Effect of nutrient conditions on selected proteins
A focus on enzymes involved in pyruvate metabolism is provided in Fig.
6. As mentioned above and shown in Table 1, pyruvate kinase was
down-regulated under low K and this effect could be alleviated by low Ca
conditions in both roots and leaves. Putrescine addition tended to
increase the content in pyruvate kinase under low K but decreased it at
high K. For other enzymes (NADP-dependent malic enzyme, aconitase,
phosphoenol pyruvate carboxylase), both low Ca and putrescine
tended to attenuate the increase in protein content observed under low K
in leaves. In roots, low Ca has little effect on these enzymes while
putrescine increased their content at high K.
Transporters and channels that were both detected and associated with
significant changes are shown in Figs. S3 and S4. As expected, a high
affinity K transporter was induced at low K in roots, and in both leaves
and roots, the subunit β2 of the voltage dependent aldo/keto
reductase-potassium channel was downregulated at low K + high Ca and
upregulated at low K + low Ca. In leaves, low K caused a decline in two
ABC cassette containing proteins (TAP1 and cABCI6) and an ammonium/urea
transporter, suggesting changes in nitrogen/protein homeostasis. In
roots, nitrate transporters were upregulated at low K + high Ca, high K
+ low Ca and were clearly downregulated by putrescine.
The two-way ANOVAs carried out to look at significant proteins were used
to visualize proteins associated with specific interactions effects
(Fig. S5). In the group of roots proteins upregulated under high K + low
Ca, proteins involved in nitrogen assimilation appeared clearly (group
1, Fig. S5a). In leaves, some enzymes of catabolism (isocitrate
dehydrogenase, aconitase) were specifically enhanced at low K + high Ca
(group 1, Fig 5b). Interestingly, many proteins of the photosynthetic
machinery (including magnesium chelatase, Rubisco and proteins of the
chloroplastic electron transfer chain) were downregulated in this
specific condition, suggesting that the effect of low K was mostly
driven by high Ca (group 2, Fig. 5b). K x putrescine interaction
analysis clearly showed that, as found above, the downregulation of
pyruvate kinase was strong under low K + high Ca, and that putrescine
addition minimized differences between low K and high K for key
catabolic enzymes (Figs. 5c-d).