Sucrose affects the developmental transition of rhizomes in Oryza longistaminata

Abstract

Oryza longistaminata, the African wild rice, can propagate vegetatively through rhizomes. Rhizomes elongate horizontally underground as sink organs, however, they undergo a developmental transition that shifts their growth to the surface of the ground to become aerial stems. This particular stage is essential for the establishment of new ramets. While several determinants such as abiotic stimuli and plant hormones have been reported as key factors effecting developmental transition in aerial stem, the cause of this phenomenon in rhizome remains elusive. This study shows that depletion of nutrients, particularly sucrose, is the key stimulus that induces the developmental transition in rhizomes, as indicated by the gradient of sugars from the base to the tip of the rhizome. Sugar treatments revealed that sucrose specifically represses the developmental transition from rhizome to aerial stem by inhibiting the expression of sugar metabolism and hormone synthesis genes at the bending point. Sucrose depletion affected several factors contributing to the developmental transition of rhizome including signal transduction, transcriptional regulation and plant hormone balance.

Keywords: Developmental transition; Gravitropism; Oryza longistaminata; Rhizome; Sucrose.

Fig. 1

Morphological changes at the bending point of the aerial stem. a Upper picture observed from soil surface showing a new tiller as ramet growing far from the parental shoot of O. longistaminata. Uncovering the soil, lower picture showing the separated tiller is connected to the parental shoot through rhizomes. b Image of aerial stem and rhizome of O. longistaminata. Aerial stem and rhizome have similar phytomer structure. Each black box represents one unit of phytomer separated by nodes (indicated by black triangle). Red arrow indicates the direction of developmental transition from rhizome to new aerial stem. c–f Gravi-response of aerial stem. Comparison of cell sizes on the upper and lower segments of pulvini (yellow scale bars = 1 cm, white scale bars = 100 µm.) Pulvinus structure and longitudinal section of aerial stems before (c) and 3 days after orienting the aerial stems to lie onto its side (d). Gray and blue rectangles indicate the upper and the lower segment. The graph shows the width and height of cells from 60 cells of 3 plants (e, f). g The node around the region with (i) and without (ii) pulvinus 3 days after orienting the aerial stems to lie onto its side. h The graph shows bending angle from five plants. DAT days after treatment, Error bars indicate ± SD. Significant differences were detected by student t test, **p ≤ 0.01, *p ≤ 0.05

Fig. 2

Morphological changes at the bending point of the rhizome. a Twisted rhizome underground with scale leaves (i), magnified picture (ii), and without scale leaves (iii). Yellow scale bar = 2 cm. b The appearance of normal bending rhizome, each rectangle colored in gray and blue indicate the upper and lower segments that were sectioned in c, d. c–f Comparison of cell sizes on the upper and lower segments of the rhizome. White scale bars = 100 µm. Rhizome structure and longitudinal section of its straight part (c) and bending part (d). The graph shows the width and height of cells from 60 cells of 3 samples (e, f). Error bars indicate ± SD. Significant difference was detected by student t test, *p ≤ 0.05

Fig. 3

Relationship between the morphological feature of rhizome and rhizome bending. a Illustration on how the number of internodes of already bent rhizome was counted. Red rectangle indicates the bending point. b Graph showing the elongated internode number in rhizome from base to bending point of rhizome. Bars within the dotted lines show 71% of rhizome has elongated 6th to 8th internodes underground. Red triangle indicates the 5-internode rhizome used for the physiological experiments (Figs. 4, 5, 6). c Graph showing the correlation between rhizome initial length and the number of days required for the rhizome to aerial stem aboveground

Fig. 4

Carbohydrates distribution in O. longistaminata rhizome. a Transverse sections of rhizomes after 30 min of staining with KI solution. Scale bar = 5 mm. Reversed plot profile of staining intensity was analyzed using the ImageJ software (Ver1.44). Error bars indicate ± SEM for four biological replicates. b Illustration of rhizome sampling condition. Sampling points were at the 1st, 3rd and 5th internode indicated by red rectangle. Suc sucrose, Glc glucose, Frc fructose. Sugar concentration in the rhizome; sucrose (c), glucose (d), fructose (e). Error bars indicate ± SD for five biological replicates

Fig. 5

Effect of sugars on the bending response of excised rhizomes. a Image of setup for sugar treatments. b, c Physiological observations of rhizomes during external sugar treatments. Scale bars = 3 cm, red triangle pointing at the tip of the rhizome. Mock (b) and with sucrose (c). d Change in the angle of upward growing rhizomes during sugar treatments. Error bars indicate ± SEM

Fig. 6

Changes in the relative expression levels of genes after sucrose treatment in the bending part of rhizome. a The sampling point of rhizome with or without sucrose treatment. b–g Expression levels of developmental transition-related and gravitropic response-related genes, OsSUT1 (b), GA20ox2 (c), GA2ox (d), OsPIN1 (e), RGP1 (f), OsSUS1 (g) and Ivr2 (h) in O. longistaminata rhizome. Black and white bar indicates without or with sucrose, respectively. Suc sucrose. Values are represented relative to the expression level of OsUBI transcript. Error bars indicate ± SD for five biological replicates. Significant differences were detected by student t test, *p ≤ 0.05

Fig. 7

GA overcomes the sucrose effect on negative gravitropism of rhizome by promoting cell elongation. a Change in the angle of upward growing rhizomes during GA and UNI treatments with or without sucrose. Error bars indicate ± SEM. b Cell height at upper and lower part of the bending position of rhizome. Twenty cells of three plants were measured. Significant difference was detected by student t test, *p ≤ 0.05. Suc sucrose, GA gibberellin, UNI uniconazole