tein sequence is well conserved among all the genes that have been identified in monocots and dicots. An
internal repeat of a hydrophilic amino acid motif GGQTRKEQLGEEGYREMGHK was found in the bar-
ley genes and may be repeated up to four times [6] (Figure 1). A similar 20-amino-acid motif is dupli-
cated in the cotton gene leaA2, corresponding to the cDNA D-32 [7], although most of the genes identi-
fied thus far contain only one of these motifs, including the gene first published, D-19 [3]. It has been
suggested that group 1 proteins function in a water-binding capacity, creating a protective aqueous envi-
ronment [6].
- Dehydrin/RAB/D-11 Family (Group 2)
Group 2 proteins have been called dehydrin [8], RAB (responsive to ABA) [9], and D-11 [3]. These pro-
teins are also overwhelmingly hydrophilic. There is a characteristic lysine-rich region with the consensus
amino acid sequence EKKGIMDKIKEKLPG, which is repeated at least two times, once at the carboxy
terminus and once internally [10]. These genes are expressed in seeds and in response to water deficit, salt
stress, and low-temperature treatments in vegetative tissues [11,12]. These genes are also ABA regulated.
The mRNA has been found in all stressed organs that have been investigated. In tomato, maize, and Ara-
bidopsis, family members have been shown to be regulated by elevated levels of endogenous ABA dur-
ing periods of dehydration in leaves [13–15]. In Craterostigma plantagineum, the protein DSP16 is lo-
calized in the cytoplasm, as determined by immunocytolocalization [16]. Close et al. [10] have observed
that dehydrins are localized in the cytoplasm and the nucleus of aleurone layers, but primarily in the cy-
toplasm of root and shoot cells.
This class of genes has been identified more frequently than any other stress-induced gene in plants.
Members of this family of proteins have been found in grasses and in dicots, and there is also immuno-
logical evidence that these proteins accumulate in Anabaena[17,18]. Tables of these genes are presented
in Close et al. [10] and Dure [19]. It has been recognized that this class of proteins can be divided into at
least two types. In type I, the internal lysine-rich signature motif is adjacent to a polyserine region. The
ABIOTIC STRESSES AND ABSCISIC ACID 737
Figure 1 Unique aspects of several of the overwhelmingly hydrophilic proteins that accumulate during pe-
riods of water deficit. (A) Conserved element that may be repeated in the Em family of proteins [3–7]. (B) Con-
served amino acid motifs found in the dehydrin/RAB/D-11 family of hydrophilic proteins. Two types have been
noted, those with the polyserine region (S) and those without [3,8–10,19,24]. (C) Conserved 11-amino-acid re-
peat found in D-7 which may form an amphipathic -helix. At position 12 in the wheel, the amino acid sequence
is repeated. Positive and negative amino acids are noted with plus and minus signs. Amino acids with an amide
group are boxed. Apolar amino acids are shown in the shaded box. Note that one face is apolar and the other
has a pattern on negatively charged, amide-containing, positively charged amino acids [3,25]. A similar pattern
of amino acids with these characteristics is repeated in D-29, although the exact amino acids are not conserved
[3,4,25,26]. (D) The D-113 family of proteins has a conserved structure with an -helix in the amino-terminal
domain followed by a random coil region at the carboxy terminus [19,26].