have been identified that are involved in antifungal activity, protection from freeze-thaw inactivation,
transport of water, RNA binding, and gene regulation.
- Antifungal Proteins
In addition to the direct effects of water stress, environmental stress may contribute to the susceptibility
of the plant to pathogens. Genes encoding osmotin and nonspecific lipid transfer proteins have antifungal
activity and are induced in response to water deficit–based stresses.
A protein was first discovered that was prominent in tobacco cells adapting to high concentrations of
salt. The accumulation of the protein is correlated with osmotic adaptation and it was therefore named os-
motin [59,60]. This protein has a signal sequence and is localized in vacuole inclusion bodies. It is a ba-
sic homologue of family 5 pathogenesis-related proteins. This protein family, including osmotin, has been
shown to have antifungal activity [60–62]. Members of this family have been shown to permeabilize the
fungal plasma membrane. Transgenic tobacco plants overproducing osmotin have been shown to be more
tolerant of fungal attack than are control plants [60]. It is proposed that there is a specific interaction be-
tween osmotin and the membrane, possibly with a portion of the molecule interacting with the membrane,
with the rest forming an ion or water channel that permeabilizes the fungal membrane [60]. It is not cer-
tain if this is the only role of osmotin or if it also plays a direct role in salt tolerance.
Proteins that have the capacity to transfer lipids from liposomes to mitochondria in vitro have been
studied in plants. A class of these proteins transfers a number of different classes of lipids and has been
called nonspecific lipid transfer proteins (nsLTPs) [63]. Three genes that are homologous to nsLTP genes
have been found to be induced by stress in aerial plant parts: two from tomato [64,65] and one from bar-
ley [66]. However, the role of these genes during stress has not been determined. Since it was determined
that nsLTPs are expressed in the epidermis of the shoot, it has been suggested that they play a role in cu-
ticle formation [67]. It has now been demonstrated that an nsLTP-like protein isolated from radish seeds
has antifungal activity; it inhibited fungal hyphae growth but did not affect spore germination [68]. There-
fore, the role of nsLTPs during stress may be in the protection of the shoot from fungal attack. However,
further studies are required to elucidate the function of nsLTP-like proteins during stress.
- Protection from Freeze-Thaw Inactivation
Another type of protection is the protection of enzymes from freeze-thaw inactivation. The cor15gene is
induced by cold acclimation in Arabidopsis[69]. The polypeptide that is encoded by this gene is targeted
to the chloroplast [70]. It is 100 times more effective than BSA at protecting lactate dehydrogenase from
freeze-thaw inactivation [71]. This preliminary evidence indicates that this gene encodes a protein with a
potential protective role in freeze-induced dehydration.
- Protein Degradation
Cellular function may also be protected during stress by preventing protein degradation or degrading pro-
teins that are no longer functional. Heat shock proteins that are induced by water deficit [72,73] may be
involved in the refolding of proteins to regain their function, or the prevention of protein aggregation [74]
during stress. Protease inhibitors induced by water deficit may protect against proteases released after cel-
lular disruption and membrane disorganization as a result of stress. The gene Bnd22, induced after pro-
longed dehydration stress in Brassica napus, has some characteristics of Künitz trypsin inhibitor, al-
though it does not have all the signature amino acids. It is also expressed in a manner that is different from
other Künitz trypsin inhibitors; it is not expressed in seeds, it is expressed only in the aerial parts of the
plant after a prolonged drought stress [75]. Further studies are needed to determine if this protein is a pro-
tease inhibitor that plays a role during water deficit. Ubiquitin, which was also shown to be induced by
water deficit [72], may target proteins for degradation that cannot regain function after water deficit.
- Major Intrinsic Proteins
A class of proteins, major intrinsic proteins, have been identified which may form transmembrane chan-
nels and be involved in the transport of ions, other metabolites, or water across membranes. These pro-
teins have six membrane-spanning domains that are postulated to form a channel. Drought-induced ex-
amples of this class of proteins have been isolated from pea [52] and Arabidopsis[76–79]. These genes
are most closely related to each other, but are also similar to nod26, tonoplast intrinsic protein (TIP) from
bean, bovine major intrinsic proteins (MIP), and glycerol facilitator protein [77]. Arabidopsishas at least
ABIOTIC STRESSES AND ABSCISIC ACID 741