Plant responses to environmental stress

Plant responses to environmental stress

Elizabeth Vierling and Janice A. Kimpel

University of Arizona, Tucson, Arizona and University of Georgia, Athens, Georgia, USA

Considerable progress is being made in identifying genes that are important for tolerance to abiotic stress and in defining stress-responsive gene promoters and signal-transduction pathways. Although genetically engineered crop plants with greater resistance to environmental stress have not yet been produced, research is at a turning point where correlative changes can now

be tested for effectiveness in conferring stress tolerance.

Current Opinion in Biotechnology 1992, 3:164-1 70

Introduction

It is well recognized that agricultural losses resulting from environmental stress are significant. The challenge for plant breeders and biotechnologists continues to be production of stress-resistant plants while maintaining acceptable yields. Recent molecular studies of plant stress responses and their relevance to engineering stress-resistant plants are the subject of this review.

Several major areas of progress can be identified. First, many stress-induced genes have been cloned and characterized and their roles in the stress response are now being clarified. Genes involved in small molecule biosynthetic pathways important for stress tolerance, including hormones, osmolytes and phytochelatins, are being identified. Second, mechanisms by which plants sense stress, thus leading to adaptive responses, are being defined. This includes elucidation of signal-transduction pathways and defi- nition of transcriptional and post-transcriptional regulatory mechanisms. Finally, restriction fragment length polymorphism (RFLP) mapping technology is now available in several plant species, providing a means of marking and tracking genetic loci associated with stress resistance. It is likely that response to stress is mediated by several genes; RFLP maps can be used to estimate additive effects and dominance of each locus associated with the phenotype.

Although the following discussion considers different stresses individually, it is important to recognize that many of the responses overlap because of similarities in the physiological changes that occur. For example, drought stress, salt stress and cold stress all in- volve problems of water availability. Because of such overlaps, exposure"'to one type of stress may induce a degree of tolerance to other stresses. Also, many

stress-induced genes are also developmental ly regulated, which indicates that physiological changes at different stages of development have similarities with those exper ienced during stress.

Osmotic stress

Osmotic stress can be caused by several different environmental factors including drought, desiccation, salt and cold. As a step toward understanding and engineering tolerance to these stresses, researchers have identified genes related to desiccation [1], genes induced by low turgor (encoding a putative ion chan- nel, a thiol protease and aldehyde dehydrogenase [2]), genes involved in the biosynthesis of compatible solutes (proline, pyrroline-5-carboxylate reductase, betaine, betaine aldehyde dehydrogenase) [1,3"], genes involved in ion transport [1,3"], and genes induced by salinity (e.g. osmotin) [1]. As the hormone abscisic acid (ABA) is involved in plant responses to osmotic stress, it is not surprising that many, though not all, of these genes are also regulated by ABA [1]. ABA- regulated gene expression is under intense investiga- tion and both cis-acting sequences and trans-acting factors important for ABA-mediated gene expression have been identified. Further advances in the areas of drought stress, salt stress and cold stress are discussed below.

Drought stress

Some of the best characterized genes expressed in response to osmotic stress are the Lea (late embryo-

Abbreviations AB~abscisic acid; bZl~leucine zipper DNA binding; CHS--chalcone synthase; 4CL--4-coumarate-coenzyme A ligase;

CPRF--common plant regulatory factor; GUS--[8-glucuronidase; HSE--heat-shock promoter element; HSP--heat-shock protein; LMW--Iow molecular weight; PI--proteinase inhibitor; RAB--responsive to ABA; RFLP--restriction fragment length polymorphism;

SOD--superoxide dismutase; UV--ultraviolet.

