The Mechanism Of The Difference In Grain Filling And Quality Among Grains Within A Panicle Of Compact-panicle Rice

Panicle shape is an important component of plant type in cereal crops due to its close association with canopy structure, thus affecting grain yield. There is a close relationship between grain density in panicle (GD) and grain yield and quality. In recent two decades, some compact-panicle rice cultivars were released and widely planted in the southeast China, and these cultivars are characterized by high yield potential attributing to the increase in floret number per panicle. However, There is a greater difference in weight and quality among grains in a panicle for the compact-panicle cultivars than loose-panicle ones. Thus it is quite important to validate the exact influence of GD on grain weight and quality and the mechanism of the difference among grains within a panicle, in order to breakdown the contradiction between grain number per panicle and quality difference among grains in a panicle. In the present study, the difference in weight and quality among grains in different positions in a panicle were determined by using rice cultivars differing in panicle type, to study the relationship between GD and the variations in grain filling and quality among grains in a panicle. Furthermore, the characteristics of two kinds of cultivars, i.e. compact-panicle and loose-panicle cultivars were compared in leaf photosynthesis, vascular bundles in panicle axis, the activities of the enzymes involved in starch synthesis, the contents of plant hormones, the effect of changing source/sink on floret development and quality formation, and the differentially expressed genes between superior and inferior florets, to reveal the mechanism of the poor grain filling and larger difference in development and quality among grains in a panicle of compact-panicle cultivars. The main results were as follows:1. Grain weight and major quality parameters were significantly correlated with GD. Grain weight (GW), amylose content (AC), brown rice length (BRL) and width (BRW) were negatively correlated with GD, while brown rice percentage (BRP) and head milled rice percentage (HMRP) were positively correlated with GD. The compact-panicle cultivars had lower GW and larger variations among grains in a panicle, compared with loose-panicle ones. Moreover, the extent of variation in quality among grains in a panicle of compact-panicle cultivars was also larger than that of loose-panicle ones, especially for grain shape (BRL, BRW, and L/W), CGP, and AC. Among these quality parameters, CGP and AC had the largest variations, being much larger than that ofgrain weight.2. The compact-panicle cultivars had a shorter, wider, and thicker flag leaf, with higher specific leaf area, SPAD value and chlorophyll content than those of loose-panicle ones. Moreover, the dry weight of culm-sheath was also higher for compact-panicle cultivars than for loose-panicle ones. In addition, compact-panicle cultivars had a reasonable canopy with a better lighting condition at mid and top plants.. Accordingly, the cultivars had a higher potential in photosynthesis ability. On the other hand, the compact-panicle cultivar was lower in leaf Pn and sugar content in leaf and leaf sheath. The re-mobilization of assimilate stored in leaf sheath was much less during grain filling for compact-panicle cultivars than for loose-panicle cultivars.3. The compact-panicle cultivars had developed vascular bundles in panicle axis, with vascular bundle number and areas of internode, culm wall, vascular bundles, and phloem being higher than those of loose-panicle cultivars. A large difference was found in the areas of vascular bundle and phloem between the rachis of superior and inferior florets of compact-panicle cultivar. The areas of vascular bundle and phloem in superior floret rachis were much larger in compact-panicle cultivars than in loose-panicle ones, while little difference was found between inferior floret rachis in two kinds of cultivars.4. There were significant differences in AGP-glucose pyrophosphorylase (APGase), starch branching enzyme (SBE) and soluble starch synthase (SSS) activities between two kinds of florets, with superior florets being higher than inferior ones at early filling stage and smaller at later stage, and the difference was larger for compact-panicle cultivars than for loose-panicle ones. For compact-panicle cultivars, APGase activity of superior florets was higher at early grain filling, and decreased during development. The inferior florets had highest APGase activity at mid grain filling. The largest difference in APGase between two kinds of florets occurred at early grain filling, with superior florets being significantly higher than inferior ones. The SSS activity of superior floret decreased with grain filling, while that of inferior florets increased at early stage and decreased at later stage. The highest SSS activities of superior and inferior florets happened at 10 and 20 d, respectively. The superior florets had a higher GBSS activity than inferior florets at early stage, which was reversed at later stage due to a dramatic decrease of superior florets. The SBE activity of superior florets was higher and lower than that of inferior ones at early and later grain filling,respectively. The highest SBE activity of superior and inferior florets occurred at early and mid grain filling stage, respectively.5. The superior florets of compact-panicle cultivars had higher and lower plant hormones (IAA, ABA, ZR, and GA3) contents than the inferior ones at early and mid grain filling, respectively. The highest hormone content of superior occurred at 10 d after flowering, while that of inferior florets happened at 15 d after flowering. The compact-panicle cultivars had a larger difference in the four hormone contents between superior and inferior florets than loose-panicle cultivars at early grain filling (5-10 d after flowering).6. The effects of changing source/sink on grain filling, activities of the enzymes involved in starch synthesis, and hormone content were dependent on different floret positions. With a increase in source/sink ratio (the floret-removal treatment) of compact-panicle cultivars, the highest grain filling rate, starch content, grain weight, and filled grain percentage of inferior florets were increased significantly, while the grain filling of superior florets changed little. Meanwhile, the CGP of superior florets decreased, and the CGP and AC of inferior florets increased significantly. Increasing source/sink ratio had great impacts on the activities of the enzymes involved in starch synthesis during grain filling. The activities of APGase, SSS, and SBE in superior florets decreased at early filling stage and increased at middle or later stages. Meanwhile, the activities of SSS and SBE in both florets at early filling stage and of APGase and GBSS in inferior florets over the whole stage were increased. On the other hand, after reducing source/sink ratio (the leaf-cutting treatment), the grain filling rate, starch content, and grain weight were significantly reduced. Meanwhile, the HMRP and AC of superior florets were increased and decreased, respectively, and the CGP of inferior florets increased significantly. The activities of APGase and SBE at early filling stage and GBSS at later filling stage were reduced in superior florets, while SSS activity at middle filling stage and SBE activity at later stage were increased. Plant hormones contents in florets during grain filling were also affected by increasing source/sink ratio. After increasing source/sink ratio of compact-panicle cultivars, the contents of four hormones in both superior and inferior florets were decreased significantly. The reduction in source/sink ratio reduced the differences in GA3 and ZR content between superior and inferior florets, and ABA, IAA, and GA3 contents in inferior florets at mid or latter grain filling were lower than those in superior florets.7. The total RNA and mRNA contents in superior florets at early grain filling were higher than those in inferior florets. Suppression subtractive hybridization (SSH) was used to construct a cDNA library containing genes differentially expressed in inferior florets. By screening the cDNA library and sequencing the enriched cDNAs, six known genes and three novel genes were identified. The six known genes, i.e. D21130, AF294580, DQ219479, AB070845, XM₄64834, and D50643, encoded acidic ribosomalprotein PO, 10 kDa prolamin, xylanase inhibitor, leucin-zipper motif, glutelin precursor, and 26 kDa globulin, respectively. The functions of three novel genes, i.e. AK071090, AK060156, and AK107966 might be related to the transposase, which was identified in Photobacterium damselae subsp. Piscicida (AK071090 and AK107966) and truncated cytochrome C oxidase protein (AK060156).