Arabidopsis plants overexpressing additional copies of heat shock protein Hsp101 showed high heat tolerance and endo-gene silencing

Hsp101 chaperone is vital for survival of plants under heat stress. We generated transgenic Arabidopsis thaliana (Arabidopsis) lines with extra copies of Hsp101 gene using diverse approaches. Arabidopsis plants transformed with rice Hsp101 cDNA driven by Arabidopsis Hsp101 promoter (IN lines) showed high heat tolerance while the plants transformed with rice Hsp101 cDNA driven by CaMV35S promoter (C lines) were like wild type plants in heat stress response. Transformation of Col-0 plants with 4633 bp Hsp101 genomic fragment (GF lines) from A. thaliana containing both its coding and the regulatory sequence resulted in mostly over-expressor (OX) lines and a few under-expressor (UX) lines of Hsp101. OX lines showed enhanced heat tolerance while the UX lines were overly heat sensitive. In UX lines, silencing of not only Hsp101 endo-gene was noted but also transcript of choline kinase (CK2) was silenced. Previous work established that in Arabidopsis, CK2 and Hsp101 are convergent gene pairs sharing a bidirectional promoter. The elevated AtHsp101 protein amount in most GF and IN lines was accompanied by lowered CK2 transcript levels under HS. We observed increased methylation of the promoter and gene sequence region in UX lines; however, methylation was lacking in OX lines.

AtHsc70-1 negatively regulates the basal heat tolerance in Arabidopsis thaliana through affecting the activity of HsfAs and Hsp101

Heat shock protein 70 (Hsp70) chaperones are highly conserved and essential proteins with diverse cellular functions, including plant abiotic stress tolerance. Hsp70 proteins have been linked with basal heat tolerance in plants. Hsp101 likewise is an important chaperone protein that plays a critical role in heat tolerance in plants. We observed that Arabidopsis hsc70-1 mutant seedlings show elevated basal heat tolerance compared with wild-type. Over-expression of Hsc70-1 resulted in increased heat sensitivity. Hsp101 transcript and protein levels were increased during non-heat stress (HS) and post-HS conditions in hsc70-1 mutant seedlings. In contrast, Hsp101 was repressed in Hsc70-1 over-expressing plants after post-HS conditions. Hsc70-1 showed physical interaction with HsfA1d and HsfA1e protein in the cytosol under non-HS conditions. In transient reporter gene analysis, HsfA1d, HsfA1e and HsfA2 showed transcriptional response on the Hsp101 promoter. HsfA1d and HsfA2 transcripts were at higher levels in hsc70-1 mutant compared with wild-type. We provide genetic evidence that Hsc70-1 is a negative regulator affecting HsfA1d/A1e/A2 activators, which in turn regulate Hsp101 expression and basal thermotolerance.

Natural populations of Arabidopsis thaliana differ in seedling responses to high-temperature stress

Little is known about adaptive within-species variation in thermotolerance in wild plants despite its likely role in both functional adaptation at range limits and in predicting response to climate change. Heat shock protein Hsp101, rapidly heat induced in Arabidopsis thaliana, plays a central role in thermotolerance in laboratory studies, yet little is known about variation in its expression in natural populations. We explored variation in thermotolerance and Hsp101 expression in seedlings from 16 natural populations of A. thaliana sampled along an elevation and climate gradient. We tested both naive controls (maintained at 22 °C until heat stress) and thermally pre-acclimated plants (exposed to a 38 °C 3-h acclimation treatment). After acclimation, seedlings were exposed to one of two heat stresses: 42 or 45 °C. Thermotolerance was measured as post-stress seedling survival and root growth. When stressed at 45 °C, both thermotolerance and Hsp101 expression were significantly increased by pre-acclimation. However, thermotolerance did not differ between pre-acclimation and control when followed by a 42 °C stress. Immediately after heat stress, pre-acclimated seedlings contained significantly more Hsp101 than control seedlings. At 45 °C, Hsp101 expression was positively associated with survival (r(2) = 0.37) and post-stress root growth (r(2) = 0.15). Importantly, seedling survival, post-stress root growth at 45 °C and Hsp101 expression at 42 °C were significantly correlated with the home sites' first principal component of climate variation. This climate gradient mainly reflects a temperature and precipitation gradient. Thus, the extent of Hsp101 expression modulation and thermotolerance appear to be interrelated and to evolve adaptively in natural populations of A. thaliana.