© Current Biology Ltd ISSN 0958-1669

Plant responses to environmental stress Vierling and Kimpet 165

genesis abundant) genes. These were first identified as being expressed late in seed development and their expression is correlated with increased ABA levels and tolerance of embryos to desiccation. There are three major groups of Lea genes and homologs of each group that are regulated by osmotic stress have been found [1]. Certain LEA-like genes/proteins have also been called RAB (responsive to ABA) or dehydrins. This is a major example of how normal physiological processes such as embryo desiccation and stress can overlap. The LEA proteins are extremely hydrophilic and it has been suggested that they protect other proteins from the effects of water loss. Ad- ditional support for this model has now been gained from studies of Ceratostigma p lantagineum (resurrec- tion plant), which is adapted to tolerate extreme desiccation. Piatkowski et al. [4] cloned ABA-responsive genes f rom this plant and found three that are related to previously described genes encoding LEA proteins (or dehydrins). Two genes not previously identified in other plant species were also found.

It is important to recognize that different plant organs or even different growth regions of the same organ respond differently to osmotic stress and also to ABA. Plant et al. [5] identified a protein that is expressed in drought-stressed aerial parts of tomato, but not in roots, and shows homology to a phospholipid transfer protein. The same group has linked ABA to the induction of this gene by showing that it is not induced in the ABA-deficient mutant of tomato, f lacca. Examining several genes, including [3-tubulin, actin, cell wall proteins (glycine-rich and hydroxyproline-rich proteins) and two unidentified water-deficit-induced sequences, Creelman and Mullet [6] documented changes in expression specific to the growth-inhibited elongating region of soybean hypocotyls. The authors speculate that the decrease of 13-tubulin and actin reflects the decreased growth rate and that changes in cell wall proteins may control wall extensibility in the elongating region.

Differential inhibition of shoot versus root growth is a well recognized response to drought stress. It appears that although ABA accumulates in both leaves and roots, the hormone only inhibits the growth of leaves. Sharp's group has extensively studied this physiological phenomenon . Their recent results [7] indicate that proline deposition is responsible for osmotic adjustment in root tips and allows continued growth of roots experiencing water deficit. These data not only indicate that osmotic adjustment is highly regulated, but also provide insight into the involvement of proline in drought tolerance.

Salt stress

Plants experiencing salt stress suffer from both reduced water availability and the accumulation of toxic ions, in particular Na +. McCue and Hanson [3"'] have written an excellent review of the problems and progress in

engineering plants for salt tolerance, particularly with regard to solute accumulation. Now that several genes involved in the biosynthesis of compatible solutes have been identified, understanding the regulation of solute product ion in different plant systems becomes the next challenge. Another important regulatory as- pect is the control of ion pumps. Genes have been identified that encode subunits of the tonoplast AT- Pase, which is thought to be essential for the energy requirements of increased Na + pumping. However, the extent to which transcriptional versus post-transcriptional processes modulate the activity of these pumps is unclear [8].

Metabolically engineering plants to have traits that may confer stress tolerance has recently been accomplished by Tarczynski et al. [9"']. They introduced the Escherichia coli gene encoding mannitol- 1-phosphate dehydrogenase (mt lD) into tobacco, and demonstrated that the transgenic plants had elevated levels of mannitol in leaf and root tissues (exceed- ing 6 btmol gram-1 fresh weight). In E. coli, mtlD is normally involved in catabolism of mannitol- l-phosphate to fructose-6-phosphate. In plants, it appears that excess fructose-6-phosphate drives the reaction in reverse, with a non-specific phosphatase rapidly and irreversibly converting the manni tol- l -phosphate to mannitol. These transgenic plants are excellent ma- terial for studies of the contribution of sugar alcohols to tolerance of salt or other osmotic stresses.

Several genes induced in response to salt stress have been identified, including RAB genes, salT and that encoding osmotin, of which some show tissue-specific expression [1]. The function of these genes remains unclear beyond the proposed function of the Lea-like RAB genes. They do not appear to be strictly salt-specific, but also respond to other osmotic stresses and in many cases to ABA. Gene induction is also involved in the adaptive change in photosynthesis in the salt-toler- ant plant Mesembryanthemum crystallinum. Bohnert and colleagues [10] have shown that the switch from C 3 to Crassulacean acid metabolism that occurs during salt stress in this plant is accompanied by new gene expression, including the induction of specific phos- phoenolpyruvate carboxylase genes.

