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BackgroundPoor filling of grains in the basal spikelets of large size panicles bearing numerous spikelets has been a major limitation in attempts to increase the rice production to feed the world’s increasing population. Considering that biotechnological intervention could play important role in overcoming this limitation, the role of cytokinin in grain filling was investigated based on the information on cell proliferating potential of the hormone and reports of its high accumulation in immature seeds.ResultsA comparative study considering two rice varieties differing in panicle compactness, lax-panicle Upahar and compact-panicle OR-1918, revealed significant difference in grain filling, cytokinin oxidase (CKX) activity and expression, and expression of cell cycle regulators and cytokinin signaling components between the basal and apical spikelets of OR-1918, but not of Upahar. Exogenous application of cytokinin (6-Benzylaminopurine, BAP) to OR-1918 improved grain filling significantly, and this was accompanied by a significant decrease in expression and activity of CKX, particularly in the basal spikelets where the activity of CKX was significantly higher than that in the apical spikelets. Cytokinin application also resulted in significant increase in expression of cell cycle regulators like cyclin dependent kinases and cyclins in the basal spikelets that might be facilitating cell division in the endosperm cells by promoting G1/S phase and G2/M phase transition leading to improvement in grain filling. Expression studies of type-A response regulator (RR) component of cytokinin signaling indicated possible role of OsRR3, OsRR4 and OsRR6 as repressors of CKX expression, much needed for an increased accumulation of CK in cells. Furthermore, the observed effect of BAP might not be solely because of it, but also because of induced synthesis of trans-zeatin (tZ) and N6-(Δ2-isopentenyl)adenine (iP), as reflected from accumulation of tZR (tZ riboside) and iPR (iP riboside), and significantly enhanced expression of an isopentenyl transferase (IPT) isoform.ConclusionThe results suggested that seed-specific overexpression of OsRR4 and OsRR6, and more importantly of IPT9 could be an effective biotechnological intervention towards improving the CK level of the developing caryopses leading to enhanced grain filling in rice cultivars bearing large panicles with numerous spikelets, and thereby increasing their yield potential.Electronic supplementary materialThe online version of this article (10.1186/s12870-018-1279-4) contains supplementary material, which is available to authorized users.

The increase of spikelet number in the panicles of modern super rice has made the architecture compact, as the extra spikelets are accommodated mostly on secondary branches than on primary branches. However, the grain yield did not improve because of poor grain filling, which was more visible in the basal spikelets than apical spikelets. The objective of this study was to examine the effect of the compactness and positional difference of spikelets in the panicle on grain filling by comparing the activity and genetic expression of starch synthesising enzymes in the developing kernels of lax-(Upahar and CR3856-45-11-2-7-2-5 (CR-45)) and compact-(Mahalaxmi and CR3856-29-14-2-1-1-1 (CR-29)) panicle cultivars. Upahar and Mahalaxmi are genetically related, whereas CR-45 and CR-29 are recombinant inbred lines. The grain carbohydrate concentration and activity of sucrose synthase (SUS) enzyme were estimated during the active period of grain filling. Further, expression of isoforms of SUS, ADP glucose pyrophosphorylase (APL and APS for large and small units respectively) and starch synthase (SS and GBSS for soluble and granule bound starch synthases respectively) were also assayed through PCR studies. The genotype approach used revealed grain SUS activity and starch concentration high and sugar concentration low in the lax-compared with compact-panicle cultivars and in the apical spikelets compared with basal ones. The margin of variation between apical and basal spikelets was higher in the compact-than the lax-panicle cultivars. Genetic expression of most of the isoforms of the enzymes was higher in the lax-than the compact-panicle cultivars as seen in RT-PCR studies. A quantitative appraisal of transcript levels of isoforms in the qRT-PCR identified greater expression of SUS3 in the basal spikelets of Upahar than that in Mahalaxmi and in CR-45 over CR-29, most prominently during the active period of grain filling. We conclude that proximal location as well as increased density of spikelets on panicles affected SUS3 expression in the basal spikelets. The metabolic dominance of a spikelet in rice panicle is dependent on the expression of the genes for different isoforms of starch synthesising enzymes, but the expression of SUS3 could be more specific than the others. SUS3 expression is most active during grain filling of the lax-panicle cultivars, but its dominance is reduced significantly in the kernels of the compact-panicle cultivars.