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.

Plant root sensing and adaptation to changes in the nutrient status of soils is vital for long-term productivity and growth. Reactive oxygen species (ROS) have been shown to play a role in root response to potassium deprivation. To determine the role of ROS in plant response to nitrogen and phosphorus deficiency, studies were conducted using wild-type Arabidopsis and several root hair mutants. The expression of several nutrient-responsive genes was determined by Northern blot, and ROS were quantified and localized in roots. The monitored genes varied in intensity and timing of expression depending on which nutrient was deficient. In response to nutrient deprivation, ROS concentrations increased in specific regions of the Arabidopsis root. Changes in ROS localization in Arabidopsis and in a set of root hair mutants suggest that the root hair cells are important for response to nitrogen and potassium. In contrast, the response to phosphorus deprivation occurs in the cortex where an increase in ROS was measured. Based on these results, we put forward the hypothesis that root hair cells in Arabidopsis contain a sensing system for nitrogen and potassium deprivation.

Auxin is a key plant hormone that regulates plant development, apical dominance, and growth-related tropisms, such as phototropism and gravitropism. In this study, we report a new Arabidopsis thaliana transcription factor, MYB77, that is involved in auxin response. In MYB77 knockout plants, we found that auxin-responsive gene expression was greatly attenuated. Lateral root density in the MYB77 knockout was lower than the wild type at low concentrations of indole-3-acetic acid (IAA) and also under low nutrient conditions. MYB77 interacts with auxin response factors (ARFs) in vitro through the C terminus (domains III and IV) of ARFs and the activation domain of MYB77. A synergistic genetic interaction was demonstrated between MYB77 and ARF7 that resulted in a strong reduction in lateral root numbers. Experiments with protoplasts confirmed that the coexpression of MYB77 and an ARF C terminus enhance reporter gene expression. R2R3 MYB transcription factors have not been previously implicated in regulating the expression of auxin-inducible genes. Also it was previously unknown that ARFs interact with proteins other than those in the Aux/IAA family via conserved domains. The interaction between MYB77 and ARFs defines a new type of combinatorial transcriptional control in plants. This newly defined transcription factor interaction is part of the plant cells9 repertoire for modulating response to auxin, thereby controlling lateral root growth and development under changing environmental conditions.

Potassium deprivation leads to large reductions in plant growth and yields. How plants sense and transduce the stress signals initiated by potassium deprivation is poorly understood. Both ethylene production and the transcription of genes involved in ethylene biosynthesis increase when plants are deprived of potassium. To elucidate the role of ethylene in low potassium signaling pathways, we used both genetic and chemical approaches. Our results showed that ethylene is important in tolerance to low potassium and for changes in both root hair and primary root growth in Arabidopsis thaliana. We show that ethylene acts upstream of reactive oxygen species in response to potassium deprivation. The expression of High-Affinity K+ Transporter5 was used as a marker of potassium deprivation and was found to be dependent on ethylene signaling. In the ethylene insensitive2-1 (ein2-1) mutant, the ethylene-mediated low potassium responses were not completely eliminated, suggesting that some potassium deprivation–induced responses are either ethylene independent or EIN2 independent. Ethylene signaling is a component of the plant's response to low potassium that stimulates the production of reactive oxygen species and is important for changes in root morphology and whole plant tolerance to low potassium conditions.

Summary: A hot pepper (Capsicum annuum) cDNA clone encoding pathogenesis‐related protein 10 (CaPR‐10) was isolated by differential screening of a cDNA library prepared from pepper leaves inoculated with tobacco mosaic virus pathotype (TMV‐P₀).CaPR‐10transcripts were induced in the incompatible interaction with TMV‐P₀orXanthomonas campestrispv.vesicatoria(Xcv) but not induced in the compatible interaction. Characterization of enzymatic properties of CaPR‐10 indicated that the recombinant protein exhibits a ribonucleolytic activity against TMV RNA, as well as against pepper total RNA, and shows its putative antiviral activity in several conditions. The CaPR‐10 protein existed at very low level in leaf tissue but was dramatically induced as soon as plants were inoculated with TMV‐P₀, and this was correlated with the increase of its ribonucleolytic activity. Immunoblot analysis and pull‐down assays using proteins extracted from pepper leaves showed that TMV‐P₀inoculation led to the phosphorylation of CaPR‐10, a modification that should affect its capacity for RNase function. We present data that the induction and subsequent phosphorylation of CaPR‐10 increased its ribonucleolytic activity to cleave invading viral RNAs, and this activity should be important to its antiviral pathway during viral attackin vivo.