Molecular basis of auxin-regulated extension growth and role of dextranase

Abstract

The first step in the extension growth ofthe plant cell is a process in which the cell wall becomes ductile or plastic, after which the actual enlargement takes place passively under the influence of turgor. The nature of this process has not been explained, although much research has been carried out concerning it. In the present report, it is shown that a specific enzyme, which is identical or nearly so with dextranase (a-1,6-D-glucan 6-glucanohydrolase, EC 3.2.1.11) and is associated with the cell walls of growing coleoptiles, plays a prominent role in this process. The action ofthis enzyme is dependent on the level ofgrowth hormone, auxin, in the tissue. Under its action, certain cell wall components are broken down to yield arabinose and glucose. These sugars are also released during autolysis ofcell wall material. The molecular linkages broken in the process are probably the arabinogalactan crosslinks of the hemicellulose matrix, which are the main constituents of the wall containing arabinose. This is substantiated by the finding that dextranase can break down arabinan and compounds containing arabinose chains with the release of arabinose, just as in the action of the enzyme on the wall. The breaking of these crosslinks will impart the necessary plasticity to the wall for cell extension to occur.The essential step in auxin-regulated extension growth (elongation) of the plant cell is plasticization ("loosening") of the cell wall. Thereafter, the cell wall may be irreversibly extended under the influence of the turgor to bring about the actual enlargement of the cell (1-3). The nature of this physical modification of the wall and the underlying process have remained unknown until now.In 1968, I undertook a study of the possible involvement of enzymes in this process. Such involvement might be in agreement with findings that certain antibiotics inhibit cell elongation and plasticization of the cell wall (4-6) and that protein synthesis, as well as synthesis of DNA and RNA, is a requirement for cell elongation (7-12). In that study, the presence of enzymes in elongating coleoptiles was investigated, and several glucanases were found to be associated with the cell walls (13).In an effort to screen out the enzyme actually involved in the process of the modification of the cell wall under the influence of the hormone, the effect of the hormone concentration in the tissue on the activity of the enzymes, at optimal pH range, was studied. The activity ofonly one glucanase was found to depend on the auxin level in the tissue and to be higher at higher auxin concentration. This enzyme is dextranase (a-1,6-D-glucan 6-glucanohydrolase, EC 3.2. 1.11) (14,15).Whether this outstanding behavior is due to a special, direct sensitivity of the enzyme to the hormone or to a more complicated situation such as the interplay of more than one enzyme need not be considered at this point. The special behavior of this enzyme, however, made it first choice for being investigated as the possible "master" enzyme in the process...

Microbial Dextran-Hydrolyzing Enzymes: Fundamentals and Applications

“…The enzymes capable of hydrolyzing dextrans are found in various microbial groups (Table 1), in animal and human tissues (51,161,170), and in coleoptiles of the genus Avena (65,66). Dextranase activity is also demonstrated in soil samples (40).…”

“…Interestingly, an enzyme identical to dextranase (EC3.2.1.11) is also associated with the cell walls of growing coleoptiles of a plant, Avena. The enzyme plays a prominent role in the growth process, hydrolyzing certain cell wall components and providing necessary plasticity to the cell walls to extend (66).…”

Calcium: The Missing Link in Auxin Action

“…Auxin-induced growth can be largely explained by a rapid elongation of cells and is correlated with acidification of the apoplast [ 88 ], activation of cell wall modifying enzymes [ 89 , 90 ] and K + uptake [ 91 , 92 , 93 ]. These elements have been rationalized in the “Acid Growth Theory” [ 94 , 95 , 96 , 97 , 98 , 99 ], which suggests that apoplast acidification is the major regulator of auxin-induced elongation by activating cell wall loosening enzymes and by providing the electrochemical gradient that drives K + uptake, which is necessary for water uptake and cell expansion.…”

Gentiobiose: a novel oligosaccharin in ripening tomato fruit

“…There are also suggestions of the presence in cell walls of an a-transglucosylase capable of attacking dextran [a-(1fi6)-D-glucan] (Heyn 1981). Similarly, a fungus, Acremonium sp.…”