Cell Wall Loosening by Expansins

Abstract

In his 1881 book, The Power of Movement in Plants, Darwin described a now classic experiment in which he directed a tiny shaft of sunlight onto the tip of a grass seedling. The region below the coleoptile tip subsequently curved toward the light, leading to the notion of a transmissible growth stimulus emanating from the tip. Two generations later, follow-up work by the Dutch plant physiologist Fritz Went and others led to the discovery of auxin. In the next decade, another Dutchman, A.J.N. Heyn, found that growing cells responded to auxin by making their cell walls more “plastic,” that is, more extensible. This auxin effect was partly explained in the early 1970s by the discovery of “acid growth”: Plant cells grow faster and their walls become more extensible at acidic pH. Auxin was hypothesized to stimulate growth, in part, by inducing plant cells to acidify their extracellular space. How an acidic pH makes walls more extensible was unclear until 1992, when the proteins that catalyze this process (later named “expansins”) were identified. Expansins alone can induce cell walls to extend, but in living cells they probably act in concert with a variety of enzymes that cut and restructure the wall. In this Update I will discuss expansin in the context of plant growth and describe emerging concepts about its roles in plant development. Growing cell walls differ from mature walls in many ways. They are generally thinner, have a different polymer composition, are not highly cross-linked by covalent bonds, and are pliant and easily deformed by mechanical forces. Such wall pliancy is important for growing cells, because the wall surface must enlarge as the cell grows. The pliancy of growing walls is special in that it enables prolonged wall extension (“creep”) and stress relaxation. Wall stress relaxation reduces cell turgor and thereby creates the driving forces needed for water uptake by growing cells. It is the key physical process limiting cell enlargement (Cosgrove, 1997). Wall pliancy sounds simple, but its underlying molecular basis is complex. It seems to be due partly to polymer physics and partly to carefully controlled reactions that alter the bonding relationships of the wall polymers. The growing wall is a composite polymeric structure: a thin weave of tough cellulose microfibrils coated with heteroglycans (hemicelluloses such as xyloglucan) and embedded in a dense, hydrated matrix of various neutral and acidic polysaccharides and structural proteins (Bacic et al., 1988; Carpita and Gibeaut, 1993). Like other polymer composites, the plant cell wall has rheological (flow) properties intermediate between those of an elastic solid and a viscous liquid. These properties have been described using many different terms: plasticity, viscoelasticity, yield properties, and extensibility are among the most common. It may be attractive to think that wall stress relaxation and expansion are largely a matter of polymer physics, but many physiological experiments indicate that there is another level of control by the cell. Cell expansion can be stimulated or inhibited within seconds, without major changes in cell wall structure or viscoelastic properties (for review, see Cosgrove, 1993). This is not to say that wall structure is irrelevant for control of growth, but rather that growing cells can evidently regulate specific “loosening” processes that result in wall stress relaxation. The ensuing expansion of the wall is undoubtedly influenced by its structure and viscoelasticity. Assay of acid-induced extension in isolated cell wall specimens. A, Scheme for preparing wall specimens for clamping in the extensometer. The growing region of the cucumber hypocotyl was excised, frozen, thawed, and abraded to obtain “native” cell walls (i.e. the cells are dead, but the walls retain active enzymes). Extension of the wall is registered electronically by a position transducer (LVDT). B, Native walls begin extending when the buffer is changed from pH 6.8 to 4.5. C, Walls inactivated with a brief heat treatment lack acid-induced extension (first arrow), but this may restored by the addition of wall proteins containing expansin (second arrow). The in vitro wall extension is thought to be a type of polymer creep involving a shearing movement of the load-bearing polymers in the wall. This pH-dependent extension is known as acid growth and has been found in growing cells of angiosperms, gymnosperms, ferns, mosses, and even some green alga with walls that resemble plant walls in ultrastructure and polymer composition (e.g. Nitella). Acid growth is not simply a matter of the physical chemistry of the wall, e.g. dissolution of the pectin network at low pH, because treatment of the wall with proteases and various protein denaturants eliminates its ability for acid growth. This implies that a cell wall protein(s) acts as a catalyst for acid growth. To identify the protein catalysts of acid growth, we devised a reconstitution assay for wa