Introduction
Stem tunneling by maize stem borers is an important constraint to
achieve the potential yield of maize varieties across the world (Malvar,
Butron, Ordas & Santiago, 2008). The maize plant protect themselves
from borer attack using constitutive and induced defenses but much more
attention has focused on studying the first ones although evidence
indicates that induced defenses have lower resource allocation costs
than constitutive defenses (Beeghly, Coors & Lee, 1997, Cardinal &
Lee, 2005, Howe & Jander, 2008, Karban, Agrawal & Mangel, 1997,
Klenke, Russell & Guthrie, 1986, Santiago, Butron, Arnason, Reid, Souto
& Malvar, 2006, Santiago, Butron, Revilla & Malvar, 2011, Santiago,
Souto, Monetti, Ordas, Ordas & Malvar, 2006, Santiago, Souto, Sotelo,
Butron & Malvar, 2003). Indeed, maize stem feeding by borers
significantly modify antibiosis against stem borer larvae, although
antibiosis changes depend on the genotype and the duration of feeding
(Cao, ButrĂ³n, Malvar, Figueroa-Garrido & Santiago, 2019, Dafoe, Thomas,
Shirk, Legaspi, Vaughan, Huffaker, Teal & Schmelz, 2013). Dafoe et al.
(Dafoe et al. , 2013) reported that growth of stem borer larvae
was significantly higher when fed with stem tissues preconditioned by 48
h of larval tunneling compared to untreated stem tissues; while Cao et
al. (Cao et al. , 2019) stated that the effect of long-term attack
by borers on the antibiotic properties of corn stems is
genotype-dependent.
Previous studies have already shown that feeding-induced changes in
plant metabolites influence the behavior and performance of conspecifics
and that influence depends on the time-lag of induction (Poelman,
Broekgaarden, Van Loon & Dicke, 2008, Santiago, Cao, Butron,
Lopez-Malvar, Rodriguez, Sandoya & Malvar, 2017, Su, Chen, Mescher,
Peng, Xie, Wang, Wu, Liu, Li, Wang & Zhang, 2018, Wang, Bezemer, van
der Putten & Biere, 2015). According to that, several authors have
pointed out to different defense mechanisms involved in the response to
long-term feeding by insects compared to those implicated in the early
response (Donze-Reiner, Palmer, Scully, Prochaska, Koch, Heng-Moss,
Bradshaw, Twigg, Amundsen, Sattler & Sarath, 2017, Gutsche, Heng-Moss,
Sarath, Twigg, Xia, Lu & Mornhinweg, 2009, Uawisetwathana, Graham,
Kamolsukyunyong, Sukhaket, Klanchui, Toojinda, Vanavichit,
Karoonuthaisiri & Elliott, 2015); more specifically, previous studies
also suggested important differences between maize stem responses to
short and long-term feeding by stem borers (Dafoe, Huffaker, Vaughan,
Duehl, Teal & Schmelz, 2011, Rodriguez, Padilla, Malvar, Kallenbach,
Santiago & Butron, 2018, Rodriguez, Santiago, Malvar & Butron, 2012).
The early stem response to feeding by corn borers was characterized by
the activation of signaling mechanisms mediated by phytohormones,
whereas these molecules were marginally involved in the long-term
response (Dafoe et al. , 2011, Rodriguez et al. , 2012). The
stem long-term response was characterized by reorganization of the
primary metabolism and a strong redox response mainly mediated by
germin-like proteins to produce anti-nutritive and toxic compounds that
reduce insect viability (Rodriguez et al. , 2018). However, no
studies have simultaneously characterized short and long-term responses
to stem attack by stem borers and the current work would be the first
attempt to do it. A direct comparison of both responses using several
genotypes under the same experimental conditions will shed light on real
differences which cannot be disentangled from experimental differences
when results from different experiments are compared.
As the components of the metabolome can be viewed as the end products of
gene expression, un-targeted metabolomics would be a valuable tool to
simultaneously monitor all biological processes operating in the plant
response to herbivory and to discover bioactive compounds involved in
plant resistance (Jansen, Allwood, Marsden-Edwards, van der Putten,
Goodacre & van Dam, 2009, Schauer & Fernie, 2006, Sumner, Mendes &
Dixon, 2003). Indeed, metabolomics has been proved as a valuable tool to
characterize plant responses to feeding by insects and to identify plant
metabolites contributing to increased susceptibility or resistance
(Agut, Gamir, Jaques & Flors, 2015, Kang, Yue, Xia, Liu & Zhang, 2019,
Liu, Hao, Hu, Zhang, Wan, Zhu, Tang & He, 2010, Marti, Erb, Boccard,
Glauser, Doyen, Villard, Robert, Turlings, Rudaz & Wolfender, 2013,
Ponzio, Papazian, Albrectsen, Dicke & Gols, 2017, Tzin, Hojo,
Strickler, Bartsch, Archer, Ahern, Zhou, Christensen, Galis, Mueller &
Jander, 2017, Wang, Qu, Zhang, Hu, Tang & Lu, 2016, Widarto, Van der
Meijden, Lefeber, Erkelens, Kim, Choi & Verpoorte, 2006). Herbivory
could induce extensive changes to both general and specialized
metabolism to prevent the allocation of energy and nutrients to
herbivore fitness, rather than to plant fitness because plant
metabolites involved in defense are not just toxic, repellent, and/or
anti-nutritive molecules, but compounds that could attract enemies of
herbivores, participate in nutrient transport and storage to make
nutrients less accessible to the insect or involved in phenology shifts
that grant herbivore avoidance (Schuman & Baldwin, 2016).
The objectives were (i) to determine changes in the level of antibiosis
of the stems induced by feeding of Sesamia nonagrioides(Mediterranean Corn Borer; MCB) larvae for two days (short-term feeding)
and nine days (long-term feeding), (ii) to characterize the metabolome
of the stem short and long-term responses to borer attack and (iii) to
look for metabolic pathways that could modulate plant resistance to MCB