Cold stress/acclimation

Increased freezing tolerance following a period of cold acclimation or cold hardening is a dramatic example of how many plant species can adapt to extremes in temperature. Biochemical changes associated with low-temperature tolerance include increases in sugars, organic acids and soluble protein, the appearance of new proteins and alterations of lipids [11].

Changes in gene expression that correlate with cold acclimation have been described in several species and cDNAs encoding cor (cold regulated) genes have been isolated [11]. In Arabidopsis, the major cor genes en-

166 Plant biotechnology

code 140, 47, 24, 15 and 6.6 kD polypeptides. Interest- ingly, Lin et al. [12"] have shown that the COR polypeptides remain soluble following boiling, which is an in- dication of their hydrophilicity. The LEA proteins have similar properties and some of the cor genes are also regulated by osmotic stress [11]. As survival of both drought and cold stress requires tolerance to dehydration, it is likely that the COR proteins counteract dehydration stress. Similar cold-induced, hydrophilic proteins have been identified in several plant species [12.].

Understanding the regulation of cor gene expression is only beginning. ABA has long been associated with cold acclimation and plant responses to osmotic stress, but its role in these responses is complex. Although ABA induces cor genes in wild-type plants at room temperature, studies using Arabidopsis ABA-deficient and ABA-insensitive mutants have shown that both types of mutants still induce cor gene expression in response to cold treatment [13",14"]. Thus, ABA and cold must act through separate, but convergent, induction pathways.

Changes in membrane lipids required to maintain membrane fluidity at cold temperatures have been demonstrated to have a positive effect on cold tolerance [11]. A dramatic demonstrat ion of the ability to alter lipids, thereby increasing cold tolerance, has been accomplished by Wada et al. [15"]. Intro- duction of desA, a gene for fatty-acid desaturation from the chilling-resistant cyanobacterium Synechocys- tis, into chilling-sensitive Anacys t i s n i d u l a n s changed the fatty acid composit ion of the membranes and en- abled photosynthesis to proceed uninhibited at 5°C. As progress continues in the identification of enzymes involved in lipid metabolism in higher plants [16], the effects of over- and under-expression of the genes encoding these enzymes should be studied.

Another possible approach to increasing freezing tolerance has been taken by Hightower et al. [17], who have introduced fish antifreeze proteins into tobacco. Anti- freeze proteins, which have not been found in plants, are composed of multiple repeats of an alanine-rich 11- amino-acid unit and act to lower the freezing point by a non-colligative mechanism. The transgenic tobacco plants showed expression of the antifreeze proteins, but increased freezing tolerance of whole plants was not measured. The ultimate goal of these workers is to test these proteins for their effectiveness in preventing ice-crystallization damage in fruits and vegetables.

Heat stress

Plants and other eukaryotes, as well as prokaryotes, produce a specific set of 'heat shock proteins' (HSPs) when tissue temperatures are increased, either grad- ually or abruptly, 5-10°C above optimal growth temperatures. Regulation of HSP expression and charac-

terization of the major HSPs in plants has progressed considerably [18,19"]. Because the heat-shock response is highly conserved evolutionarily, studies of HSP function in other organisms have also contributed to understanding the response in plants.

A major regulatory point in HSP expression is the transcriptional activation of the HSP genes. The promoter elements required for induction of these genes are well characterized in plants and are similar to those in other eukaryotes [18]. The heat-shock promoter element (HSE) has been successfully used to drive heat-induced expression of several different genes in transgenic plants. Scharf et al. [20.] have now cloned genes for the transcription factors from tomato (and Arabidopsis, L Nover, personal communication) which bind the HSE. These factors contain a DNA-binding domain similar to other eukaryotic HSE-binding factors. Surprisingly, in contrast to other eukaryotes, tomato contains at least three distinct factors, all of which have the same HSE-binding domain but are highly divergent over the rest of the protein. Determining the regulatory significance of this complexity will be important for manipulating HSP gene expression.

