Constitutive and inducible trypsin proteinase inhibitor production incurs large fitness costs in Nicotiana attenuata

Abstract

Plant trypsin proteinase inhibitors (TPIs) are potent herbivore-and jasmonate (JA)-induced defenses, but support for the commonly invoked explanation for their inducible expression, namely their associated fitness costs, has been elusive. To determine whether the expression of TPIs incurs fitness costs, we expressed 175 bp of the seven-domain pi from Nicotiana attenuata in an antisense orientation in a TPI-producing genotype (WT) of N. attenuata to reduce TPI expression. Moreover, we expressed the full-length seven-domain pi in a sense orientation under control of a constitutive promoter to restore TPI activity in a natural genotype unable to produce TPIs because of a mutation in its endogenous pi gene. Lifetime reproductive output was determined from high and low TPI-producing plants of the same genetic background with and without JA elicitation and grown in the same pot to simulate natural competitive and nutrient regimes. Transformants with either low or no TPI activity grew faster and taller, flowered earlier, and produced more seed capsules (25-53%) than did neighboring TPI-producing genotypes, and JA elicitation increased TPI production and decreased seed capsule production further. Growth under high light levels only marginally reduced these fitness costs. Results were similar regardless of whether TPI activity was suppressed or restored by transformation: the larger the difference in TPI activity between neighbors, the larger the difference in seed capsule production (R 2 ‫؍‬ 0.57). TPI production is costly for a plant's components of fitness when grown under realistic competitive regimes and is consistent with the hypothesis that inducibility evolved as a cost-saving mechanism.

Nitrogen Supply Influences Herbivore-Induced Direct and Indirect Defenses and Transcriptional Responses in Nicotiana attenuata

“…TrypPIs are likely to make large demands on a plant's N budget, but quantitative measures have not been made. However, TrypPI expression is known to be responsible for decreased growth and seed production in N. attenuata (Glawe et al, 2003;Zavala et al, 2004), which may reflect N-based tradeoffs between growth and TrypPI production. Surprisingly, the expression levels of TrypPI transcripts in OS-elicited LN plants were significantly higher than those in HN plants (Fig.…”

“…We elicited plants by treating mechanical wounds with M. sexta OS and measured the timing and magnitude of the JA and SA burst, which are reliable signatures of the plant's ''recognition'' that likely organize much of the tailoring of the defense responses. In addition to wounding, OS applications, and herbivore attack, JA and MeJA applications are also known to elicit the accumulation of some herbivore-induced defense metabolites in N. attenuata, such as nicotine (Winz and Baldwin, 2001), TrypPI (Van Dam et al, 2001;Glawe et al, 2003;Zavala et al, 2004), VOCs (Halitschke et al, 2000;Lou and Baldwin, 2003), caffeoylputrescine, chlorogenic acid, and diterpene glycosides (DTGs; Keinänen et al, 2001;Lou and Baldwin, 2003). We measured the elicited mixture of both systemic and local responses in two phyllotactically adjacent leaves.…”

“…The plants typically persist for three growing seasons after fires Preston and Baldwin, 1999) and during this time, the soil N supply decreases dramatically as postfire succession and leaching depletes soil reserves (Lynds and Baldwin, 1998). One of its induced direct defenses, nicotine, is known to make large demands on the plant's N budget (for review, see ; and others, such as trypsin proteinase inhibitors (TrypPI), are also likely to require large resource investments (Glawe et al, 2003;Zavala et al, 2004). Hence N. attenuata represents an ideal system with which to examine the effects of N supply rate on herbivore-induced signaling pathways, the transcriptional reorganization, and the direct and indirect defenses these pathways elicit.…”

A Comparison of Two Nicotiana attenuata Accessions Reveals Large Differences in Signaling Induced by Oral Secretions of the Specialist Herbivore Manduca sexta

“…Alternatively, they could reflect a greater availability of nitrogen for nicotine biosynthesis due to the lack of nitrogen-intensive TPI production in AZ. However, given that AZ produces constitutively higher amounts of nicotine, whereas ET and JA can only influence induced nicotine production, and that such high constitutive nicotine production was not seen in a line of the UT accession that was silenced for TPI (Zavala et al, 2004a), we assume that changes in certain trans-or ciselement(s) in the regulation of nicotine biosynthesis have enabled AZ to produce more nicotine than UT.…”

“…How the TPI null allele was fixed in AZ is unclear because this mutation could have considerably lowered the ability of AZ to survive under herbivory pressure. Using TPI-silenced transgenic plants, Zavala et al (2004a) demonstrated that TPI production is costly under normal conditions because plants having lower TPI levels grow faster and have higher fitness due to the re-allocation of nitrogen; however, under selection pressure from herbivory, TPI-containing plants realize greater fitness due to the effectiveness of this defense (Zavala et al, 2004a(Zavala et al, , 2004b. One hypothesis is that after the TPI mutation event in AZ, there was very low selection pressure from herbivores; therefore, this null TPI allele was quickly fixed because it provided plants with higher fitness.…”