3.4.2. Overexpression of CaHSP18.1a Enhances Plant Tolerance to Drought Stress

CaHSP18.1a is a molecular chaperone, though its response to drought stress is still unclear. To further study its function under drought and salt stress, *CaHSP18.1a* transgenic *Arabidopsis thaliana* and WT seedlings were drought treated (Figure 6A). After water control treatment was conducted for 10 d on 3-week-old plants with consistent growth, WT plants showed severe wilting; the leaves turned yellow, while overexpression plants grew better than WT plants (Figure 6A). These results indicated that *CaHSP18.1a* increases drought tolerance of transgenic *Arabidopsis*. In addition, the MDA content and REL were increased in both the WT and OE lines, whereas the MDA content of transgenic seedlings was obviously lower than that of WT plants (Figure 6B,C). Thus, the degree of damage in OE plants was lower than that in WT plants. The SOD, CAT, and peroxidase (POD) activity showed an upward trend in both the WT and OE lines, but the activity level of SOD, CAT, and POD in transgenic seedlings was notably higher than that in WT plants (Figure 6D,F,G). While ascorbic acid peroxidase (APX) activity increased, there was, however, no visible difference between *CaHSP18.1a*-OE and WT lines (Figure 6E). The expression levels of the 18 stress-related genes were induced to varying degrees by drought stress. However, the expression of *AtHsfA2*, *AtHSPC30*, and *AtAPX1* exhibited almost no change in WT seedlings, and all of them were strongly increased in *CaHSP18.1a*-OE lines after drought stress. In addition, the expression levels of other genes were higher in transgenic lines compared to the WT plants after drought stress (Figure 7). Thus, the 18 stress-related genes examined may be involved at different levels in the response of *CaHSP18.1a*-OE lines to drought stress.
