Modeling pollen tube growth: Feeling the pressure to deliver testifiable predictions

The frequency and amplitude of oscillatory pollen tube growth can be altered by changing the osmotic value of the surrounding medium. This has motivated the proposition that the periodic change in growth velocity is caused by changes in turgor pressure. Using mathematical modeling we recently demonstrated that the oscillatory pollen tube growth does not require turgor to change but that this behavior can be explained with a mechanism that relies on changes in the mechanical properties of the cell wall which in turn are caused by temporal variations in the secretion of cell wall precursors. The model also explains why turgor and growth rate are correlated for oscillatory growth with long growth cycles while they seem uncorrelated for oscillatory growth with short growth cycles. The predictions made by the model are testifiable by experimental data and therefore represent an important step towards understanding the dynamics of the growth behavior in walled cells.     

Changes in nuclear electrolytes of Chironomus thummi salivary gland cells during development

A technique is described for the measurement of Na⁺, K⁺ and Mg²⁺ in cytoplasmic samples and nuclei from chironomid salivary glands. [Na], [K] and [Mg] were followed from the penultimate (L₃) through the last larval instar (L₄) until pupation, in the two developmental types, R and B, of Chironomus thummi. In nuclei, [Mg] falls from 78 mM in the middle of the L₃ to about 27 mM at the L₃ → L₄ molt, a level which is maintained thereafter. [Na] falls from 180 mM to around 60 mM at the L₃ → L₄ molt and falls further to about 38 mM in the prepupa; the pattern of change differs between the developmental types. [K] increases at both molts, from a level of 131 mM to about 150 mM at the L₃ → L₄ molt and from about 115 mM to about 130 mM at the pupal molt, the developmental types differing again. The cytoplasmic ion content measured during the prepupal stage runs parallel to the nuclear samples but always contains about 18 mM more Na and 12 mM less K than the nuclei. In the nuclei the ∑[Na + K] has a steady downward trend but rises slightly in the prepupa. The Na/K ratio follows a complex course related to the molt cycles. Analysis of variation allows to discern subclasses of nuclei with a particularly high Na/K ratio; the frequency of such nuclei is followed through development. It is argued (a) that the observed alterations in cation composition are of a kind and magnitude which under experimental conditions can induce puffs in certain chromosomal segments, and (b) that shifts in ion concentration may be implicated in the control of DNA replication.