¥ierling [19"] has recently reviewed molecular and functional data on HSPs in plants. Four classes of HSPs common to all eukaryotes, HSP90, HSP70, HSP60 and low molecular weight (LMW) HSPs, have been characterized. Proteins from the first three groups are be- lieved to function as 'molecular chaperones ' [21] by binding to other proteins and maintaining them in a conformation necessary for correct folding, inter- action with other cellular components, or transport across membranes . For example, HSP60, also known as the ribulose bisphosphate (RuBP) carboxylase binding protein, functions in the assembly of RuBP carboxylase [21]. BiP, a homolog of HSP70 that is found in the endoplasmic reticulum, has recently been identified as participating in seed storage protein deposition [22,23]-- another example of molecular chaperone activity. Exciting data relating to the mechanism of HSP70 action has been gained from crystallization of the amino-terminal portion of the protein, which exhibits ATPase activity essential for function [24"'].

The LMW I-fSPs may also act as molecular chaperones, although direct evidence is lacking. Plants appear to be unique among eukaryotes in having LMW HSP homologs not only in the cytoplasm, but also in the endoplasmic reticulum [19"] and chloroplast [25]. This is an interesting parallel to HSP70, homologs of which are found in these three compartments as well as in mitochondria.

It is assumed that HSP expression is necessary for sur- viving heat stress, presumably to provide molecular chaperones, although most of the supporting data are derived from studies of HSPs or homologous proteins that are present constitutively. There are no published studies in which HSP expression has been enhanced or reduced by gene manipulations in transgenic plants.

Oxidative stress

Oxidative stress occurs when the concentrations of reactive oxygen species, such as the superoxide radical (O2'-) , hydrogen peroxide, or the hydroxyl radical (OH) , increase in cells [26"]. Plants have evolved several mechanisms to protect against these poten- tially damaging molecules, including the synthesis of scavenger sulphydryl compounds and enzymes such as peroxidases, catalases and superoxide dismutases (SODs). Increases in the activity of these enzymes are strongly correlated with imposition of stress (e.g. infection, ultraviolet (UV) irradiation, herbicide applica- tion, elevated ozone and sulphur dioxide in the air, and chilling) [27]. In yeast, E. coli and Drosophila, these enzymes are critical for aerobic g r o w t h - - d o e s it follow that constitutive or over-expression of these genes will improve plant resistance to stress?

Initial work by Tepperman and Dunsmuir [28] indi- cated that overexpression of a chloroplast Cu/Zn-SOD did not improve resistance to superoxide toxicity. Re- suits from human and animal studies also suggest that overproduction of SOD can cause detrimental effects. However, there are at least three known forms of SOD, suggesting that other forms might provide some level of protection. Bowler et al. [29"] have overexpressed a Mn-SOD enzyme targeted to both mitochondria and chloroplasts of tobacco. In tissues of the transgenic plants with elevated chloroplast Mn-SOD activity, protection against l ight-dependent paraquat damage was significantly increased. In the dark, however, tissues that only moderately overproduced Mn-SOD were ac- tually more sensitive to herbicide damage. The phenotype is probably determined by the overall ratio of the oxygen radical and hydrogen peroxide, which react non-enzymatically to form the extremely toxic hydroxyl radical. The oxygen radical is used by SODs as a substrate in a reaction that releases hydrogen peroxide. ThUs, SODs can dramatically affect the ratio of these reactive oxygen species. The authors suggest that the best approach may be to engineer plants with both a SOD and a peroxidase as the formation of hydroxyl radicals should be quite rare in tissues producing both enzymes.

