Morphological and molecular mechanisms of floral nectary development in Chinese Jujube

Background: Chinese jujube (Ziziphus jujuba Mill.), also called Chinese date, is one of the oldest and widely cultivated fruit trees with great economic values, which, at least, can be attributed to the melliferous flower with highly developed nectary that can secret huge amount of nectar in a rather tiny floral size. However, the morphological nature, metabolic products, developmental process, as well as molecular and regulatory mechanisms of jujube nectary remain largely unknown.

Results: Here, we selected Z. jujuba 'Dongzao' as a system to address these questions. We uncovered that the jujube nectary is an annular or donut-shaped secretory protrusion that surrounds the base of the carpels, along with emerald and glistening hues, which can produce a bulk honey with many metabolic compounds (e.g. saccharides and flavonoids) that has a high nutritional value and benefit for human health. The development of jujube nectary is a dynamic process of earlier cell division followed by later cell expansion. We also identified putative genes associated with the nectary development and found that the CRABS CLAW (CRC) ortholog (ZjCRC) is the key to nectary development: the gene is highly expressed in nectary; ectopic expression of it in the Arabidopsis crc-1 mutant rescued the lost nectary (also the carpel and silique defects). We also demonstrated that a MADS-box transcription factor ZjAGAMOUS1 (ZjAG1) is required for the direct activation of ZjCRC expression.

Conclusions: Taken together, our results not only provide a comprehensive portrait of the jujube nectary, but also pave the way to effective utilization of jujube and other woody crops.