Late‐embryogenesis‐abundant (Lea) genes are induced in immature embryos by ABA and osmotic stress. Using the ABA‐biosynthesis inhibitor norflurazon in combination with ionic and non‐ionic stress, the expression of three distinct groups of Lea transcripts in immature barley embryos was investigated, these groups being embryoand aleurone‐specific B15C mRNAs, embryo‐specific B19 inRNAs, and rab/dehydrin mRNAs, which encode stress‐related proteins that are also expressed in vegetative tissues. The expression of B15C and rab/dehydrins in response to mannitol was not affected by the addition of norflurazon, which decreased the endogenous ABA levels by 89%; this observation is consistent with an ABA‐independent pathway. The expression of B19 was only slightly dependent on ABA. In contrast, induction of the Lea genes by salt (NaCI) required at least 25ümol m−3 endogenous ABA. Together, these results are evidence for different signal transduction pathways linking ionic and non‐ionic osmotic stress to Lea mRNA accumulation in barley embryos.

Whole leaves and guard-cell protoplasts of the C3 plant Vicia faba L. (broad bean) were separately extracted following a period of illumination or following a period of darkness. Kinetic parameters of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), Vmax and Km(PEP.Mg), were determined as a function of assay pH (7.0 or 8.1), the presence of 5 mM glucose-6-Pfree (Glc-6-P, an activator), and the presence of 5 mM malatefree (an inhibitor). On the basis of these parameters, guard-cell PEPC was distinguished from that of whole leaf, indicating either that guard cells contain a unique isoenzyme of PEPC or a different complement of isoenzymes or--and less likely--that the obligatorily different methodologies for the leaf (intact organ) and the guard-cell (protoplast) enzymes altered them specifically. The values of Vmax were relatively unchanged, regardless of assay conditions or tissue pretreatment. The values obtained for whole-leaf PEPC Vmax were restricted to a small range (52.4 +/- 5.9 (SD) to 64.4 +/- 4.8 (SD) mumol.g fresh mass-1.h-1; the high value coincided with the presence of Glc-6-P, and the low value was obtained in the presence of malate. Guard-cell PEPC Vmax was also restricted to a small range: 7.48 +/- 0.89 (SD) pmol.guard-cell pair-1.h-1 (pH 8.1, light, +Glc-6-P) to 5.79 +/- 0.60 (SD) pmol.guard-cell pair-1.h-1 (pH 7.0, dark, +malate). Depending on effectors, and particularly pH, large changes in Km(PEP.Mg) were calculated (whole-leaf PEPC: 0.03 to 3.84 mM; guard-cell PEPC: 0.06 to 3.43 mM).(ABSTRACT TRUNCATED AT 250 WORDS)