NtSUS2 silencing alter the topology and the connectivity of the guard cell metabolic network during dark-to-light transition
We next evaluated the effect of NtSUS2 silencing on guard cell metabolism at metabolic network level, in which metabolites are the nodes and the link is the strength of the connection among them, determined by Pearson correlation coefficient (-0.5 >r > 0.5). Correlation-based networks demonstrate that light imposition changed substantially the topology and the connectivity of the networks, with opposite trends observed between WT and the transgenic lines (Figures 7A-F). Guard cell WT metabolic network changed from a highly integrated and connected network under dark condition to a less connected and fragmented network in the light (Figure 7A-B). This is evidenced by the 2.2-fold higher network density, the higher average number of links and the lower number of both connected components and isolated nodes in WT in the dark, when compared to WT in the light (Table S1). Furthermore, WT metabolic network was 1.4-fold and 1.5-fold more connected than L3 and L13 metabolic networks in the dark, respectively. By contrast, L3 and L13 metabolic networks were respectively 1.3-fold and 1.9-fold more connected than WT in the light (Figures 7A-F; Table S1).
Another clear difference observed among WT and transgenic lines is related to the heterogeneity of the network, a topological characteristic of complex networks (Jeong et al., 2000; Pinheiro & Hartmann, 2017). Higher heterogeneity value reflects the tendency of the network to have few nodes highly connected (Doncheva et al., 2012). This parameter is lower and higher in WT than both transgenic lines under dark and light conditions, respectively (Table S1), indicating that few nodes have high degree of connection in WT in the light. We next evaluated the appearance of new hubs and the preferential attachment in each genotype during dark-to-light transition. A hub-like node was considered as those with number of links above the average of the network under dark condition within each genotype. The number of hub-like nodes dropped from 22 to 3 in WT and from 25 to 17 in L3, whilst L13 increased from 28 to 37 after dark-to-light transition (Table S1). The preferential attachment was substantially different between the genotypes, in which WT, L3 and L13 have respectively 1, 9 and 26 nodes with higher number of links in the light than the average found in the respective genotype in the dark. Furthermore, the number of new hubs that appeared in the light is also higher in the transgenics, in which 2, 8 and 11 new hubs were found in WT, L3 and L13, respectively (Table S1).