Biochemical and molecular characterisation of exogenous cytokinin application on grain filling in rice

Binay Bhushan Panda,Birendra Prasad Shaw,Lamboder Behera,Sushant Kumar Dash,Sudhanshu Sekhar

doi:10.1186/s12870-018-1279-4

Binay Bhushan Panda, Birendra Prasad Shaw + Show 3 more

Open Access

https://doi.org/10.1186/s12870-018-1279-4

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Abstract

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.

Highlights

Poor 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
The study indicated that CK metabolism could be of much importance in the process concerned with grain filling in rice
It is well reflected from a high activity level of cytokinin oxidase (CKX) and a greater expression of the CKX isoforms in the poorly filled basal spikelets compared with that in the well filled apical spikelets of the compact-panicle rice cultivar OR-1918

Summary

Introduction

Poor 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. The rate of 2.7% increase in annual yield achieved in 1980s has got drastically reduced to 1.1% evaluated until 1990s [3] This is in sharp contrast to the fact that the world population is going to increase to 9.6 billion by 2050 [4] from the current figure of 7.3 billion [5]. The effort to increase the number of spikelets on panicles was made to enhance the sink capacity with the aim of breaking the yield ceiling Breeding effort in this regard resulted in cultivars with large panicles bearing numerous spikelets, including the NPT of the International Rice Research Institute (IRRI) [10] and ‘super hybrid rice’ in China [11, 12]. Poor filling of grains is seen in the spikelets of other large panicle rice varieties/cultivars [15, 16]

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