Sulfate Uptake and Assimilation – Whole Plant Regulation

Abstract

Sulfur remains an important issue on the agenda for crop plant nutrition. In addition to avoidance of sulfur deficiency, which will impact on yield and quality, there are requirements for adequate fertilization of crops for resistance to biotic and abiotic stresses. Equally importantly, there are clear consequences for efficient nitrogen utilization and there are interactions with micronutrient acquisition (selenium and molybdenum). Substantial advances have been made at the cellular level, dissecting the signal transduction pathways linking cellular nutritional status with expression of sulfur regulated genes and pathways. However cellular processes need to be placed in the context of whole plant regulation of sulfur uptake and assimilation, which encompasses developmental, spatial and environmental factors, and which facilitates optimum growth and fecundity (and yield in the case of crops) with available sulfur supply. During development, adequate sulfur must be acquired for optimum growth, ideally with any excess being sequestered into re-mobilizable temporary stores. As the plant develops, efficient utilization of sulfur will require organ to organ transfer, and additionally degradation pathways, metabolic inter-conversions and multiple trans-membrane and vascular tissue mediated transport steps for both inorganic and organic sulfur compounds. For crops, efficient transfer of sulfur to harvested sink tissues and its incorporation into protein are important agronomic traits. Insufficient sulfur to meet the demand for growth results in a number of plant responses, targeted at optimising uptake and use of available sulfur. Notable early and specific responses are the up-regulation of transporters and key steps of the assimilatory pathways in sulfur-deficient tissues, and the allocation of resources to stimulate growth of root tissues compared to the shoots. These responses involving root proliferation and transporter functionality are adaptations to improve pedospheric sulfur acquisition. A long standing question has concerned the existence and nature of inter-organ signals of nutrient status. It is possible that local sulfur availability, coupled with intrinsic cell specific programmed function, is sufficient to mediate local gene and pathway expression, influence organ responses and effect whole plant sulfur management without inter organ signals. Developmental cues will influence organ specific pathways, most clearly demonstrated in processes of leaf senescence and associated nutrient remobilization. Conversely, the recognition of possibly mobile phloem located miRNAs may be indicative of long distance regulatory mechanisms. Similarly, root proliferation will almost certainly have a hormonal basis.