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Plant roots exhibit remarkable developmental plasticity in response to local soil conditions. It is shown here that mild salt stress stimulates a stress-induced morphogenic response (SIMR) in Arabidopsis thaliana roots characteristic of several other abiotic stresses: the proliferation of lateral roots (LRs) with a concomitant reduction in LR and primary root length. The LR proliferation component of the salt SIMR is dramatically enhanced by the transfer of seedlings from a low to a high NO3− medium, thereby compensating for the decreased LR length and maintaining overall LR surface area. Increased LR proliferation is specific to salt stress (osmotic stress alone has no stimulatory effect) and is due to the progression of more LR primordia from the pre-emergence to the emergence stage, in salt-stressed plants. In salt-stressed seedlings, greater numbers of LR primordia exhibit expression of a reporter gene driven by the auxin-sensitive DR5 promoter than in unstressed seedlings. Moreover, in the auxin transporter mutant aux1-7, the LR proliferation component of the salt SIMR is completely abrogated. The results suggest that salt stress promotes auxin accumulation in developing primordia thereby preventing their developmental arrest at the pre-emergence stage. Examination of ABA and ethylene mutants revealed that ABA synthesis and a factor involved in the ethylene signalling network also regulate the LR proliferation component of the salt SIMR.

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Abiotic stresses are a primary cause of crop loss worldwide. The convergence of stress signalling pathways to a common set of transcription factors suggests the existence of upstream regulatory genes that control plant responses to multiple abiotic stresses. To identify such genes, data from published Arabidopsis thaliana abiotic stress microarray analyses were combined with our presented global analysis of early heat stress-responsive gene expression, in a relational database. A set of Multiple Stress (MST) genes was identified by scoring each gene for the number of abiotic stresses affecting expression of that gene. ErmineJ overrepresentation analysis of the MST gene set identified significantly enriched gene ontology biological processes for multiple abiotic stresses and regulatory genes, particularly transcription factors. A subset of MST genes including only regulatory genes that were designated 'Multiple Stress Regulatory' (MSTR) genes, was identified. To validate this strategy for identifying MSTR genes, mutants of the highest-scoring MSTR gene encoding the circadian clock protein CCA1, were tested for altered sensitivity to stress. A double mutant of CCA1 and its structural and functional homolog, LATE ELONGLATED HYPOCOTYL, exhibited greater sensitivity to salt, osmotic and heat stress than wild-type plants. This work provides a reference data set for further study of MSTR genes.

The apparent association of ornithine decarboxylase (ODC) with rapid cell proliferation in developing tomato (Lycopersicon esculentum Mil. cv.Pearson ms-35) fruits has been previously described. Further evidence is provided by the use of two ODC inhibitors, a-difluoromethylornithine (a-DFMO) and a-methylornithine (a-MO). Fruit development was inhibited by these inhibitors if applied during the period of intensive cell division. When applied in vitro, the two inhibitors were shown to inhibit the activity of ODC but not that of arginine decarboxylase (ADC). When applied in vivo, a-DFMO, a catalytic irreversible inhibitor, caused 97.1% reduction of ODC activity in the dialyzed extract from the treated ovaries, while it had no effect on ADC. On the other hand, a-MO, a reversible inhibitor, did not reduce the activity of these two enzymes in the dialyzed extracts when applied in vivo. The dialysis procedure probably removed a-MO from the enzyme fraction. Putrescine, the product of both ODC and ADC, aleviated the inhibition of fruit development but did not restore ODC activity to the control level. These results suggest that in the young developing tomato fruit, ODC is the enzyme responsible for the synthesis of putrescine, which is essential for the early stages of fruit development. The reduced activity of ODC elicited by putrescine suggests a mechanism of feedback regulation by enzyme repression or release of an ODC anti-enzyme.Polyamines are widely distributed in nature, but their precise role in cellular processes is not always fully understood. They are associated with cell proliferation, tissue regeneration, and malignancy (1,6,7,17,18). Most of the information on the biosynthetic pathways of putrescine, spermidine, and spermine, their regulation, and the possible site of action has been obtained from studies with microorganisms and mammalian cells (1). Such information is lacking for plant systems. Several reports (4,8,14,15,(20)(21)(22) do, however, describe the presence of various polyamines in plants and the occurrence of enzymes involved in polyamine biosynthesis, e.g. ODC3 and ADC. It is commonly accepted that ADC is the enzyme responsible for the production of putrescine in plants and that ODC is of lesser importance (14,15,(20)(21)(22)).We have recently described an apparent association between elevated ODC activity and rapid cell proliferation in two plant systems: tomato ovaries during the first 10 d after pollination and ' The work was performed in partial fulfillment of the requirements for the PhD thesis of E. C.'To whom correspondence should be addressed. 'Abbreviations: ODC, ornithine decarboxylase; ADC, arginine decarboxylase; a-DFMO, a-difluoromethylornithine; a-MO, a-methylornithine.tobacco XD cells growing in suspension culture during the logarithmic phase of growth (9). ADC was also present in tomato ovaries. However, its activity did not change during the logarithmic phase of growth, being one-fourth of this maximal level of ODC (E. Cohen, S. (Malis) Arad, Y. M. Heimer, and Y. Mizra...