Ultraviolet light stress

Some plants achieve protection from the damaging effects of UV irradiation by synthesizing flavonoid pig- ments. The first enzyme in the committed pathway to flavanoid pigment production is chalcone synthase (CHS). Previous work [30] identified a cis-acting DNA sequence in the parsley CHS promoter, unit 1, that contains two separate regions (box I and box II) necessary for UV light inducibili W of CHS. Weisshaur et al. [31"] determined that unit 1 is both necessary and sufficient for UV light inducibility, and they isolated three cD- NAs encoding trans-acting factors that bind to the box II core sequence (ACGTGGC). Preliminary anal-

Plant responses to environmental stress Vierling and Kimpel 167

ysis indicates that all three cDNAs encode a leucine zipper DNA-binding (bZIP) motif. These bZIP mo- tifs are similar to all other identified bZIP regions of higher plants. The sequences recognized by all these plant bZIP domains contain the core ACGT sequence present in box II. Consequently, the parsley trans-factors have been named common plant regulatory factors (CPRF)-I, CPRF-2, and CPRF-3. How all these plant trans-acting factors regulate very different promoters despite recognizing the same four-base-pair sequence remains to be resolved. These authors hypothesize that at least two cis elements are required for activation in response to a stimulus, one of which is typically an ACGT type. Perhaps most intriguing, CPRF-1 is itself induced by UV light. Maximal accumulation of CPRF-1 occurs at the same time as the maximal increase in CHS mRNA levels, consistent with a causal relationship be- tween CPRF-1 synthesis and CHS mRNA accumulation.

In addition to UV-light inducibility, CHS is induced upon pathogen challenge and is also developmen- tally regulated. Studies by Wingender et al. [32] with soybean and by Fritze et al. [331 with snapdragon de- fine the CHS promoter as a linear array of separable cis-acting elements, subsets of which are responsive to the various stimuli. The binding affinities and distribu- tion of the trans-acting factors that interact with these elements are highly conserved, as promoter function may be correctly induced in heterologous expression systems (soybean in parsley, snapdragon in tobacco). Despite this strong functional conservation among the known trans-acting factors, there is no strong sequence conservation in the identified cis-acting elements of the CHS promoters.

Arabidopsis thaliana offers an opportunity to dissect the mechanism of induction of these genes by the use of mutants. Feinbaume et al. [34] have developed a set of transformed plants carrying either the full length CHS promoter fused to a [3-glucuronidase (GUS) re- porter gene or any one of a set of 5' deletions of the promoter fused to GUS. Seeds from these plants are being mutagenized and the p rogeny screened for unusual (e.g. high, low or non-inducible) levels of GUS activity. The first screen revealed one mutant with much reduced expression of both the endogenous CHS gene and the CHS-GUS fusion gene. A similar approach is being used to dissect the molecular mechanisms by which other stresses are perceived.

The enzyme 4-coumara te-coenzyme A ligase (4CL), which acts at the branchpoint where the phenyl- propanoid backbone is directed into several end- product-specific pathways, is also induced by stress. In parsley, expression of the gene encoding 4CL is induced by UV irradiation, wounding and pathogen infection, and is also under developmental regulation. The cis-acting element in the parsley 4CL-1 promoter that controls stress inducibility is distinct from that controlling developmental regulation [35]. As seen for the CHS promoter, factors interacting with the parsley 4CLq promoter appear highly conserved, as the developmental and stress-inducible regulation can be

168 Plant biotechnology

expressed in transgenic tobacco. In a surprising ob- servation [35], stress responsiveness of this gene was found to require the presence of cis-acting elements not only in the promoter region of the gene but also in the coding region.

Wounding

Genes coding for repair or protective functions are induced in response to wounding. These include cell-wall structural proteins [36] and enzymes, lignin, suberin, flavonoid [37] and isoprenoid synthetic enzymes [38], fructosidases (for energy mobilization) [39] and proteinase inhibitors (PIs). The induction of tomato PIs, which are effective against many chewing insects, has been studied by Ryan's laboratory (Wash- ington State) for the past twenty years. Last year they reported the discovery of a previously unknown type of hormone in plants, a small, 18-amino-acid peptide [40"]. The peptide, named systemin, is responsible for the systemic induction of the PI genes that occurs following wounding of a single leaf. This landmark discovery, coupled with the finding that methyl jasmonate also systemically induces the PI genes [41"], led Ryan to propose a signal transduction pathway for wounding (International Society for Plant Molecular Bi- ology Meeting October 1991, Tucson, Arizona, USA). In his model, systemin is released at the wound site upon cell disruption and moves throughout the plant. It binds to receptors on the plasma membrane , causing the release of linolenic acid which is further metabo- lized to jasmonic acid (the soluble form of methyl jasmonate). Jasmonic acid then acts directly or indirectly to trigger activation of the wound-responsive genes.

