Mechanical Stimulation Decreases Auxin and Gibberellic Acid Synthesis but Does Not Affect Auxin Transport in Axillary Buds; It Also Stimulates Peroxidase Activity in Petunia × atkinsiana

Abstract

Thigmomorphogenesis (or mechanical stimulation-MS) is a term created by Jaffe and means plant response to natural stimuli such as the blow of the wind, strong rain, or touch, resulting in a decrease in length and an increase of branching as well as an increase in the activity of axillary buds. MS is very well known in plant morphology, but physiological processes controlling plant growth are not well discovered yet. In the current study, we tried to find an answer to the question if MS truly may affect auxin synthesis or transport in the early stage of plant growth, and which physiological factors may be responsible for growth arrest in petunia. According to the results of current research, we noticed that MS affects plant growth but does not block auxin transport from the apical bud. MS arrests IAA and GA₃ synthesis in MS-treated plants over the longer term. The main factor responsible for the thickening of cell walls and the same strengthening of vascular tissues and growth arrestment, in this case, is peroxidase (POX) activity, but special attention should be also paid to AGPs as signaling molecules which also are directly involved in growth regulation as well as in cell wall modifications.

Keywords: cell wall lignification; plant architecture; plant hormone synthesis; thigmomorphogenesis.

Figure 1

Shoot growth in petunia subjected to MS depending on stress intensity. Statistical analysis was made in each term separately. A total of 15 plants in each block from each treatment were measured in each of the five terms (α − 0.05). The lowercase letters present statistical differences (α − 0.05) between the treatments.

Figure 2

(a) IAA content (ng·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM], analyzed separately, of petunias subjected to MS depending on stress intensity. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α − 0.05). (b) IAA content (ng·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM], analyzed separately, of petunias subjected to MS depending on stress duration. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α − 0.05). The lowercase letters present statistical differences (α − 0.05) between the treatments.

Figure 2

(a) IAA content (ng·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM], analyzed separately, of petunias subjected to MS depending on stress intensity. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α − 0.05). (b) IAA content (ng·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM], analyzed separately, of petunias subjected to MS depending on stress duration. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α − 0.05). The lowercase letters present statistical differences (α − 0.05) between the treatments.

Figure 3

Auxin influx (LAX1 and AUX1) and efflux (PIN1) carriers immunoanalysis in stems of control (A,B,E,F,I,J) and stroked 160 times per day (C,D,G,H,K,L) plants. Red arrows present LAX1, AUX1, and PIN1 antibody signals in SAM buds and provascular strands of the main stem (A,C,E,G,I,K) and no signal (blue arrow) in the axillary bud (B,D,F,H,J,L).

Figure 4

(a) Gibberellic acid (GA₃) content (ng·g⁻¹ DW) in the apical meristem of stems [SAM], analyzed separately, of petunias subjected to MS depending on stress intensity. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α = 0.05). (b) Gibberellic acid (GA₃) content (ng·g⁻¹ DW) in the apical meristem of stems [SAM], analyzed separately, of petunias subjected to MS depending on stress duration. The specimen was collected from 15 plants in each block from each treatment in each of the five terms (α − 0.05). The lowercase letters present statistical differences (α − 0.05) between the treatments.

Figure 5

(a) POX activity (µg purpurogallin·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM] of petunias subjected to MS depending on stress intensity. The specimen was collected from 15 plants from each treatment in each of the five terms; α − 0.05. (b) POX activity (µg purpurogallin·g⁻¹ DW) in the apical meristem of stems [SAM] and roots [RAM] of petunias subjected to MS depending on stress duration. The specimen was collected from 15 plants from each treatment in each of the five terms; α − 0.05 The lowercase letters present statistical differences (α − 0.05) between the treatments..

Figure 6

Histological determination of cell wall lignification in vascular bundles of stems of P. atkinsiana. (A–D)—longitudinal section of the stem of control plants. Tracheary elements are not visible in vascular bundles. (E–H)—longitudinal section of the stem of plants subjected to MS 160 times. Tracheary elements are clearly visible in vascular bundles. Arrow-tracheary elements. (I,J)—cross-section of the stems of control (I) and subjected to MS 160 times (J). Usually, plants subjected to stress develop reinforcing tissue, such as collenchyma or sclerenchyma, resulting in the arrest of developing growth. (I)—cross-section of the stem of control plants. Sclerenchyma cells are not visible. (J)—cross-section of the stem of plants subjected to MS 160 times. Sclerenchyma cells are clearly visible. Arrow—sclerenchyma cells.