Stomata control the exchange of CO 2 and water in land plants. For this reason, plants evolved to quickly respond the surrounding environment and endogenous cues in order to maintain their photosynthetic rates, but avoiding excessive water loss. Although guard cell has been used as model for characterization of signaling pathways, mainly regarding abscisic acid (ABA) response, several important questions about its functioning remain elusive. Recently, transcriptomics, proteomics, and metabolomics studies carried out using guard cells as model have contributed substantially for our understanding on how guard cells sense and respond to relative air humidity, CO 2 , ABA, and sucrose. Comparatively, proteomics and metabolomics platforms need substantial improvement to increase the number of analytes detected. However, with the introduction of metabolomics-based technologies, several studies have been published increasing our knowledge on guard cell function. Here, we review these new exciting findings as well as discuss the importance of using these new data to improve prediction when modelling stomatal behavior.

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Background and Aims Calcium (Ca) plays an important role to increase drought tolerance in plants. We evaluated the effect of water restriction and Ca supply on the expression of Ca transporters, on Ca acquisition, allocation and its effects in eucalypt responses to drought. Methods Drought-tolerant eucalyptus plants (Eucalyptus urophylla x Eucalyptus camaldulensis - clone VM01) were grown in a hydroponic system in the presence or absence of Ca and subjected or not to water restriction using polyethylene glycol. Ca concentration by plants, gas exchange, plant growth, and the expression of the ACA11 and MCA1 Ca transporters were evaluated. Results Water restriction increased the expression of ACA11 (vacuole Ca transporter) and decreased the expression of MCA1(endodermal Ca transporter), in eucalypt roots. Water restriction increased Ca content in plant roots and Ca supply ameliorated the negative effects of water stress. Conclusion Water restriction alters the expression of Ca transporters in eucalypt roots altering the Ca allocation pattern and improving root growth and water status. Although caution must be taken, our findings bring novel evidence of an important mechanism that favors survival of drought-tolerant woody plants under water deficit.