IAWA
Kajii
Journal
et al. –35Laticifers
(2), 2014:of109–115
Ficus
109
LATICIFERS OF FICUS CARICA AND THEIR POTENTIAL ROLE
IN PLANT DEFENSE
C. Kajii 1, T. Morita 2 and K. Kuroda 1, *
1 Graduate
School of Agriculture, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan
Technology Research Center, Hiroshima Prefectural Technology Research Institute,
2835 Mitsu, Akitsu-cho, Higashihiroshima 739-2402, Japan
*Corresponding author; e-mail: kurodak@garnet.kobe-u.ac.jp
2Agricultural
ABSTRACT
The distribution and structure of laticifers in Ficus carica L. were investigated in
a current-year branch and an 8-year-old trunk, using serial sections stained with
safranin-fast green or nile blue. In the 8-year-old trunk, laticifers were found not
only in the cortex, secondary phloem, and pith, but also in the secondary xylem.
The laticifers in the phloem and xylem were of the branched, non-articulated
type. In addition, horizontal laticifers extending from the phloem through the
cambium to the xylem were found in some rays. Laticifers penetrating the cambial initial layer elongated in the cambial zone without cell division. Activation
of latex production occurred after wounding of cortex and phloem, and the
latex exuded from the cut site of laticifers into the surrounding wounded tissue
and broken cells. The potential role of laticifers in defense of F. carica against
certain pathogens is discussed.
Keywords: Latex, fig tree, defense, Ceratocystis ficicola.
INTRODUCTION
Laticifers are defined as cells or series of connected cells containing a fluid called
latex and forming systems that permeate various tissues of the plant body (Evert 2006).
Latex is known to contain a variety of proteins and enzymes. In particular, the latex of
the genus Ficus contains ficin (cysteine proteases), indicating appreciable proteolytic
activity, which in turn has a role in defense against herbivores and pathogens (Williams
et al. 1968; Konno 2011). We focused on the distribution of laticifers in the common
edible fig tree (Ficus carica L.) that might prevent invasion by an ambrosia beetle,
Euwallacea interjectus, the vector of a vascular wilt fungus, Ceratocystis ficicola (Kajii
et al. 2013). Laticifers of the genus Ficus are mentioned frequently in the literature together with other Moraceae species (Vreede 1949; Metcalfe 1967; Davies et al. 1982;
Rachmilevitz & Fahn 1982; Van Veenendaal & Den Outer 1990; Kang et al. 2000;
Palhares et al. 2007). However, descriptions of the laticifers in the secondary phloem
and secondary xylem of F. carica are fragmentary (Ogata et al. 2008). The purpose of
the present study is to describe the distribution and structure of laticifers in the branch
and trunk of F. carica, and to estimate the potential contribution of laticifers to blocking the activity of the pests.
© International Association of Wood Anatomists, 2014
Published by Koninklijke Brill NV, Leiden
DOI 10.1163/22941932-00000052
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IAWA Journal 35 (2), 2014
MATERIALS AND METHODS
An 8-year-old trunk of Ficus carica was harvested in the orchards of Hiroshima Prefecture. Current-year branches of F. carica were collected from the Hiroshima Prefectural
Technology Research Institute after making horizontal incisions with a knife to induce
latex excretion. All specimens were fixed with FAA (formalin:acetic acid:30% ethanol,
5:5: 90 v/v). After the specimens were washed under tap water overnight, transverse,
tangential, and radial sections (20 µm thick) were cut from those specimens with a
sliding microtome. Sections were stained with safranin-fast green (Johansen 1940)
or nile blue (Conn 1977), or kept without staining, and then were processed for light
microscopy (Nikon, Eclipse 80i). Some of the thick sections (100 µm thick) including
laticifers were embedded in Spurr media (Spurr Low-Viscosity Embedding Kit) after
the presence of laticifers was checked under the light microscope. Then, 3-µm-thick
sections were made with a rotary microtome. Sections were observed with a differential
interference microscope.
RESULTS AND DISCUSSION
Laticifers were observed in the cortex (Fig. 1), secondary phloem, and pith (Fig. 2) of
both the current-year branches and the 8-year-old trunk. In addition, they were found
in the secondary xylem and cambium of the 8-year-old trunk.
The laticifers in the phloem and pith were of the branched, non-articulated type
(Fig. 3 & 4), as reported by Rachmilevitz and Fahn (1982). Laticifers in the secondary xylem extended in a horizontal direction within the rays (Fig. 5). Wheeler et al.
(1989) described that non-articulated laticifers are not uncommon in the xylem rays of
Apocynaceae, Asclepiadaceae, Euphorbiaceae, and Moraceae. In our material, laticifers found in the secondary xylem sometimes were branching and extending axially
interspersed among the fibers (Fig. 6). The incidental presence of radial laticifers in
Ficus is mentioned by Koek-Noorman et al. (1984) but not that of axial laticifers. Axial
non-articulated laticifers have not yet been reported in the tissue of F. carica, although
they are known to exist in the secondary xylem of Artocarpus species (Moraceae)
(Topper & Koek-Noorman 1980).
