Plant Molecular Biology 41: 207–216, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Nectarin I is a novel, soluble germin-like protein expressed in the nectar of
Clay Carter, Richard A. Graham and Robert W. Thornburg
Department of Biochemistry and Biophysics, Iowa State University, 2212 Molecular Biology Building, Ames, IA
50011, USA (
author for correspondence)
Received 14 May 1999; accepted in revised form 27 July 1999
Key words: germin, germin-like proteins, nectar, Nicotiana, oxalate oxidase
We have identiﬁed a limited number of proteins secreted into the nectar of tobacco plants. Nectarin I is the most
highly expressed nectar protein and has a monomer molecular mass of 29 kDa. The other major nectar proteins are
expressed at lower levels and have monomer molecular masses of 41, 54, and 65 kDa respectively. Nectarin I was
puriﬁed and antiserum was raised against the protein. Under nondenaturing conditions, Nectarin I has an apparent
molecular mass of >120 kDa. The expression of Nectarin I was restricted to nectary tissues and to a much lower
levelin the ovary. No Nectarin I was found in petals, stems, leaves, or roots or other ﬂoral tissues. The expressionof
Nectarin I was also developmentally regulated. It is expressed in nectary tissues only while nectar is being actively
secreted. Subsequently, the N-terminus of puriﬁed Nectarin I was sequenced. Sequence identity showed Nectarin I
is related to wheat germin. Although hydrogen peroxide is readily detectable in tobacco ﬂoral nectar, we were
unable to demonstrate any oxalate oxidase activity for Nectarin I. A partial cDNA encoding the mature Nectarin I
N-terminus was isolated and used to probe a Nicotiana plumbaginifolia genomic library. The Nectarin I gene was
isolated and the translated sequence was consistent with both N-terminal and internal cyanogen bromide-derived
amino acid sequence. The gene contains a single 386 nt intron and encodes a mature protein of 197 amino acids.
Many plants require insect or avian pollinators to ob-
tain efﬁcient seed set. Dicotyledonous plants often
attract these pollinators with offerings of ﬂoral nectar
that is secreted into the ﬂoral tube at the base of the
ovary. Nectar is a rich source of sugars and amino
acids and provides a reward to pollinators, thereby
increasing the fecundity of those plants that provide
nectar. The secretion of nectar is usually under devel-
opmental control beginning when the ﬂowers open.
After pollination, the nectar is frequently resorbed
(Burquez and Corbet, 1991). In addition, nectar se-
Journal Paper J-18306 of the Iowa Agriculture and Home
Economics Experiment Station, Ames, Iowa. Project 3340.
The nucleotide sequence data reported will appear in the
EMBL, GenBank and DDBJ Nucleotide Sequence Databases under
the accession number AF132671.
cretion increases as the ﬂower is visited by pollinators
(Smith et al., 1990).
The composition of nectar has been widely stud-
ied. Nectar is an aqueous combination of a signiﬁcant
number of substances. Chief among these are sucrose,
glucose, and fructose. Other carbohydrates including
arabinose, galactose, mannose, gentiobiose, lactose,
maltose, melibiose, trehelose, melezitose, rafﬁnose,
and stachyose have also been identiﬁed in nectars of
some ﬂowers (Baker and Baker, 1981). Various types
of nectars can be ordered into three groups according
to sugar content: sucrose prevalent, glucose and fruc-
tose prevalent, and equal amounts of sucrose, glucose,
and fructose (Roshchina and Roshchina, 1993). Inter-
estingly, sugar concentrations vary greatly depending
on the type and location of the nectary (Roshchina
and Roshchina, 1993). A relationship exists between
the sugar composition of the nectar and the amount of
vascular tissue underlying the nectary (Esau, 1977). If