P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and, consequently, plants cannot grow without a reliable supply of this nutrient. Pi is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways (Theodorou and Plaxton, 1993). After N, P is the second most frequently limiting macronutrient for plant growth. This update focuses on P in soil and its uptake by plants, transport across cell membranes, and compartmentation and redistribution within the plant. We will concentrate on P in higher plants, although broadly similar mechanisms have been shown to apply in algae and fungi.Daniel P. Schachtman, Robert J. Reid, and S.M. Aylin

Potassium (K ⁺ ) is an essential nutrient required by plants in large quantities, but changes in soil concentrations may limit K ⁺ acquisition by roots. It is not known how plant root cells sense or signal the changes that occur after the onset of K ⁺ deficiency. Changes in the kinetics of Rb ⁺ uptake in Arabidopsis roots occur within 6 h after K ⁺ deprivation. Reactive oxygen species (ROS) and ethylene increased when the plants were deprived of K ⁺ . ROS accumulated in a discrete region of roots that has been shown to be active in K ⁺ uptake and translocation. Suppression of an NADPH oxidase in Arabidopsis ( rhd2 ), which is involved in ROS production, prevented the up-regulation of genes that are normally induced by K ⁺ deficiency, but the induction of high-affinity K ⁺ transport activity was unchanged. Application of H ₂ O ₂ restored the expression of genes induced by K ⁺ deficiency in rhd2 and was also sufficient to induce high-affinity K ⁺ transport activity in roots grown under K ⁺ -sufficient conditions. ROS production is an early root response to K ⁺ deficiency that modulates gene expression and physiological changes in the kinetics of K ⁺ uptake.

Plants often grow in soils that contain very low concentrations of the macronutrients nitrogen, phosphorus, potassium, and sulfur. To adapt and grow in nutrient-deprived environments plants must sense changes in external and internal mineral nutrient concentrations and adjust growth to match resource availability. The sensing and signal transduction networks that control plant responses to nutrient deprivation are not well characterized for nitrogen, potassium, and sulfur deprivation. One branch of the signal transduction cascade related to phosphorus-deprivation response has been defined through the identification of a transcription factor that is regulated by sumoylation. Two different microRNAs play roles in regulating gene expression under phosphorus and sulfur deprivation. Reactive oxygen species increase rapidly after mineral nutrient deprivation and may be one upstream mediator of nutrient signaling. A number of molecular analyses suggest that both short-term and longer-term responses will be important in understanding the progression of signaling events when the external, then internal, supplies of nutrients become depleted.