Abstract
BackgroundThe SWEET (Sugars Will Eventually be Exported Transporters) gene family is a recently identified group of sugar transporters that play an indispensable role in sugar efflux, phloem loading, plant-pathogen interaction, nectar secretion, and reproductive tissue development. However, little information on Saccharum SWEET is available for this crop with a complex genetic background.ResultsIn this study, 22 SWEET genes were identified from Saccharum spontaneum Bacterial Artificial Chromosome libraries sequences. Phylogenetic analyses of SWEETs from 11 representative plant species showed that gene expansions of the SWEET family were mainly caused by the recent gene duplication in dicot plants, while these gene expansions were attributed to the ancient whole genome duplication (WGD) in monocot plant species. Gene expression profiles were obtained from RNA-seq analysis. SWEET1a and SWEET2s had higher expression levels in the transitional zone and maturing zone than in the other analyzed zones. SWEET1b was mainly expressed in the leaf tissues and the mature zone of the leaf of both S. spontaneum and S. officinarum, and displayed a peak in the morning and was undetectable in both sclerenchyma and parenchyma cells from the mature stalks of S. officinarum. SsSWEET4a\\4b had higher expression levels than SWEET4c and were mainly expressed in the stems of seedlings and mature plants. SWEET13s are recently duplicated genes, and the expression of SWEET13s dramatically increased from the maturing to mature zones. SWEET16b’s expression was not detected in S. officinarum, but displayed a rhythmic diurnal expression pattern.ConclusionsOur study revealed the gene evolutionary history of SWEETs in Saccharum and SWEET1b was found to be a sucrose starvation-induced gene involved in the sugar transportation in the high photosynthetic zones. SWEET13c was identified as the key player in the efflux of sugar transportation in mature photosynthetic tissues. SWEET4a\\4b were found to be mainly involved in sugar transportation in the stalk. SWEET1a\\2a\\4a\\4b\\13a\\16b were suggested to be the genes contributing to the differences in sugar contents between S. spontaneum and S. officinarum. Our results are valuable for further functional analysis of SWEET genes and utilization of the SWEET genes for genetic improvement of Saccharum for biofuel production.
Highlights
The SWEET (Sugars Will Eventually be Exported Transporters) gene family is a recently identified group of sugar transporters that play an indispensable role in sugar efflux, phloem loading, plant-pathogen interaction, nectar secretion, and reproductive tissue development
The paralogous OsSWEET11/Os8N3/Xa13 and OsSWEET14/Xa25 are targeted by the pathogenic bacterium Xanthomonas oryzae pv. oryzae (Xoo) and hijacked for nutritional gain to support their own growth, which results in bacterial blight [22]
Identification of SWEET genes in sugarcane and other species Based on the reference sequences of SWEETs from previous studies in Oryza sativa [37] and Arabidopsis thaliana [38], the SWEET genes were searched from 10 representative plant genomes downloaded from phytozome 10 [39]
Summary
The SWEET (Sugars Will Eventually be Exported Transporters) gene family is a recently identified group of sugar transporters that play an indispensable role in sugar efflux, phloem loading, plant-pathogen interaction, nectar secretion, and reproductive tissue development. The first two gene families of sugar transporter, MSTs and SUTs, have been extensively studied in higher plants in the last two decades [4, 9,10,11,12,13,14,15,16], whereas the SWEET gene family was recently identified as sugar effluxers [7] based the role of its members in the transport of hexose or sucrose across cell membranes. Eom [23] and Chen [24] recently reviewed the function of the SWEET family proteins in diverse biological and physiological processes including phloem loading, leaf and senescence fructose conservation, pollen nutrition and seed filling, pathogen susceptibility, nectar secretion
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