Switch from Apoplasmic to Symplasmic Phloem Unloading during Storage Roots Formation and Bulking of Sweetpotato

Abstract

675 RESEARCHS torage roots yield differs among sweetpotato (Ipomoea batatas L.) varieties from nearly 60 t ha −1 in high-yielding varieties to <40 t ha −1 in low-yielding varieties. Both high-yielding and low-yielding varieties are easily overgrown (Duan et al., 2018), especially under excessive fertilization, which is common in crop production. When overgrowth appears, the distribution of assimilates between aboveground tissues and storage roots is not coordinated. Thus, storage root yield is notably decreased (Chen et al., 2012), mainly due to the low transportation rates of assimilates from leaves and stems to storage roots. Assimilate transportation from leaves and stems to storage roots is much faster in highyielding than in low-yielding varieties of sweetpotato, which results in a higher harvest index (Liu et al., 2015a). A further study indicated that the unloading of assimilates in storage roots is a major limitation for photosynthate transportation in sweetpotato varieties (Liu et al., 2015b). To design neoteric methods that improve storage roots yield and reduce resource waste caused by ABSTRACT To clarify the unloading pathway of assimilates in storage roots of sweetpotato (Ipomoea batatas L.), a combination of methods including electron microscopy, movement of the phloemmobile symplasmic tracer carboxyfluorescein, and assays of invertase and sucrose synthase activities were explored to investigate this pathway from storage roots formation to harvest. The sieve element-companion cell complex was symplasmically connected to surrounding parenchyma cells by plasmodesmata and isolated before storage root bulking (40-140 d after planting). Numerous plasmodesmata appeared among phloem parenchyma throughout storage root bulking. Images of carboxyfluorescein movement indicated that the dye was restricted to phloem and released into surrounding tissues before and after storage root formation, respectively. Sucrose synthase activity increased continually during storage root bulking, and it was much higher than that of insoluble acid invertase. Although this remained low and changed little, that of soluble acid invertase increased and remained high. The starch content in storage roots increased during bulking, but sucrose content decreased. Thus, the predominant unloading pathway switched from apoplasmic to symplasmic during storage root formation and bulking. This switch resulted in enhanced sink potential of storage roots, evidencing opportune sink-source relationships in sweetpotato.

Change in Sucrose Cleavage Pattern and Rapid Starch Accumulation Govern Lily Shoot-to-Bulblet Transition in vitro

“…Our RNA-Seq data regarding the differential expression of unigenes in various treatments annotated to AtSWEET11 and AtSWEET12 also suggest the possibility in the abovementioned hypothesis ( Supplementary Figure 6 ). In fact, SuSy contributes more to the synthesis and storage of starch, while CWIN produces more sugars, such as glucose, for cell division in the early developmental stages, probably via the sugar signaling pathway instead of via the provision of carbon ( Weber et al, 2005 ; Baroja-Fernandez et al, 2009 ; Sonnewald and Fernie, 2018 ; Liu et al, 2019 ; Liao et al, 2020 ). These findings are in agreement with our observations of the sugar and starch contents shown in Figure 4 .…”

Tuber and Tuberous Root Development

“…However, at very early stages of tuberization and storage root formation, the apoplasmic mode switches to a symplasmic unloading transport mode mediated by plasmodesmata (PD). This switch could be demonstrated through fluorescent tracer studies for potato, sweetpotato, and cassava (84,90,140). This transition is accompanied by reduced cell wall invertase and increased sucrose synthase activities (8,81,90).…”

“…Changes in the mode of phloem unloading seem to be a common change occurring during storage organ formation. Noninduced potato stolons and the fibrous roots of sweetpotato and cassava are initially characterized by the mode of apoplasmic unloading, in which sucrose and other assimilates have to be exported into and reimported from the apoplast by carrier proteins (84,90,140). However, at very early stages of tuberization and storage root formation, the apoplasmic mode switches to a symplasmic unloading transport mode mediated by plasmodesmata (PD).…”

Regulatory and functional divergence among members of Ibβfruct2, a sweet potato vacuolar invertase gene controlling starch and glucose content

“…Acid invertase (including VIN and cell wall invertase) activity can be induced by wounding treatment ( Matsushita and Uritani, 1974 ). VIN activity increases after storage root formation, continues to intensify, and remains at high levels during storage root bulking; this activity is much higher than that of neutral invertase and insoluble acid invertase and plays important roles in regulating sucrose unloading in storage roots ( Liu et al., 2019 ). Ibβfruct2 is an important VIN-encoding gene in sweet potato and is expressed in sprouting shoots, immature leaves, stems, and storage roots ( Wang et al., 2005 ).…”