I came across this article, published about a year ago, in Plant Physiology and Biochemistry and was intrigued:
Insight into small RNA abundance and expression in high- and low-temperature stress response using deep sequencing in Arabidopsis
doi: 10.1016/j.plaphy.2014.09.007
The experiment was small RNA-seq on heat-stressed (HT .. 36C for one day), normal temperature (NT) and low-temperature (LT .. 4C for one day ) Arabidopsis plants. The basic observation that I found intriguing was that the high-temp stress library had quite a few 24mers, while the cold and normal temp. libraries had a lot fewer. (See Figure 1 from the paper below):
Does this imply that heat stress activates the 24nt siRNA pathway?
The major issue with the experiment however is a lack of replication and independent verification: Just one library per treatment so there are no biological replicates to assess the reproducibility, so the result I think is provisional at best.
However, there are some earlier reports that also show effects of heat stress on bulk small RNA functions. Ito et al. (2011) showed that the Arabidopsis ONSEN retrotransposon was transcribed and reverse-transcribed in response to heat stress, but that actual transpositions were prevented by the het-siRNA pathway. Zhong et al. (2013) showed that heat stress de-activated the trans-acting siRNA pathway (which mostly makes 21 and 22mer siRNAs that can also behave like heterochromatic siRNAs). I think that understanding the role of heat-induced small RNA profile changes will be quite interesting.