Systemic nature of drought-tolerance in common bean

Abstract

The response to drought at the physiological and molecular levels was studied in two common bean varieties with contrasting susceptibility to drought stress. A number of genes were found to be up-regulated in the tolerant variety Pinto Villa relative to the susceptible cultivar, Carioca. The products of these genes fell in different functional categories. Further analyses of selected genes, consisting of their spatial differential expression and in situ mRNA accumulation patterns displayed interesting profiles. The drought-tolerant variety displayed a more developed root vasculature in drought conditions, when compared to the susceptible tropical bean Carioca. The in situ localization of three selected genes indicated the accumulation of their corresponding mRNAs in companion cells, sieve tubes and in developing phloem, suggesting that these, and/or the encoded proteins could constitute phloem-mobile signals. Indeed, a number of transcripts that are induced in response to water deficit accumulate in the phloem in other plant species, suggesting a general phenomenon. Moreover, the analysis of drought stress in plant varieties with contrasting tolerance to such stimulus will help to determine the role of differential expression of specific genes in response to such phenomenon, as well as other biochemical, morphological and physiological features in both cultivars.

Acknowledgements

The present work was supported by CONACyT México (27/2004 SAGARPA to B.X.-C. and 50769 to R.R.-M).

Figures and Tables

Figure 1 Model for the systemic response to drought in plants. An initial stimulus (hardening of the soil) is perceived by the root cap. ABA (blue rectangles) is then synthesized and transported through the transpiration stream (blue arrows) to mature leaves, where it induces stomatal closure. On the other hand, ABA is perceived in most cell types, first at the level of the roots and then systemically, where a signal transduction cascade through protein phosphorylation ensues. Here, transcription factors activate in turn genes coding for proteins or enzymes with a protective function. Some of the resulting signals (low molecular weight compounds, proteins and/or RNA) may be transported long-distance through the phloem (red lines) from mature leaves to developing tissues, resulting in a truly systemic response to drought stress. The blue arrow indicates ABA and its transport to systemic tissues, while the red arrows denote signals induced by ABA and transported through the phloem to other tissues. The upper red arrow close to the shoot apex indicates inhibition of growth and flowering.

Table 1 Genes induced by drought stress expressed in vascular tissue

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