Unequal distribution of osmiophilic particles in the epidermal periplasmic space of upper and lower flanks of gravi-responding rye coleoptiles

Lateral redistribution of auxin is not the means for gravitropic differential growth of coleoptiles: A new model

Gravicurvature in water- and auxin (IAA)-incubated coleoptiles of rye (Secale cereale L.) is similar, despite a general strongly enhancing effect of exogenous IAA on the overall (cell) elongation of these organs. Longitudinally split coleoptiles or isolated longitudinally halved coleoptiles (horizontally positioned as upper or lower halves) respond gravitropically in the same way as water-incubated intact coleoptiles, irrespective of whether the halves are incubated in distilled water or IAA. A new model for the principal mechanism of regulation of gravitropic growth is proposed which depends on, yet does not involve, the redistribution of IAA as the means for gravistimulated differential growth, as postulated by the Cholodny-Went hypothesis (CWH). It is based on a gravimediated temporarily restrained infiltration of IAA-induced wall-loosening factors into the growth-limiting outer epidermal walls of the concave organ flank.

Apoplast as the site of response to environmental signals

When the life cycle of plants is influenced by various environmental signals, the mechanical properties of the cell wall are greatly changed. These signals also modify the levels and structure of the cell wall constituents and such modifications are supposed to be the cause of the changes in the wall mechanical properties. These changes in the cell wall, the major component of the apoplast, can be recognized as the response of plants to environmental signals. The analysis of the mechanism leading to the response suggests that the apoplast is involved not only in the response but also in the perception and transduction of environmental signals in concert with the receptors of signals located on the plasma membrane. Thus, the apoplast plays a principal role in the communication of plants with the outer world and enables the plants to adapt themselves and survive in the environment full of stresses.