B. Decreased Water Status
Several abiotic stresses have a common element between them, namely decreased cell water status. They
include water deficit (“drought,” “water stress”), salinity, and low temperature. Although each of these
stresses imposes unique perturbations, numerous proteins are induced in common, and these stresses ap-
pear to share some signaling pathways. Hence, these stresses have been combined under one topic area,
and each will be considered in turn.
- Water Deficit
PHYSIOLOGY. Water deficit affects plants on several levels. Numerous studies have demonstrated
that cell expansion and growth are among the first processes to decline under water deficit. With pro-
gressive water deficit, photosynthesis is adversely affected, and eventually assimilate partitioning. On the
cellular level, membranes and proteins can be damaged by a reduction in hydration (see later) and an in-
crease in reactive oxygen species or peroxidation.
Resistance to water deficit is manifested in four general ways: timing of growth to avoid water
deficit, morphological adaptations, physiological adaptations, and metabolic alterations. The first three
are complex processes and are incompletely understood, but significant progress has been made in un-
derstanding specific metabolic alterations.
Quantitative and qualitative changes in the synthesis of proteins have been reported to occur in plants
in response to water deficit. Reductions in polyribosome stability have been reported [100–103], as well
as changes in transcription [104]. Many, but not all, de novo synthesized proteins also appear in response
to ABA application, supporting the role of ABA as a mediator in some water deficit–related responses
(see later). Isolation of the genes responsible for these proteins has been accomplished in a number of
species by cDNA cloning techniques. The proteins have been placed in several gene families on the ba-
sis of sequence homology and similar expression patterns. Numerous abbreviations have arisen in de-
scribing these genes, including rab (responsive to ABA), RD (responsive to desiccation), ERD (early de-
hydration inducible), lea (late embryogenesis abundant), and em (early maturation). The nomenclature
used to describe water deficit stress is also diverse: drought, drought stress, water stress, and osmotic
stress have all been used, but these may not be physiologically equivalent.
Seed maturation has been used as a model for metabolic alterations resulting from desiccation or de-
hydration. While this approach has identified many desiccation-related proteins, seed maturation repre-
sents a very specific type of water deficit and occurs simultaneously with other developmental events. As
such, seed maturation will be considered only to a limited context.
WATER DEFICIT–INDUCIBLE PROTEINS
LEA Proteins. LEA genes were first described in a survey of cDNAs from developing cotton
seeds [105]. As analyzed by Dure [106], 18 different groups have been recognized on the basis of se-
quence homology both within cotton and between other species. Four groups, termed D-19, D-113, D-7,
and D-11 (see later), have been characterized to varying extents. Little functional significance has been
established for these groups, although Dure’s analysis [106] suggests that their secondary and tertiary
structures indicate that they could act as “reverse chaperones,” facilitate counterion storage, or to main-
tain the hydration status of proteins and membranes.
LEA D-11/RAB/Dehydrins. This group has been considerably studied at many levels [107]. De-
hydrins are widespread, occurring in every higher plant species examined. Sizes range from 9 to 200 kDa,
with a highly conserved 15-amino-acid consensus (EKKGIMDKIKEKLPG) located near the carboxy ter-
minus. This consensus is repeated up to 11 times, depending on the specific dehydrin. An analysis of the
sequences and induction stimuli suggests that several subclasses exist, some of which are principally low-
temperature inducible rather than inducible strictly by water deficit, salinity, or ABA [107].
Secondary structure predictions indicate that dehydrins could form an amphipathic -helix, which
has been proposed to interact with and stabilize proteins or membranes [106,107]. Low water status re-
duces the hydration of biomolecules such as proteins, which can lead their denaturation [108] and to the
disruption of membranes [109]. Dehydrins have been proposed to ameliorate these consequences by re-
ducing hydrophobic aggregations or inappropriate interactions [107]. Although their specific function is
yet to be demonstrated, several genetic studies have indicated a role in water deficit or cold tolerance. For
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 667