In acidic soils, Al primarily affects root growth, and several hypotheses exist regarding the mecha-
nism(s) of injury [190]. Plants apparently wield a variety of exclusion mechanisms, but the evidence for
tolerance mechanisms is contradictory [190]. Some data exist for metallothionein-like proteins, as well as
inducibility of PAL and proteinase inhibitors, but these are probably not the primary means of resistance
[190]. Research indicates that Al-resistant Arabidopsismutants utilize the exudation of Al-chelating or-
ganic acids or perhaps alkanization of the rhizosphere [191,192]. Undoubtedly, several enzyme biosyn-
thetic and transport pathways must be activated or induced for such activity.
F. UV Radiation
- Ultraviolet-Absorbing Compounds
The responses of plants to UV radiation are of increasing concern because of the depletion of UV-ab-
sorbing ozone in the upper atmosphere. Research on the changes in protein synthesis related to UV has
centered on the transcription and translation of enzymes involved in the flavonoid and anthocyanin
biosynthetic pathways. Aside from their roles in plant defense against pathogens and as pigments,
flavonoids also absorb UV. Chalcone synthase (CHS) and to some extent phenylammonia lyase (PAL)
are the best studied. Wingender et al. [193] examined the promotor region of the chsgene from parsley
and determined that two elements exist for the induction of CHS by UV in addition to an element for the
elicitor induction. Subsequent work indicated that UV photoreceptors are responsible for the initial per-
ception, and additional photoreceptors are required for anthocyanin or flavonoid biosynthesis in parsley
[194].
Given the induction of CHS and PAL, it is perhaps unsurprising that other biotic stress genes are also
induced by UV. Conconi et al. [195] noted the UV induction of several jasmonic acid–inducible genes
and speculated that UV-induced lipid peroxidation may stimulate the octadecanoid pathway leading to JA
(see earlier). Any function of these proteins against UV damage is unlikely, and Conconi et al. [195] sug-
gested that diversion of C and N into these proteins may result in lowered fitness.
- DNA Repair
A second area of interest is DNA repair mechanisms. As is well known, UV radiation induces various le-
sions in DNA. The best studied are cyclobutane-type pyrimidine dimers, which have been the only type
of DNA lesions reported in plants [194]. The dimers can be repaired via photoreactivation (photolyase),
excision repair, or recombinatorial repair [196,197]. The latter type of repair has not been reported in
plants [194], and very little research has apparently been reported on excision repair in plants. Photore-
activation has been reported in several species (e.g., gingko [198], tobacco [199], pinto bean [200], and
maize pollen [201]. Pang and Hays [202] reported on the presence of a photolyase activity in Arabidop-
sis thaliana. They indicated that the putative photolyase has a requirement for visible light, with an opti-
mum of between 375 and 400 nm, which is similar to that of maize pollen photolyase. Pang and Hays
[202] further suggested that Arabidopsismay actually have two photolyases, one similar to that found in
E. coliand one similar to that reported in pinto bean.
IV. CAVEATS
A final caution should be made: changes in expression at the transcriptional level do not necessarily
equate to changes at the translational level. For example, LaRosa et al. [41] reported that in tobacco, os-
motin mRNA is strongly induced by NaCl, water deficit, wounding, ABA, ethylene, and tobacco mosaic
virus. In contrast, osmotin protein levels are weakly stimulated by all but NaCl and water deficit. In ad-
dition, Artlip et al. [203] reported on the appearance of a dehydrin transcript without the appearance of
the corresponding dehydrin protein. Clearly, posttranscriptional regulation exists and needs to be consid-
ered before drawing conclusions based solely on transcript data.
An exception to this caution may be transcription factors, which frequently display apparent dispar-
ities between transcription and translation. Rather than posttranscriptional regulation, many transcription
factors undergo rapid degradation or targeted proteolysis for activation. This is well known from cell cy-
cle research, where mutants deficient in specific transcription factor degradation are seriously compro-
mised. A report by Becker et al. [204] noted the necessity for a functional proteasome complex for in-
674 ARTLIP AND WISNIEWSKI