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The availability of potassium to the plant is highly variable, due to complex soil dynamics, which are strongly influenced by root-soil interactions. A low plant potassium status triggers expression of high affinity K+ transporters, up-regulates some K+ channels, and activates signalling cascades, some of which are similar to those involved in wounding and other stress responses. The molecules that signal low K+ status in plants include reactive oxygen species and phytohormones, such as auxin, ethylene and jasmonic acid. Apart from up-regulation of transport proteins and adjustment of metabolic processes, potassium deprivation triggers developmental responses in roots. All these acclimation strategies enable plants to survive and compete for nutrients in a dynamic environment with a variable availability of potassium.

Protein reserves in the cereal endosperm are sequentially degraded to small peptides and amino acids during germination and these are translocated across the scutellum to support growth of the embryo. Peptide transport in the germinating barley grain is mediated by specific carriers localized to the plasma membrane of the scutellar epithelium. In isolated barley embryos peptide transport is rapidly inhibited by amino acid concentrations comparable with those found in the post-germination barley grain. However, this inhibition of HvPTR1 activity is not effected at either the transcriptional or translational level. The protein phosphatase inhibitor okadaic acid repressed transport of Ala-[14C]Phe, but not [14C]Ala, into the barley scutellar epithelium. In vivo [32P]orthophosphate labelling studies of barley scutellar tissue in combination with immunoprecipitation studies using antiserum raised to HvPTR1 showed that HvPTR1 (66 kDa) is phosphorylated in the presence of amino acids. Immunopurified HvPTR1 was further demonstrated to be phosphorylated on serine residues. Digestion with the N-glycosidase enzyme PNGase F results in a shift in the molecular mass of the protein by 10 kDa, indicating that HvPTR1 is an N-linked glycoprotein. These results provide strong circumstantial evidence that HvPTR1 peptide transport activity in the germinating barley grain is regulated at the post-translational level by phosphorylation in response to rising levels of amino acids emanating from the endosperm as a result of storage protein breakdown and mobilization. This is potentially an important element in balancing the flux of organic nitrogen and carbon from the endosperm to embryo during germination and seedling establishment.

Alterations in the root shape in plant mutants indicate defects in hormonal signalling, transport and cytoskeleton function. To quantify the root shape, we introduced novel parameters designated vertical growth index (VGI) and horizontal growth index (HGI). VGI was defined as a ratio between the root tip ordinate and the root length. HGI was the ratio between the root tip abscissa and the root length. To assess the applicability of VGI and HGI for quantification of root shape, we analysed root development in agravitropic Arabidopsis mutants. Statistical analysis indicated that VGI is a sensitive morphometric parameter enabling detection of weak gravitropic defects. VGI dynamics were qualitatively similar in auxin-transport mutants aux1, pin2 and trh1, but different in the auxin-signalling mutant axr2. Analysis of VGI and HGI of roots grown on tilted plates showed that the trh1 mutation affected downstream cellular responses rather than perception of the gravitropic stimulus. All these tests indicate that the VGI and HGI analysis is a versatile and sensitive method for the study of root morphology.