tae-miR399-UBC24 module regulates starch degradation inArabidopsis thaliana in response to freezing stress
Pi can regulate plant carbohydrate metabolism, for example, in the dark,
phosphorus deficient Arabidopsis will reduce photosynthesis, increase
starch and sucrose content (Ciereszko, Johansson, & Kleczkowski, 2005;
Muller, Morant, Jarmer, Nilsson, & Nielsen, 2007). OsSPX1 is a
Pi-dependent OsPHR2 activity inhibitor, which plays an important role in
the Pi signal pathway of rice (Z. Y. Wang et al., 2014), and the
decreased expression of OsSPX1 affects the expression of genes
involved in carbohydrate metabolism and sugar transport (K. Zhang et
al., 2016). Starch degradation is involved in cold-induced soluble sugar
accumulation (Ruelland et al., 2009). In this study, at 4°C and -10°C,
the starch content of OEmiR399 Arabidopsis plants was significantly
lower than that of WT, while the soluble sugar content was significantly
higher than that of WT, which indicated that overexpressing tae-miR399
at low temperature promoted starch degradation in Arabidopsis (Fig. 9A
and B). The amylase in plants is mainly divided into α-amylase and
β-amylase. In this study, during cold acclimation, the activity of
α-amylase and β-amylase in OEmiR399 Arabidopsis plants were higher than
that in WT (Fig. 10A and B). However, during freezing stress, only
β-amylase activity of OEmiR399 Arabidopsis plants was higher than that
of WT (Fig. 10B). These results indicates that during cold acclimation,
overexpressing tae-miR399 can promote both α-amylase and β-amylase
activities to regulate starch degradation in Arabidopsis, while in
freezing stress, overexpressing tae-miR399 only promote β-amylase to
mediate starch degradation in Arabidopsis. It is worth noting that the
expression of AtAPL3 and AtBAM3 in OEmiR399 Arabidopsis
plants were similar to the changes in its own enzyme activity (Fig. 10D
and F). These results indicate that the activities of α-amylase and
β-amylase in OEmiR399 Arabidopsis plants may be mainly regulated byAtAPL3 and AtBAM3 . Starch phosphorylation is another way
to degrade starch. It has been controversy about whether starch
phosphorylase is involved in the starch degradation under low
temperature stress. At 2°C, it was no significant change in the enzyme
activity of starch phosphorylase in Arabidopsis thaliana (R.
Yano, Nakamura, Yoneyama, & Nishida, 2005). However, the study by
(Berrocal-Lobo et al., 2011) showed that the transcription level and
protein level of SEX4 were significantly up-regulated at 4°C, which is
consistent with the starch degradation trend. In this study, we found
that AtSEX1 did not participate in the regulation of starch
degradation in Arabidopsis under low temperature stress, but the
expression of AtSEX4 in OEmiR399 Arabidopsis plants was induced
during the cold acclimation (Fig. 10G and H). These results suggest that
during cold acclimation, maybe starch phosphorylase also participate in
the starch degradation of Arabidopsis.