Morphology as Clue to Developmental Regulation: Stomata

Abstract

Incipient stomata might have appeared in thalloid alga-like land plants as sporophytic structures homologous to gametangial conceptacles of their isomorphic gametophytes and developed in association with vascular tissue and cuticle. Historically, stomatal evolution is correlated with growth habits and synecological events like the early terrestrial plant expansion from wetland to dryland habitats or the appearance of arboreal and grass canopies. The general tendency of elaboration and aggregation of stomatal complexes was reversed with reduction of vegetative growth at such phylum initiating events as the origin of angiosperms. Stomata links ambient environment with leaf surface air layer and the air of intercellular spaces, balancing the reduction and oxidation reactions involved in photosynthesis. Adaptive responses to environmental change require adjustments of transpiration rates through stomatal density regulation, such as revealed in Ricotia lunaria ecotypes of mesic and xeric slopes of a canyon, with the differences in Stomatal Index comparable to those obtained by experimental exposure to mildly elevated CO2 levels. Long term responses include morphological elaboration of stomatal complexes and their aggregates providing for developmental stability, scarcely impaired by occasional aberrations. Distribution of stomatal complexes correlates with vascularizaton and wax biosynthesis controlled by auxin and ABA hormones respectively at the cell differentiation level. Stomatal transcription factors respond to the interfering auxin/brassinosteroid/ABA signaling mediated by kinaze receptors and their ligands. Stomatal developmental feedbacks are probably conferred by the pH activated protease and oxidative stress activated (’mitogen activated’) protein kinase cascades. This way the hormonal response to environmental pressure is converted into the chemical free energy potential of phosphorylation contributing to internal energy of structural innovation. Universality of this scheme may explain correlation of stomatal development with the whole plant growth habit, ecology and evolution.