Partial Shade Balances Sucrose-Starch Metabolism via Compensation by PF Leaves
Plants are autotrophic organisms, and sugar metabolism in plants represents their growth regulation, stress tolerance response, and productivity. Therefore, monitoring carbohydrate status and metabolic activities in plants could be crucial for estimating the influence of various light conditions on the production and allocation of photoassimilates. The two most important carbohydrate pools that result from carbon assimilation during photosynthesis include the storage pool (starch) and export pool (sucrose). Sucrose, an important output of photosynthesis, is synthesized by the sucrose phosphate synthase (SPS). In contrast, sucrose synthase (SS) is involved in catalysis, both the degradation and synthesis of sucrose in plants (Du et al., 2020). Similarly, starch is the main storage carbohydrate in plants. Its metabolism through amylases (α and β) and debranching enzymes (DBEI, DBEIIa, and DBEIIb) are crucial for stress response and sugar biosynthesis under stress conditions (Du et al., 2020; Stitt & Zeeman, 2012; Streb & Zeeman, 2012). Previously, researchers reported the increased transportation and starvation of sugar under environmental stress that promotes plant growth and enhances stress capacity (Lemoine et al., 2013; Paparelli et al., 2013). Our investigations demonstrated that partial light conditions (PL) enhanced the soluble sugar and sucrose levels than normal light conditions (NL) (Fig. 4). We speculated the mutual balancing and compensation of sugar levels between the two sides of leaves (PF and PS) under partial light conditions, which might be a reason for delayed leaf senescence under PL conditions. It has been extensively reported that starch is hydrolyzed into sugar in response to stress (Amiard et al., 2005; Burke, 2007; Thalmann et al., 2016). Similarly, partial shading (PS) significantly induced starch degrading enzymes during the first ten days after flowering. This decline in soluble starch content and sucrose synthase activity in PL leaves triggered the sucrose synthesis in PF leaves and its transport towards PS leaves to compensate for the initial breakdown of starch. Furthermore, light receptor PHYB has been reported to promote starch accumulation (Han et al., 2017; Sun et al., 2017). Consistent with these reports, our idea showed that the increased expression of ZmPHYB1c in PF enhanced starch and sugar biosynthesis through starch degradation in PF, which compensated PS leaves. This mechanism helped maintain leaf greenness in maize under partial shading (Li et al., 2020). It probably created a synergy between the two side leaves that increased SPS and SS activity in PF leaves to maintain the sucrose and starch content in PS leaves, which helped in delaying senescence. On the other hand, the decrease in photosynthetic rate preceded leaf yellowing under all conditions, which might have induced leaf senescence through decreased sugar levels (Quirino et al., 2000). Similar results were reported in a previous report in which sugar degradation in complete shading resulted in leaf yellowing and increased relative electrolyte leakage (van Doorn, 2008).
Subsequently, we traced the translocation through the carbon labeling to verify the regulation of sugar synthesis between PF and PS, and the distribution of photoassimilates under partial and normal light conditions. A significant increase of 13C abundance in the leaves of partial shading rather than normal light in day 1 to day 10 indicated the higher sugar production under PL (Fig. 4). A similar situation for a maximum peak of 13C was observed in ryegrass shoot day 1 (Butler et al., 2004). In addition, 13C abundance in different parts of the maize plant was significantly elevated in the middle and bottom leaves in PL at day 1 under partial shading (Peuke et al., 2001). Combined with the evidence from increased Pn (Table 2), up-regulated and higher contribution of soluble sugar and sucrose synthesis by PF than PS (Fig. 3A, C-E) and, the higher13C abundance in the stem during initial ten days and then in-ear during late 30 days (Fig. 4) under PL inferred that partial shade enhanced the photosynthetic carbon fixation function of PF leaves. Furthermore, we confirmed that about 5% of 13C was transferred from PF into PS, and 70% of the 13C was supplied to the middle and lower leaves when the ear was not fully formed after flowering. It is shown that an increase in sugar level and balance of sugar transfer between the PF and PS leaves helped the leaves to maintain greenness under a partial light environment.
To further verify this hypothesis at the molecular level, we analyzed the expression pattern of sugar transporter genes, i. e., SUT, that help to diffuse sucrose into phloem through the apoplast pathway (Riesmeier et al., 1992; Sauer, 2007). Another essential sucrose transporter, “SWEET”, transfers the sucrose from parenchyma to apoplast (Chen et al., 2012). Keeping in view the role of SUT and SWEET genes in sucrose transport, we observed the expression pattern of ZmSUT1a and ZmSWEET11.1 under normal and partial light conditions. In line with our speculation and pre-mentioned results, both genes (ZmSUT1a andZmSWEET11.1) showed significantly higher expression under the PL than NL conditions. Interestingly, the ZmSUT1a andZmSWEET11.1 were up-regulated considerably during the first ten days in PF leaves. However, from day 25th and onward, the two genes showed significantly higher expression in PS leaves, indicating that sugar transport was increased towards the PS leaves. Overall, it verifies our hypothesis about the balancing of sugar in PS leaves by PF leaves under shading conditions that could delay senescence in both side leaves, which increases the outflow of sugar into the ear, as seen from the delayed grain filling period (Fig. 4D, 6C, 8). We suggest that partial shade stimulation may have enhanced the photosynthetic carbon sequestration function of PF leaves to compensate for the carbon assimilation in PS leaves and improved carbon balance and resistance on both sides. Altogether, these results suggested an increase in sugar levels and sugar balance between PF and PS was required for the delayed leaf senescence in maize.