Pena-Cortes et al. [42] showed that wound-inducibility of PI genes in potato and tomato is mediated by ABA. Using wild-type and ABA-deficient lines transformed with PI promoter-GUS coding region con- structs, they demonstrated that activation of the P1 promoter by wounding, both locally and systemically, requires ABA. ABA must therefore be included in the signal transduction scheme, perhaps with the role of mediating the jasmonic acid signalling.

The PI genes are also developmental ly regulated, but this regulation involves cis-acting elements that are separable from the wound-responsive elements, and their developmental expression is not affected by ABA [42,43].

i:

Conclusions

By manipulating gene expression and metabolic pathways in transgenic plants, it is now possible to deter- mine how many of the changes correlated with stress are important to stress tolerance. Nonetheless, ability to manipulate stress tolerance remains a long-term

problem because of the complexi W and number of changes that occur under stress conditions. Instances where transfer of a single gene will positively affect tolerance are likely to be limited, complicating the job of plant improvement. For the biotechnologist, problems in metabolic engineering are also significant, as very clearly outlined by McCue and Hanson [3"'] and demonstrated by the transgenic-plant work manipulating SODs [28,29"]. However, in addition to the identification of many genes that are clearly involved in stress tolerance, characterization of more gene promoters has enhanced our ability to direct gene expression both temporally and spatially in or- der to effect very specific changes in transgenic plants. Current plant transformation technology should enable relatively rapid hypothesis testing in model species and final testing in different crop species. A promising new approach involves using RFLPs to map quantitative trait loci [44"] associated with stress tolerance. This is an important technique for the future, both in terms of manipulating such traits in a breeding program and understanding the genetic nature of stress tolerance. The use of wild species to introgress desirable traits not present in cultivated species remains a goal of plant breeders. For traits that are controlled by many genes, RFLP maps may accelerate the introgression process. Both transgenic plant studies and RFLP-based studies aimed at understanding and improving plant stress tolerance are n o w underway in many laboratories.

Acknowledgements

EV thanks Drs J Cushman, H Bohnert and M T h o m a s h o w for helpful discussions.

References and recommended reading

Papers of particular interest, publ i shed within the annual period of review, have been highlighted as:

of special interest • . of outs tanding interest

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22. BOSTON RS, EONTES EBP, SHANK BB, WROBEL RL: I n c r e a s e d E x p r e s s i o n o f t h e Maize I m m u n o g l o b u l i n B i n d i n g Pro- t e i n H o m o l o g b-70 i n T h r e e Z e i n R e g u l a t o r y Mutants . Plant Cell 1991, 3:497-505.

23. MAROCCO A, SANTUCCI A, CERIOLI S, MOTTO M, DIFONZO N, THOMPSON R, SALAMINI F: T h r e e H i g h - l y s i n e Mu ta t i o n s C o n t r o l t h e Level o f ATP-b ind ing HSP70-1ike P ro t e in s i n t h e Maize E n d o s p e r m . Plant Cell 1991, 3:507-515.

24. FLAHERTY KM, McKAY DB, KABSCH W, HOLMES KC: Sinlilar- • . i ty o f t h e T h r e e - d i m e n s i o n a l S t ruc tu r e s o f Ac t in and

t h e ATPase F r a g t n e n t s o f a 70-kDa Hea t S h o c k Cogna te P ro t e in . Proc Natl Acad Sci USA 1991, 88:5041-5045.

