Influence of a weak DC electric field on root meristem architecture

Abstract

Background and aims: Electric fields are an important environmental factor that can influence the development of plants organs. Such a field can either inhibit or stimulate root growth, and may also affect the direction of growth. Many developmental processes directly or indirectly depend upon the activity of the root apical meristem (RAM). The aim of this work was to examine the effects of a weak electric field on the organization of the RAM.

Methods: Roots of Zea mays seedlings, grown in liquid medium, were exposed to DC electric fields of different strengths from 0.5 to 1.5 V cm(-1), with a frequency of 50 Hz, for 3 h. The roots were sampled for anatomical observation immediately after the treatment, and after 24 and 48 h of further undisturbed growth.

Key results: DC fields of 1 and 1.5 V cm(-1) resulted in noticeable changes in the cellular pattern of the RAM. The electric field activated the quiescent centre (QC): the cells of the QC penetrated the root cap junction, disturbing the organization of the closed meristem and changing it temporarily into the open type.

Conclusions: Even a weak electric field disturbs the pattern of cell divisions in plant root meristem. This in turn changes the global organization of the RAM. A field of slightly higher strength also damages root cap initials, terminating their division.

Fig. 1.

Schematic view of DC electric field application to plant roots growing gravitropically downward in a chamber filled with liquid medium (grey). A, aerating tube; B, anode; C, cathode; D, plastic plate with seedlings attached.

Fig. 3.

The response of maize root apices subjected to various treatments, as seen on longitudinal central sections. The control root, growing without application of the electric field (A), shows a closed meristem organization, with a clearly demarcated root cap junction separating the root cap and root body. A similar pattern is present in the root exposed to an electric field of 0·5 V cm⁻¹ (B). Mammillary root apex developing 24 h after disconnecting the field of 1 V cm⁻¹ (C and D). Formation of a new cap in the root treated with the same field strength as in C, but sampled 48 h after disconnecting the field (E). Scale bar = 100 µm (A–D), 200 µm (E).

Fig. 2.

Response of different maize roots to application of a DC electric field of 1 V cm⁻¹ over 3 h. The root on the left was photographed 3 h after beginning the field application (A) and that on the right 24 h after disconnecting the field (B). The root in B has just started growing downward again. The extent of root growth deviation from the vertical course, caused by the field, is indicated by the angle drawn on the photograph on the left.

Fig. 4.

Longitudinal sections of the root apices exposed to a DC electric field of 1 V cm⁻¹ (A, B, D) and 1·5 V cm⁻¹ (C). At 48 h after disconnecting the field, the new root cap develops, which is manifested by clear bending of the RCJ clonal border (arrows) (A). Five days after disconnecting the field, the complex of two root caps is still present. The new root cap formation is well advanced and the old cap is pushed aside (B). At 24 h after disconnecting the field, the cells stop dividing in the collumellar region and become axially elongated. The displacement of the amyloplast toward the anode is clearly visible in these cells. (C). On the edge of the newly formed root cap, the cells of the old root cap are crushed and become compressed (D). Scale bar = 40 µm (A, D), 100 µm (C) and 200 µm (D).