Heat promotes hypocotyl elongation through auxin-mediated
transcriptional modulation of SlUVI4 and SlCCS52B.
We increased incubation temperature during the emergence of tomato
seedlings, and observed that 33°C
treatment significantly promoted hypocotyl elongation of cultivar ‘Heinz
1706’ by 1.6- and 1.3-fold at NMS and MS growth medium compared to the
normal growth temperature 25°C, but did not affect the hypocotyl
thickness (Figure 6A, B, C). Cell cycle analysis revealed that 33°C
treatment improved endoreduplication in hypocotyl cells compared to 25°C
resulting in increases of EI values from 0.85 and 0.94 at 25°C to 0.99
and 1.00 at 33°C on NMS and MS growth medium, respectively (Figure 6D).
Heat induced promotion of endoreduplication was more pronounced at NMS
medium than MS medium, which was in accord with the rate of improvement
on hypocotyl elongation. We also measured hypocotyl growth and cell
cycle progression of tomato cultivars ‘Moneymaker’ and ‘Alisa Craig’,
and observed the same phenomena on hypocotyl elongation and
endoreduplication after high temperature treatment except a slight
decrease of hypocotyl diameter of ‘Alisa Craig’ grown on MS medium at
33°C comparted to that at 25°C (Figure S7). qRT-PCR analysis revealed
that 33°C treatment significantly inhibited the transcription ofSlUVI4 , but did not affect SlCCS52B transcription. The
transcription of SlCCS52A1 was not affected by 33°C treatment,
while the transcript abundance of SlCCS52A2 had a slight increase
suggesting a weak role in the regulation of endoreduplication (Figure
6E).
To investigate functions of plant hormones in heat treatment, we firstly
applied different concentrations of auxin (IAA), brassinosteroids (BRs),
gibberellins (GAs) and ethylene precursor
1-aminocyclopropane-1-carboxylic acid (ACC) in growth medium to analyze
hypocotyl growth. We observed slight enhancement of hypocotyl elongation
responding to low concentration of IAA (Figure 7A-C; Figure S8A, B). BR
and GA did not have consistent effects on hypocotyl growth (Figure
S8C-F). Higher concentration of ACC had more significant effects on
hypocotyl elongation and thickening (Figure S8G, H). Cell cycle analysis
in hypocotyls revealed no significant difference between the control and
0.1nM IAA treatment (Figure 7D). qRT-PCR analysis showed thatSlUVI4 transcriptions were significantly reduced, andSlCCS52B transcription had a slight reduction without statistical
significance (Figure 7E), suggesting that reduced SlUVI4transcription and steady SlCCS52B transcription might be required
to maintain endoreduplication in auxin induced hypocotyl elongation.
Both SlCCS52A1 and SlCCS52A2 transcription were
significantly reduced suggesting that they might not be as important asSlCCS52B in hypocotyl elongation. All the data above suggested
that elevated temperature promoted hypocotyl elongation possibly due to
auxin-mediated repression of SlUVI4 transcription and maintenance
of SlCCS52B transcription.