Callose implication in stomatal opening and closure in the fern Asplenium nidus

P>The involvement of callose in the mechanism of stomatal pore opening and closing in the fern Asplenium nidus was investigated by examination of the pattern of callose deposition in open and closed stomata, and by examination of the effects of callose degradation and inhibition or induction of callose synthesis in stomatal movement. Callose was identified with aniline blue staining and a callose antibody and degraded via beta-1,3-d-glucanase. Callose synthesis was inhibited with 2-deoxy-d-glucose and induced by coumarin or dichlobenil. Stomatal pore opening and closing were assessed by estimation of the stomatal pore width. The open stomata entirely lacked callose, while the closed ones displayed distinct radial fibrillar callose arrays in the external periclinal walls. The latter displayed local bending at the region of callose deposition, a deformation that was absent in the open stomata. Both callose degradation and inhibition of callose synthesis reduced the stomatal ability to open in white light and close in darkness. By contrast, callose synthesis induction considerably improved stomatal pore opening and reduced stomatal closure in same conditions. The present data revealed that: during stomatal closure the external periclinal guard cell walls experience a strong mechanical stress, probably triggering callose synthesis; and that callose participates in stomatal movement.