Ethylene induces epidermal cell death at the site of adventitious root emergence in rice

Abstract

In deepwater rice (Oryza sativa), adventitious root primordia initiate at the nodes as part of normal development. Emergence of the roots is dependent on flooding of the plant and is mediated by ethylene action. Root growth was preceded by the induced death of epidermal cells of the node external to the tip of the root primordium. Cell death proceeded until the epidermis split open. Through this crack the root eventually emerged. Induced death was confined to nodal epidermal cells covering the tip of the primordia. Our results suggest that this process facilitates adventitious root emergence and prevents injury to the growing root. Cell death was inducible not only by submergence but also by application of 1-aminocyclopropane-1-carboxylic acid, the natural precursor of ethylene and it was suppressed in the presence of 2,5-norbornadiene (bicyclo[2.2.1]hepta-2,5-diene), an inhibitor of ethylene action. Adventitious root growth and epidermal cell death are therefore linked to the ethylene signaling pathway, which is activated in response to low oxygen stress.

Figure 1

Adventitious roots at the third node of a deepwater rice shoot. Adventitious root initials are covered by the nodal epidermis during normal growth (left) or have emerged after treatment with 150 μm ethephon for 24 h (right).

Figure 2

Cross-sections of the third node stained with Evans blue to indicate dead cells. A, Staining of the nodal vasculature connected to a root primordium, but not of the protoxylem elements of the root primordium itself. B, A single epidermal cell is stained (see Fig. 3A, stage II). All parenchymal cells of the node and all cells of the root primordium itself are not stained, indicating that the primordium does not yet possess a root cap. Asterisk indicates the tip of the root primordium. C, More cells are stained (see Fig. 3A, stage III), but staining is still restricted to the epidermis and is not seen in either the nodal parenchyma cells or in the root primordium. Asterisk indicates the tip of the root primordium.

Figure 3

Degree Evans blue staining of the nodal epidermis above root primordia at the third node of submerged rice plants. A, Stage I indicates no staining of the nodal epidermis covering an adventitious root primordium; stage II indicates a patchy staining pattern of the nodal epidermis; stage III indicates areal staining of the nodal epidermis; and stage IV staining includes a cracked nodal epidermis with or without the root initial growing through the opening. B, At each time point, the staining patterns as exemplified in A were determined above each root primordium at five to eight nodes with 12 to 15 root primordia per node and attributed to stage I, II, III, or IV. The numbers are expressed as a percentage and add up to 100% for each time point. Analysis was carried out at the third node of intact rice plants that were submerged for up to 18 h.

Figure 4

Degree of epidermal Evans blue staining above root primordia at the third node of isolated stem sections. A, Stem sections were treated with 10 mm ACC for the times indicated. Results are averages of at least 15 stem sections analyzed in three independent experiments. Each stem section contained 12 to 15 adventitious root primordia. B, Stem sections were treated with 10 mm ACC and 50 μL/L NBD for the times indicated. The staining patterns are given as percentages of stage I to stage IV as described in Figure 3A. Results are averages of at least five stem sections analyzed.