appear to act in the same pathway as SALT OVERLY SENSITIVE5 (SOS5), mutants of which are defective in a cell surface adhesion protein and show a similar phenotype to the fei1 fei2 double mutant.

Because ethylene plays an important role in regulating plant cell expansion, the authors explored the role of ethylene biosynthesis in the fei1 fei2 double and sos5 single mutant. They found that the roots of fei1 fei2 double mutants and sos5 single mutants appear nearly wild type when they are grown in the presence of an analog of 1-aminocyclopropane-1-carboxylic acid (ACC; a precursor of ethylene). A compound that inhibits ACC synthase also rescues fei1 fei2 and sos5, but not cob, indicating that the effects of these ethylene synthesis blockers are specific for the FEI/SOS5 pathway. Intriguingly, the mutants produce normal amounts of ethylene, so the activity of ACC synthase does not appear to be affected. In addition, two compounds that block ethylene perception do not affect the phenotypes of the mutants and neither do mutations in the ethylene sensing pathway.

These data suggest that Arabidopsis possesses another ethylene sensing pathway or that ACC itself is the signal for the FEI/SOS5 pathway. The latter is supported by the finding that the FEI proteins can interact directly with two members of the Arabidopsis ACC synthase family in yeast two-hybrid assays. This study from Xu et al. opens up an exciting new area of exploration into the regulation of cell wall biosynthesis and its interaction with ACC and/or ethylene biosynthesis.