Common bean is cultivated under drought conditions in several developing countries, where drought is one of the most important factors that limit productivity. Common bean cultivars have been selected in Mexico that display tolerance to water deficit in field conditions, such as ‘Pinto Villa’. ‘Pinto Villa’ and ‘Carioca’ under moderate drought conditions achieved a comparable, nonsignificantly different RWC (70% and 68%, respectively); however, while both cultivars showed a reduction in the foliar area, ‘Carioca’ plants displayed additional morphological changes such as dwarfism and premature flowering. When severe stress (RWC < 50%) was applied in both plants, ‘Carioca’ plants reached their point of permanent wilting, since after irrigation under those severe conditions, no plant recovery was observed, in contrast to the normal recovery of ‘Pinto Villa’ under the same stress. ‘Pinto Villa’ is remarkable not only because of its tolerance to drought, but also for its high yields in these conditions (c. 70% of the yield in full irrigation). Under normal irrigation, ‘Carioca’ plants displayed a high photosynthetic rate, which was severely reduced when drought stress was applied. Interestingly, ‘Pinto Villa’ could maintain, even though at low level, its photosynthetic capacity. The mechanisms by which this variety can resist this stress condition are not known, but several can be proposed to be important; molecular and physiological evidence presented here indicates that protection of the photosynthetic machinery and differential water mobilization could be important in explaining such tolerance. The upregulated AQP and dehydrin encoding transcripts are directly related to water use. Their function positively impacts on water use by allowing the maintenance of intracellular water potential for basic functions. In particular, the tonoplast AQP would allow the vascular system to remain functional, thus, by translocating photoassimilates to sink tissues and allowing the plant growth. Regarding the upregulation of the SPK-3 protein kinase (the most abundant upregulated transcript), which has been described as necessary for induction of abscisic acid (ABA)-responsive genes and judging by its role in signal transduction, it is provocative to assign it an important role in the activation of proteins at post-translational level, by activating via phosphorylation yet unknown proteins. The chloroplast small Hsp and the chloroplast thioredoxin, could have an important role in the maintenance of the photosystems under stress; indeed, ‘Pinto Villa’ reduced its photosynthesis to 26% under drought stress, while ‘Carioca’ showed a decrease to only 3.5% of its normal capacity under normal irrigation. Finally, the induction of profilin in response to water deficit has not been reported in other biological systems; however, given its role in cytoskeleton regulation, its role in maintaining cell shape could be important in coping with water stress. The characterization of a group 3 late embryogenesis protein (Barrera-Figueroa et al., 2007) in ‘Pinto Villa’ indicated a rapid induction under drought conditions, however, this mRNA was not identified in the library, thus suggesting that ‘Carioca’ does respond to drought stress using similar gene-encoding late embriogenesis abundant (LEA) proteins. By contrast, the upregulated AQP, accumulates differentially in root cell types of the tolerant cultivar. Exactly the opposite situation was found in leaves, in which no AQP transcript was detectable in ‘Pinto Villa’, compared with ‘Carioca’, under full irrigation. However, as indicated by Northern blot analysis, the transcript accumulation in ‘Carioca’ is higher in drought-stressed leaves than in ‘Pinto Villa’. A function in water conservation in the conducting tissue, mostly in roots, could be invoked for this particular AQP, and it may contribute, among other drought-related proteins, to the contrasting phenotypes of both cultivars under drought and watered conditions. Interestingly, the overexpression of tonoplast AQPs in Arabidopsis had beneficial effects on salt-stress tolerance, as indicated by superior growth status and seed germination (Peng et al., 2007), suggesting a similar role for drought protection in this tolerant cultivar. The expression profiles of these mRNAs are so contrasting that would be interesting to screen different varieties and relate its spatial accumulation with drought tolerance. Both AQPs display the same deduced amino acid sequence, suggesting a conserved role, but also that the adaptation of ‘Pinto Villa’ to drought conditions could have occurred recently in evolution. Thus, substitutions between both genes should be present in noncoding regions such as the promoter and, or 5′ and 3′ UTRs. Significant homologies were found when these AQPs were compared with those already reported in common beans. The phylogenetic analysis presented in this work, obtained by maximum parsimony, placed the AQP from ‘Pinto Villa’ and ‘Carioca’ in the same clade of a group of eukaryotic tonoplast AQPs. Indeed, a highly homologous A. thaliana AQP is involved in ammonium transport in vacuoles, probably equilibrating urea concentrations between cellular compartments. Thus, in addition to its likely function as water channel, the AQP described may also be involved in the transport of other solutes. However, the assigned subcellular localization of this AQP to tonoplast by sequence comparison has to be confirmed experimentally. The construction of this phylogenetic tree is a useful tool to theoretically assign a subcellular location of other AQPs by comparing its deduced amino acid sequence. The localization of the ‘Pinto Villa’ and ‘Carioca’ AQP mRNAs also indicated that these are differentially regulated, and support the notion that in situ localization of transcripts and proteins may be helpful in assigning a particular role during a stimulus. Indeed, AQP mRNA accumulated to highest levels in mesophyll in ‘Carioca’, while in ‘Pinto Villa’ no signal was detected. At the macroscopic level, the leaves of ‘Carioca’ are pubescent, while those of ‘Pinto Villa’ are thinner, indicating in an indirect way the amount of water contained in that organ, which in turn can be attributed, at least in part, to the AQPs. When drought stress was applied to both cultivars, AQP mRNA accumulation in ‘Carioca’ decreased significantly while some signal was detectable in Pinto, albeit restricted to the phloem. The increase of tonoplast water permeability by expression of this particular AQP in the vascular system, specifically in the root phloem, may allow more efficient water conservation during dehydration stress, and allow the movement to aerial tissues. This fact indicates the systemic nature of stress tolerance, indeed, in situ RNA hybridization of Hsp- and dehydrin-encoding genes indicate an important localization in vascular tissue (data not shown). It must be emphasized that while the global concentrations of AQP mRNA are still higher in stressed leaves from ‘Carioca’ than those from ‘Pinto Villa’, the failure to induce such a gene in the phloem may render ‘Carioca’ more susceptible to water deficit. In stem and roots, again a similar pattern of AQP accumulation was observed in which the induction of AQP appears to be restricted to the phloem in the tolerant variety. This strategy would allow the plant to maintain the integrity of the vascular system, allowing the transport of water, minerals, photoassimilates, and long-distance signaling, as described in nonstressed plants (Ruiz-Medrano et al., 1999; Xoconostle-Cázares et al., 1999). The resulting homeostasis allows the plant to complete its cycle, leading to seed production. No signal was detected either in developing xylem as a consequence of water deprivation; however, a significant signal was detected in the cambium and immature phloem cells in the vascular boundary, indicating that these undifferentiated vascular cell types are already compromised to synthesize AQP. This in turn suggests that the adaptation to drought is an early event in cell differentiation. The tolerance to drought stress is a quantitative trait (Seki et al., 2003) in which the differential regulation of AQP genes seems to be important. The biological function of another set of genes associated with drought tolerance is currently being investigated. Together, these results will allow us to understand the physiological adaptations that highly drought-tolerant plant display both in laboratory conditions and in the field.