**4. Conclusions**

*Drought - Detection and Solutions*

and GPDH were induced by drought stress [49].

among the 1000 most highly expressed genes [87].

drought. Genes encoding for scavenging reactive oxygen species such as POD, PRX,

*Prunus mongolica* is a plant native to the Gobi Desert. In this species a total of 3365 differentially expressed transcripts (DETs) for water stress were identified, counting 1876 transcripts upregulated and 1489 transcripts downregulated. Among these, 42 transcripts coding for 5 aquaporin subfamilies were found. Also, 15 potential plasma membrane intrinsic proteins (PIPs) were upregulated and 1 downregulated. Interestingly, 177 transcripts related to ROS scavenging were identified. Approximately 28% of them were predicted as SOD. Several significant pathways were identified to be related to drought tolerance of *P. mongolica*. These pathways included transcription factors and plant hormone signal transduction, starch and

sucrose metabolism, and cysteine and methionine metabolism [31].

*Artemisia sphaerocephala* is a species found in sand dunes in the deserts of Northwest China. A transcriptomic analysis in this species identified 108 unigenes related to drought stress tolerance. These had homology to 17 kinases, 2 potential chaperones, 52 enzymes, 6 transporters and channels, and 3 aquaporins. Even though, transcription factor were not identified, 25 out of the 108 unigenes were

*Zygophyllum xanthoxylum* is a succulent halophythe adapted to arid environments. In this plant a total of 1723 DEGs were identified as upregulated in leaves of plants under osmotic stress (−0.5 MPa) during 24 h. Up to 53 DEGs related to ROS scavenging were also identified as upregulated, most of them encoded for glutathione S-transferase (GST) and peroxidases (POD). Also, 31 DEGs homologous to transporters were upregulated in the roots under salt and osmotic stress. However, 23 DEGs related to photosynthesis were downregulated under osmotic stress [88]. *Cleistogenes songorica* is a C4 xerophyte widely distributed in the arid regions of Northwest China. A mining study for LEA genes identified at least 44 putative LEA proteins. They were named CsLEA1 to CsLEA44 and classified into eight subfamilies. These LEAs were characterized, and two unusual LEA stress-related domains, water stress and hypersensitive response (WHy), and LEA14-like desiccationrelated proteins were detected. Most LEAs within the same family showed similar structures and properties. All the CsDehydrin proteins contained the YKS segments which are essential for keeping the capacity to adjust their conformation and maintain cellular homeostasis under stress conditions. Among the cis-regulatory elements of the LEA genes, more than three G-Boxes were registered for each *dehydrin*, *LEA\_2*, and *SMP* genes and more than two MBS for *dehydrin* and *LEA\_2*

*Prosopis juliflora* is a species native to arid lands of Mexico and widely distributed in arid and semiarid regions of Central and South America. In this species 6874 DEGs were found under salt and drought stress. Approximately 42.6% (2932 DEGs) had homology to genes with GO annotation. Among these, 1339 DEGs were upregulated, while 1596 were downregulated. More DEGs under salt and drought stress were found in roots than those of leaves. Under drought stress, there was more upregulation than downregulation in leaves, while in roots there was more downregulation. A total of 30 unigenes were recognized as exclusively responsive to drought stress. One of these genes encoding for *Arabidopsis* ortholog ABR1 was downregulated. Two upregulated genes encoded for transcription factors involved in diminishing intracellular H2O2 levels. Also, three highly upregulated DEGs encoded for "pq-loop repeat family protein transmembrane family protein" which has been assigned with a potential role in stress tolerance. Many drought-responsive genes were tissue specific. For example, 1040 genes were specifically expressed in root tissues. Up to 805 genes were commonly regulated by drought and salt stress in root tissue. Upregulated genes included those coding for dehydration-responsive

**68**

subfamily gene [89].

Drought stress tolerance is a complex quantitative trait that involves several mechanisms and multiple genes to produce the defense responses of the plants for dealing with environmental conditions of drought. Studies in model plants have found some of the main mechanisms, genes, and proteins that are activated upon drought stress. However, not enough studies have been carried out in plants native to arid lands. The new sequencing technologies and bioinformatic tools applied to some plants native to desert areas have allowed to identify many of the already characterized genes and proteins involved in drought stress tolerance. The most important is that this kind of studies has shown that myriads of drought-related transcripts are not characterized and may belong to novel genes. These may be part of the already known pathways of plants responses to drought or may participate in novel ways to tolerate drought, and after further studies may be used to improve drought tolerance in crops.
