Events of high temperatures adversely affect the germination of the pollen of rice, and then fertilization of the ovary.
To study the mechanisms of response to elevated temperatures , we have characterized the transcriptome of rice ovaries during pollination and fertilization in the tolerant varieties N22, by means of RNA-Seq. The sampling of pistils was carried out in collaboration with the International Rice Research Institute (IRRI , Philippines), and was done from plants subjected to either standard temperatures (29°C) and high temperatures (38°C). N22 was chosen as it is one of the varieties more resistant to heat stress, maintaining a high fertility even under these conditions. About 450 genes were differentially expressed (fold change >2, FDR 0.05) in response to stress from high temperatures. The expression levels of the most interesting candidate genes were confirmed by real-time quantitative RT-PCR (qRT-PCR).
The specific objective of this project is to isolate genes that may be involved in the response and tolerance to high temperatures, in the tissues of the pistil and pollen tube during pollination and fertilization. Therefore, the activities carried out so far allowed us to achieve this goal: the eleven percent of the genes that are differentially expressed in the pistils of N22 under conditions of high temperatures, encode for putative heat shock proteins and other chaperones involved in thermal stress response. Among the transcription factors, we isolated members of the WRKY family (all repressed by high temperatures) and ERF/AP2 , HSF, MYB, MADS-box and B3 families. Furthermore, six genes fall within an important Quantitative Trait Locus (QTL ) implicated in tolerance to high temperatures.
The most interesting candidate genes were subsequently studied in three varieties of rice sensitive to high temperatures. The gene expression was monitored by qRT-PCR in anthers, ovaries not pollinated and pollinated ovaries, either at standard temperatures (29°C ) and high temperatures (38°C). Preliminary data suggest that a gene coding for a chaperone of the type " peptidyl prolyl isomerase " is expressed at significantly lower levels, compared to N22, in some of the sensitive cultivars, and that in the other sensitive cultivars carries instead significant amino acid changes in the encoded protein.