X-ray crystal structure data showing that the amino-terminal half of HSP70 is structurally similar to actin. Continued progress in structural analysis of HSP70 is essential to defining the mechan i sm of molecular chaperone action.

25. CHEN Q, VIERLING E: A n a l y s i s o f C o n s e r v e d D o m a i n s Ident i fxes a U n i q u e S t ruc tu r a l Fea tu re o f a C h l o r o p l a s t Heat S h o c k Pro te in . Mol Gen Genet 1991, 226:425-431.

26. SCANDALIOS JG: R e s p o n s e o f P l a n t A n t i o x i d a n t D e f e n s e G e n e s to E n v i r o n m e n t a l S t ress . In Genomtc Responses to Environmental Stress. Edited by Scandalios JG. San Diego: Academic Press, Inc; Adv Genetics 1990, 28:1-41.

A thorough review of the biochemistry of oxidative stress and the response of plants to it. Currem knowledge of enzymology, espe- cially of catalase and superoxide dlsmutase, and molecular biology in plants is summarized.

27. TSANG EWT, BOWLER C, HEROUART D, VAN CAMp W, VILLARROEL R, GENETELLO C, VAN MONTAGU M, INZE D: Di f f e ren t i a l R e g u l a t i o n o f S u p e r o x i d e D i s m u t a s e s i n P l a n t s E x p o s e d to Env i roaxmen ta l S t ress . Plant Cell 1991, 3:783-792.

28. TEPPERMAN JM, DUNSMUIR P: T r a n s f o r m e d P l a n t s w i t h Ele- v a t e d Levels o f C h l o r o p l a s t S u p e r o x i d e D i s m u t a s e a re n o t M o r e R e s i s t a n t to S u p e r o x i d e Toxic i ty . Plant Mol Biol 1991, 14:501-511.

29. BOWLER C, SLOOTEN L, VANDENBRANDEN S, DE RYCKE R, BOTrERMAN J, SYBESMA C, VAN MONTAGU M, INZE D: M a n g a n e s e S u p e r o x i d e D i s m u t a s e C a n R e d u c e Cel lular D a m a g e Media ted b y O x y g e n Radica ls i n T r a n s g e n i c P lan t s . EMBO J 1991, 10:1723-1732.

A good demonstrat ion of the progress in plant transformation: Mn- SOD genes are targeted to either mitochondria or chloroplasts.

30. SCHULZE-LEFERT P, DANGL JL, BECKER-ANDR£ M, I-IAHLBROCK K, SCI-IULZ W: Induc ib l e In Vivo F o o t p r i n t s Deft_tie Se- q u e n c e s N e c e s s a r y fo r UV Ligh t Ac t iva t ion o f t h e Pars- l ey C h a l c o n e S y n t h a s e Gene . EMBO J 1989, 8:651-656.

31. WEISSHAAR B, ARMSTRONG GA, BLOCK A, DA COSTA E SILVA O, HAHLBROCK K: Ligh t - induc ib l e a n d C o n s t i t u t i v e l y Ex- p r e s s e d DNA-b ind ing P r o t e i n s R e c o g n i z i n g a P lan t P r o m o t e r E l e m e n t w i t h F u n c t i o n a l R e l e v a n c e i n Light R e s p o n s i v e n e s s . EMBO J 1991, 10:1777-1786.

Interesting comparison of the DNA-binding proteins of higher plains that recognize a similar core sequence (ACGT).

170 Plant

32.

biotechnology

WINGENDER R, ROHRIG H, HORICKE C, SCHELL J: Cis-regula- t o ry Elements Involved i n Ultraviolet Light Regulat ion an d Plant Defense. Plant Cell 1990, 2:1019-1026.

33. FRITZE K, STAIGER D, CZAJA I, WALDEN R, SCHELL J, WING D: Deve lopmenta l a n d UV Light Regula t ion o f t he Snap- d r a g o n Chalcone Synthase Promoter . Plant Cell 1991, 3:893-905.

