four genes in this family, each with different expression characteristics. Two members of this family are
induced by decreased turgor, TMPand TMP[78,79]. It is predicted that the expression pattern reflects
functional specialization of the various family members [77]. During drought stress, these channels may
facilitate transport of ions, metabolites, or even water. Maurel et al. [80] demonstrated that -TIP is a wa-
ter-channel protein when expressed in Xenopusoocytes. Although -TIP is not expressed during water
stress, these results present some interesting possibilities for the drought-induced members of this class
of proteins. It must be determined if the water stress–induced proteins are also water-channel proteins and
in which membranes these proteins are located.
- RNA-Binding Protein
A gene from maize, MA16, with many characteristics of the leas, contains a glycine-rich repeat and is de-
velopmentally regulated in seeds and induced in dehydrated vegetative tissues [81]. Unlike the LEAs, this
protein contains a consensus sequence, RGFGFVTF, which is conserved in RNA-binding proteins. Lude-
vid et al. [82] used in vitro ribohomopolymer binding assays to confirm that MA16 has RNA-binding ac-
tivity. In these studies, MA16 preferentially binds poly(G). There are many different functions for ri-
bonucleoproteins, including regulation of alternative splicing, pre-mRNA processing, and mRNA
translation and stability. At this time the stress response cannot be predicted because the RNAs that are
associated with MA16 in vivo have not been determined. But it is possible that this protein has an im-
portant regulatory or protective function during plant stress.
- Gene Regulation
In addition to signaling changes in the environment, the signal transduction pathway must include the in-
duction or activation of transcription factors and activators that are required for transcription induction.
A transcription factor that is involved in the induction of some of the ABA-induced genes has been iden-
tified and isolated [83]. This protein, EmBP-1, is a bZIP-type transcription factor and recognizes DNA
sequences containing the DNA sequence CACGTGGC. A gene, Alfin-1, has been isolated from salt-tol-
erant alfalfa cells that has zinc finger motifs and may be a sequence-specific DNA-binding protein [84].
An H1 histone-like protein [85] and a nonhistone chromosomal protein [86] have also been shown to be
induced by drought stress. The roles of these proteins have not been elucidated.
D. Genes for Which a Function Has Not Been Predicted
In addition to the genes that have a function that can be predicted from the DNA sequence and amino acid
sequence, there are also many stress-induced genes for which no function has been predicted. A few of
these types of genes are discussed here.
- LEAGroup 6
With the identification of rab28[87], a new leagroup can be formed. Two representatives of group 6 have
been identified, one from cotton, D-34 [3], and the other from maize, rab28[87]. The predicted gene
product is different from the other LEA proteins, which are overwhelmingly hydrophilic; it has a balanced
hydrophobicity plot. RAB28 is predicted to have four to six -helices and a globular structure. Functional
predictions have not been made.
- RD22
A gene, rd22, induced by water stress, salinity, and ABA in Arabidopsis[88], was determined to be sim-
ilar to an unidentified seed protein (USP) from Vicia faba[89]. RD22 is induced early during seed de-
velopment but not during the late stages of embryogenesis, like USP. The homology between RD22 and
USP is found in the carboxy-terminal portion of the protein [88]. RD22 has an amino-terminal hy-
drophobic region with five repeated sequences in the amino terminus. The consensus is TnVnVGnG-
GVnnnnnnKGK, which is predicted to contain a sheet–turn–sheet structure [88]. The significance of
this structure or the function of this gene product is not known.
- Germinlike Proteins
A root-specific protein that is induced in barley by salt stress was found to be similar to germin [90]. Ger-
min is a protein of unidentified function that accumulates during the onset of growth following germina-
742 BRAY