Thomas Leustek

Histidine biosynthesis in plants

Summary.The study of histidine metabolism has never been at the forefront of interest in plant systems despite the significant role that the analysis of this pathway has played in development of the field of molecular genetics in microbes. With the advent of methods to analyze plant gene function by complementation of microbial auxotrophic mutants and the complete analysis of plant genome sequences, strides have been made in deciphering the histidine pathway in plants. The studies point to a complex evolutionary origin of genes for histidine biosynthesis. Gene regulation studies have indicated novel regulatory networks involving histidine. In addition, physiological studies have indicated novel functions for histidine in plants as chelators and transporters of metal ions. Recent investigations have revealed intriguing connections of histidine in plant reproduction. The exciting new information suggests that the study of plant histidine biosynthesis has finally begun to flower.

IS GRP78 A SENSOR OF CELLULAR SECRETORY ACTIVITY

In eukaryotes, secreted proteins and membrane proteins are transported through two membrane-bound compartments consisting of the endoplasmic reticulum (ER) and Golgi complex. In the lumen of the ER they undergo covalent modification, folding and assembly which are prerequisites for efficient transport (1). These events are mediated by a complement of resident ER proteins. Studies on the development of secretory capacity indicate that cells respond to increases in the synthesis of secretory proteins by coordinately regulating the expression of resident ER proteins (2). The glucose regulated proteins (GRP), GRP781 and GRP94, may provide important insight into the mechanisms by which coordinate regulation of ER resident proteins may be achieved. The GRPs are transcriptionally activated by a variety of stress agents which inhibit processing and transport of secretory proteins from the ER (3,4). While little is known of the function of GRP94, GRP78 is known to bind stably to unprocessed, misfolded or unassembled proteins whose presence in the ER stimulates GRP expression (5). The GRPs also play an essential role under non-stress conditions since expression is constitutive in all mammalian tissues but is increased in cells active in protein secretion (2), and GRP78 is highly conserved in eukaryotes (6). To explain the regulation of GRPs and other resident ER proteins several groups have suggested the existence of a feedback mechanism whereby cells monitor the level of exported proteins in the ER and adjust the level of resident ER proteins accordingly (2,5). GRP78 has been proposed to function as such an ER sensor. In this paper I review the evidence for this proposal including genetic and biochemical similarities to the heat shock proteins (HSPs) and specifically to the 70 kD heat shock family (HSP70) of which GRP78 is a member.

Advances in understanding sulfur utilization efficiency in plants

Sulfur (S) is an essential mineral nutrient required for plant growth and development. It is found in amino acids, vitamins/cofactors, electron transport chains, and a host of stress response compounds. S fertilization in agriculture is required only under specific circumstances, but the S nutrition status of crops has a profound effect on growth and resistance to stresses. Plants have evolved integrated mechanisms to sense S status and to respond to ambient conditions. This book chapter summarizes recent progress on clarifying mechanisms that impact S utilization efficiency, including sulfate uptake and mobilization, S sensing, signaling, and the mobilization of stored S reserves.

SULTR1;2 in S Nutrient-Status Control in Arabidopsis

An essential life process for plants is the assimilation of sulfur, which is available to plant roots as sulfate that is taken up into cells where it is reduced and assimilated. Plants can sense the availability of sulfur, but the sensor has not been identified. Two Arabidopsis thaliana mutants were isolated (termed sel1-15 and sel1-16), that show increased expression of a sulfur-deficiency-activated gene beta glucosidase 28 (BGLU28) when grown on medium with high sulfate content (Zhang et al. 2014). The mutants are missense alleles of SULTR1;2 encoding a high affinity sulfate transporter. Although the mutants are defective in sulfate transport they show higher expression of BGLU28 and other sulfur-deficiency-activated genes even when they are treated with a very high dose of sulfate such that the intracellular concentration of sulfate and its metabolite glutathione (GSH) are identical to wild type. The reduced sensitivity to inhibition of gene expression is also observed in sel1 mutants fed with the sulfur assimilation products cysteine and GSH. The results suggest that SULTR1;2 may have a second role in addition to its known function as a high affinity sulfate transporter. It may also have a regulatory role in response to sulfur nutrient status.