Genetic Evidence for an Osmoregulatory Function of Glycinebetaine Accumulation in Barley

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Abstract
Betaine (glycinebetaine) accumulates in several plant families in response to salt or water stress. Although betaine is hypothesised to be central to cellular osmoregulation, there is little direct evidence for this. Betaine level in barley was therefore altered genetically by creating two F4 isopopulations with different mean betaine levels, and the osmoregulatory behaviour of these populations and of their parents was examined under saline conditions. To minimise linkage effects, the isopopulation procedure included several parents and two rounds of crossing. Mean betaine levels of the two populations in unstressed conditions were 23 and 32 μmol g-1 dry weight; measurements of various morphological and developmental characters indicated that the populations were otherwise genetically comparable. Salinising with 300 mM NaCl caused a decline of 0.5 MPa in solute potential (Ψs) and an 8- to 10-fold increase in betaine level in both isopopulations. The absolute difference in betaine level between the two populations remained constant as salt concentration was raised to 300 mM. Although selected only for differing betaine level, the parents and isopopulations differed also for Ψs; the high-betaine genotypes maintained a Ψs 0.1 MPa lower than the low-betaine genotypes at all salt levels. Furthermore, in both populations and parents, betaine level was linearly related to Ψs (r2 Ͱ4; 0.97). These observations are consistent with coordinated genetic control of betaine level and Ψs and imply that betaine accumulation is a mandatory component of osmoregulation in barley.