HTT2 promotes plant thermotolerance in Brassica rapa

Numerous regulatory genes participate in plant thermotolerance. In Arabidopsis, HEAT-INDUCED TAS1 TARGET2 (HTT2) is an important thermotolerance gene that is silenced by ta-siR255, a trans-acting siRNA. ta-siR255 is absent from heading Chinese cabbage (Brassica rapa ssp. pekinensis). Our previous attempt to overexpress the endogenous BrpHTT2 gene of heading Chinese cabbage (B. rapa ssp. pekinensis) failed because of cosuppression. In theory, heading Chinese cabbage can overexpress Arabidopsis HTT2 to improve thermotolerance in the absence of ta-siR255-mediated gene silencing and the weak potential of coexpression.

To test the potential application of HTT2 in improving crop thermotolerance, we transferred p35S::HTT2 to heading Chinese cabbage. We tested the leaf electrical conductivity, hypocotyl elongation, and survival percentage of p35S::HTT2 plants subjected to high-temperature (38 °C) and heat-shock (46 °C) treatment. The leaf electrical conductivity of p35S::HTT2 seedlings under high temperature decreased but did negligibly change under heat shock. The hypocotyl length of p35S::HTT2 seedlings increased under high temperature and heat shock. The survival rate of p35S::HTT2 seedlings increased under heat shock. BrpHsfs, a subset of heat-shock factor genes, were upregulated in p35S::HTT2 plants under high-temperature and heat shock conditions. In the field, transgenic plants with HTT2 appeared greener and formed leafy heads earlier than wild-type plants.

Exogenous HTT2 increased the survival rates of heat-shocked heading Chinese cabbage by promoting thermotolerance through decreasing electrical conductivity and extending hypocotyl length. Our work provides a new approach to the genetic manipulation of thermotolerance in crops through the introduction of exogenous thermotolerance genes.