Reduced starch availability and degradation reduces growth
during the night.
Even in control conditions, the growth of sh2 was impaired (Table
1, Figure 8d). Starch accumulates gradually during the photoperiod in
the mature zone of the leaf and is remobilized at night by amylase
activity providing the energy supply for growth processes in the growth
zome (Smith and Stitt, 2007; Stitt and Zeeman, 2012; Thalmann et al.,
2016). Consequently, it is not the starch content itself, but the
ability to sustain a steady supply of soluble sugar during the night
which is crucial for plant growth (Rasse and Tocquin 2006). Starch
degradation during the day is induced under stress conditions (Yano et
al., 2005, Thalmann et al., 2016). The sh 2 mutant had reduced
starch levels in the mature leaf tissues (Figures 4c, and 8a) and
accumulated higher soluble sugar levels during the day (Figure 5a and
8a), but these concentrations decreased rapidly to very low levels early
in the night (Figure 8c). Caspar et al. (1985) linked the rapid
declining sugar levels in the early night observed in another starchless
mutant to nonproductive respiration and suggested that respiration
during the night is proportional to the availability of soluble sugars
rather than the energy demand for growth. We showed that the growth
impairment during the night is due to the low soluble sugar
concentrations in sh2 and could be fully restored to WT rates by
sucrose feeding (Figure 8e and f).
Overall, we showed a strong effect of altered energy and antioxidant
metabolism in the maize leaf proteome in response to drought. Subsequent
functional analysis of sh2 allowed us to unravel the central role
of starch synthesis in growth regulation and stress defense.
Acknowledgments
We would like to thank Karin Schildermans, Tim Willems, and Azmi
Abdelkrim for their help during optimizing and performing the proteome
analysis, as well as Els Prinsen for the abscisic acid measurements and
Danny Huybrecht and Sevgi Oden for the technical support in the lab
work.