It is well documented that non-articulated, branched laticifers develop from single
initials or primordia during development of the embryo, and then elongate as the plant
grows (Mahlberg 1961, 1963; Cass 1985; Murugan & Inamdar 1987; Rudall 1987, 1994;
→
Figure 1–7. Laticifers in stem tissues of Ficus carica. – 1 & 2: Axial laticifers (L) in the cortex
(1) and pith (2) of a current-year branch. Cross sections. Nile blue staining. – 3: An axial laticifer
(L) in the secondary phloem. Tangential section. Safranin-fast green staining. – 4: A branching
laticifer (L) in the secondary phloem. Tangential section. Nile blue staining. – 5: A long horizontal laticifer extending in the ray tissue of the xylem (L). Radial section. Safranin-fast green
staining. – 6: Connection or branching of the horizontal and axial laticifer (L) in the secondary
xylem. Radial section. Safranin-fast green staining. – 7: A horizontal laticifer (L) extending in
the secondary phloem, cambium, and the secondary xylem. Cross section. Rectangular area (*)
see Fig. 8. — Scale bars in 3, 4, 6 & 7 = 50 µm, in 2 = 100 µm, in 1 & 5 = 200 µm.
Kajii et al. – Laticifers of Ficus
1
3
111
2
4
6
5
7
Abbreviations of the two plates: A = axial parenchyma; C = cortex, Ca = cambium; CZ = cambial
zone; L = laticifer; Ph = phloem; Pi = pith; S = sieve tube; ; W = wound; X = xylem.
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IAWA Journal 35 (2), 2014
8
9
8
10B
10A
10C
Figure 8–10. Laticifers in stem tissues of Ficus carica. – 8: High magnification of a horizontal
laticifer existing in the cambial zone. – 9: A laticifer in the secondary phloem (L) turned at a
right angle and extending horizontally in the xylem. Radial section. – 10: Cortex of currentyear branches. After wounding, laticifers actively secreted latex (L) (A) and laticifers around
the wound were stained purple (B). In non-treated areas, latex was not visible (C). Radial section (A) and cross section (B, C). Nile blue staining. — Scale bars in 8 = 10 µm, in 9 & 10A =
200 µm, in 10B & C = 500 µm.
Kajii et al. – Laticifers of Ficus
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Van Veenendaal & Den Outer 1990; Roy & De 1992; Da Cunha et al. 1998). It generally
is presumed that the laticifers penetrate the rays from the pith (Evert 2006). It has also
been recorded that laticifers traverse the vascular cambium in some Asclepiadaceae,
Euphorbiaceae, and Moraceae (Blaser 1945; Artschwager 1946; Vreede 1949; Rudall
1989). To understand the differentiating process of laticifers in Ficus, we observed the
structure of the laticifers in the cambial region in detail with thin sections embedded
in plastic (Fig. 7).
Under the differential interference microscope, laticifers were observed extending
horizontally in the secondary phloem and xylem traversing the cambial initial layer
(Fig. 8, arrow). In addition, we found that some longitudinal laticifers in the secondary
phloem turned at a right angle and then extended horizontally into the xylem (Fig. 9,
arrow). These findings suggest that longitudinal laticifers in the phloem and horizontal laticifers in the secondary xylem are continually formed. For the few examples of
laticifers that extend between the phloem and xylem and are changing their direction at
a right angle, further discussion on the site of origin will be difficult. Tangential septa
were not found in the horizontal laticifers in the cambial zone (Fig. 8, arrow). This
shows that laticifers did not form by cell division in the cambial zone. That is, the cell
wall of laticifers must be elongating in the cambial zone, probably with the repetitive
addition of wall substances. Fujii and Sudo (1988) described that the laticifer walls in
the matured ray tissue of Pimelodendron amboinicum (Euphorbiaceae) shows a loose
texture and is composed of different layers. Although the process of laticifer wall
formation in Ficus has not been observed, their data suggest that laticifers accumulate
wall substances with the continual elongation at cambial zone.
Laticifers were found more frequently in the secondary phloem than in the secondary
xylem of specimens of both ages (current-year- and 8-year-old). In specimens wounded
just before harvest by a thin blade, laticifers and latex that was exuded from the incision
wound into adjacent broken cells (Fig. 10A, B) stained purple to pink with nile blue–
much stronger so than specimens without incision wounds (Fig. 10C). This indicates
that laticifers actively secreted latex in reaction to wounding. The latex may have a role
in preventing pest invasion. This defense system, however, does not always function
in the case of fig trees infected with a wilt pathogen, Ceratocystis ficicola, vectored by
the ambrosia beetle, Euwallacea interjectus. As described in our previous report (Kajii
et al. 2013) invasion of that vector beetle was successful from the necrotic area of the
phloem where latex exudation had stopped, although the invasion had failed from the
sound phloem. The function of laticifers in xylem is not clear because the activities of
the beetle and pathogenic fungus were not prevented in the sapwood of fig trees (Kajii
et al. 2013). Further observations will be necessary to assess whether the laticifers in
phloem and xylem can be effective as defense system in fig trees, or whether certain
pathogens and beetles have developed strategies to brake this defense system.
Acknowledgement
We wish to acknowledge valuable discussions with Dr. T. Fujii on the structure and distribution of
laticifers in trees.
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IAWA Journal 35 (2), 2014
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Accepted: 12 February 2014