Soybean and Bees

According to the first scenario mentioned by Heil (2011), carbohydrates are uploaded as
sucrose from the phloem to the secretory tissue where they are stored and/or processed
(Kram and Carter, 2009; Wenzler et al., 2008). It is widely known that, during active secretion,
sucrose is metabolized by cell wall invertases, producing hexose-rich nectars and creating
the required source-sink relationships (Frey-Wyssling et al., 1954; Agthe, 1951; Zimmermann,
1953). More recently it was established that genes coding for complete sucrose
biosynthesis are up regulated in A. thaliana nectaries (Kram et al., 2009), and the expression
patterns of genes involved in starch metabolism allow a clear separation of an anabolic phase
before anthesis and a catabolic phase during secretion in nectaries of ornamental tobacco
(Ren et al., 2007).
According to Zimmermann (1953) and Heil et al. (2005), sucrose can also be eliminated from
nectar by post-secretory hydrolysis, which is mediated by invertases that are secreted into
the nectar itself. Ruhlmann et al. (2010) discovered an apoplastic invertase required to create
the sink status for active nectar secretion. A mutant line, which lacked this activity was
referred by Ruhlmann et al. (2010) and Kram and Carter (2009). This enzyme was associated
to reduced levels of starch accumulation within the nectary, demonstrating that apoplastic
invertases might also play a central role in the uploading of sucrose from the phloem and its
subsequent storage in the nectariferous tissue.
Nonetheless, Gaffal et al. (2007) and Ren et al. (2007) demonstrate that starch accumulation
can only account for a part of the sugar that is secreted during the peak activity of floral nectaries.
Moreover, Pacini et al. (2003) alerts that extra floral nectaries have not been reported
to store starch and that all carbohydrates are likely to come directly from the phloem, and
nectar formation and secretion depend on vesicle-based mechanisms. Matile (1956) and Heil
et al. (2004) remember that floral nectaries are phylogenetically derived from extra floral
nectaries, then direct transport from the phloem seems to represent the original mechanism,
whereas starch accumulation could be an alternative strategy for the secretion of large
amounts of sugar, during the peak activities of floral nectaries, as stated by De La Barrera and
Nobel (2004)
Anyway, there are a lot of open questions regarding where non-carbohydrate nectar constituents
are produced, where and how they are added to the prenectar and how they are
secreted, according to Heil (2011). This author theorizes that, considering the abundance
and chemical diversity of nectary proteins and the lack of reports of many of these nectarins
from other tissues, synthesis in the nectary tissue seems probable.
In fact, the secretory cells of Vigna unguiculata EFNs contain protein-rich inclusions (Kuo and
Pate et al., 1985) and all Nectarin genes that correspond to nectar proteins in the FN of orna-
62 SoybeAn and bees