34. FEINBAUME ILL, STORZ G, AUSUBEL FM: High In tens i ty a nd Blue Light Regulated Exp re s s ion o f Chimer ic Chaicone Syn thase Genes in Transgenic A r a b i d o p s i s t h a l i a n a Plants. Mol Gen Genet 1991, 226:449-456.

35. DOUGLAS CJ, HAUFEE KD, ITES-MORALES M-E, ELLARD M, PASZKOWSKI U, HAHLBROCK K, DANGL JL: Exonic Se- q u e n c e s are Requi red for Elicitor a n d Light Activation of a Plant Defense Gene, bu t P r o m o t e r Sequences are Sufficient for Tissue Specific Express ion . EMBO J 1991, 10:1767-1775.

36. SHOWALTER AM, ZHOU J, RUMEAU D, WORST SG, VARNER JE: Tomato Extens in and Extensin- l ike cDNAs: Structure an d Express ion i n Response to Wounding . Plant Mol Biol 1991, 16:547-565.

37. KEITH B, DONG X, AUSUBEL FM, FINK GR: Differen- tial Induc t ion o f 3-deoxy-n-arab ino-heptu losonate 7- p h o s p h a t e s y n t h a s e genes in A r a b i d o p s i s t h a l i a n a by Wound ing a n d Pathogenic Attack. Proc Natl Acad Sci USA 1991, 88:8821-8825.

38. gANG Z, PARK H, LACY GH, CRAMER CL: Differential Acti- va t ion of Potato 3-hydrox-y-3-methylglutaryl Coenzyme A Reductase Genes by Wound ing a n d Pa thogen Chal- lenge. Plant Cell 1991, 3:397-405.

39. STURM A, CHRISPEELS MJ: cDNA Clon ing o f Carrot Extra- cel lular ~-fructosidase a n d its E x p r e s s i o n in Response to Wound ing and Bacterial Infect ion. Plant Cell 1990, 2:1107-1119.

40. PEARCE G, STRYDOM D, JOHNSON S, RYAN CA: A Polypept ide • . F r o m Tomato Leaves Induces Wounddnduc ib le Pro-

t e inase Inhibi tor Prote ins . Science 1991, 253:895-898. A landmark paper that introduces a class of hormonal molecules never previously described for plants.

41. FARMER EE, RYAN CA: In te rp lan t Communica t ion : Air- b o r n e Methyl J a s m o n a t e Induces Synthes is o f Pro- t e inase Inhibi tors in Plant Leaves. Proc Natl Aca d Sci USA 1990, 87:7713-7717.

Identifies a second volatile compound (ethylene being the first) that mediates interplant communication. Methyl jasmonate released from sagebrush can induce PI synthesis in nearby tomato plants.

42. PENA-CORTES H, WILLMITZER L, SANCHEZ-SERRANO JJ: Ab- scisic Acid Mediates Wound Induc t ion but no t Developmental -specif ic Express ion o f the Pro te inase Inh ib i to r H Gene Family. Plant Cell 1991, 3:963-972.

43. WINGATE VPM, RYAN CA: Uniquely Regulated Pro te inase Inh ib i to r I Gene in a Wild Tomato Species. Plant Physiol 1991, 97:496-501.

44. PATERSON AH, DAMON S, HEWITr JD, ZAMIR D, RABINOWITCH HD, LINCOLN SE, LANDER ES, TANKSLEY SD: Mendel ian Factors Under ly ing Quantitat ive Traits in Tomato: C o m p a r i s o n Across Species, Genera t ions and Environ- merits. Genetics 1991, 127:181-197.

Shows feasibility of breeding for quantitative traits using quantitative trait loci identified by RFLP mapping. Suggests that variation in a polygenic trait (e.g. soluble solids in fruits) may be the result of allelic variation in orthologous genes.

E Vierling, Department of Biochemistry, Life Sciences South, Univer- sity of Arizona, Tucson, Arizona 85721, USA.

JA Kimpel, Department of Agronomy, Miller Plant Sciences Building, University of Georgia, Athens, Georgia 30602, USA.

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Plant responses to environmental stress