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The bio-diversities of plants and insects are largely due to their complicated interactions. To escape or survive from attacks by herbivorous insects, plants are not only equipped with physical barriers and toxic compounds (such as cuticles, trichomes and thorns), but also initiate an intricate network of signal recognition and transduction upon insect challenge. To adapt to their host plants, insects also have developed multi-layered mechanisms, including highly specialized oral cavity for feeding and complex digestive systems for enzymatic processing of toxin-laden diets. In addition, herbivorous insects contain active molecules in their oral secretions (OS) which either trigger or interfere with plant defense during herbivory.
Cotton bollworm (Helicoverpa. armigera) and fall armyworm (Spodoptera frugiperda) are chewing and devastating pests in agriculture. Besides their preferred hosts, they are also found in the fields of many other crop species. The green mirid bug (Apolygus lucorum) is a mesophyll feeding insect. It injects digestive enzymes from its oral cavity into plant mesophyll cells and sucks the digested plant tissues. This bug is not sensitive to Bt toxin. With the wide cultivation of Bt crops and the decrease of pesticide application, the plant bug has become a major pest in the field. We use Cotton bollworm, fall armyworm and green mirid bug as the test insects to study their interactions with the main crops cotton plants (Gossypium hirsutum), maize (Zea mays) and with the model plant Arabidopsis. Our research goals are to elucidate the molecular basis of plant defenses against insect and insect strategies for fitness and to develop new strategies for pest control.

1. The molecular basis of recognition between plant and insect.

Herbivorous insects contain active molecules in their oral secretions (OS) which either trigger or interfere with plant defense during herbivory. For example, certain fatty acid-conjugates (FACs) and lipases in the OS of caterpillars can be recognized by plants and act as elicitors to induce plant defense response. Even the proteolytic fragments of chloroplastic ATP synthase β-subunit from Fabaceae plants in insect OS are able to elicit plant defense. Besides elicitors, the insect-released molecules that disturb host plant defense response are defined as insect effectors. The first reported effector-like protein in an herbivorous insect is Glucose Oxidase (GOX) from Helicoverpa zea, which inhibits nicotine accumulation in tobacco. A set of salivary glands secreted proteins from Green Peach Aphid (Myzus persicae) have distinct locations in its infected plants, indicating that these proteins may be directly involved in plant-Aphid interaction. However, compared with the well-studied microbe pathogen effectors the reports from insect effectors are limited and the mechanism of how insect effectors counteract host plant defense remains largely enigmatic. We are interested in the molecular basis of recognition of insect damages by plants and inhibition of plant defense responses by insects.

2. The regulation of plant defense signaling transduction.

In plants, the initial signal perception and transduction are essential for an appropriate defense against biotic stress. Plants can recognize different insect damages and make accurate response. The early responses include changes in membrane potential and calcium concentration, reactive oxygen compounds (ROS) burst etc. which ultimately trigger various defense signal transduction. Jasmonate (JA) is the main defense phytohormone in activating defense against biotic attacks including chewing insects. CORONATINE INSENSITIVE1 (COI1), a component of the ubiquitin E3 ligase SCFCOI1, is the first reported jasmonoyl-L-isoleucine (JA-Ile) receptor. JASMONATE-ZIM-domain (JAZ) proteins bind to transcription factors such as MYC2 to restrict JA signal output. The contents of JA and JA-Ile in plant cells are maintained at a low level in the absence of stress and rise rapidly upon external stimuli. JA-Ile promotes COI1-JAZ interaction and triggers JAZs degradation, releasing transcription factors to activate downstream defense genes. But how the membrane potential, calcium signal and ROS burst integrate into JA signaling pathway are remain elusive. Our interest is to investigate the early response of plants to insect herbivory and decipher the initial regulation mechanisms of plant defense signaling transduction.

3. Alternative splicing in plant defense pathway and their regulation.

Usually, most analysis of transcriptome data mainly focus on the gene expression level. In fact, in addition to changes in transcription level, plants also have changes in post-transcriptional processing of mRNA as well as translation and posttranslational modification upon multiple stimuli. Alternative splicing refers to a process in which a single gene produces multiple mRNA variants, including the combination of different exons, the removal or retention of different introns, which is widely existed in the process of gene transcription of plants and animals [41]. Alternative splicing mainly takes five different forms: intron retention (IR), 5 '-end (A5SS), 3' -end (A3SS), exon jump (ES) and mutex exon (MXE).
Alternative splicing increases protein diversity, affects protein potential functions. It is an important gene regulation involved in various plant physiological activities. It modulates key steps in plant growth and development, such as seed germination and flowering and it also relates to plant responses to environmental stress including phosphate uptake and pathogen resistance. What we are interested in is the changes of alternative splicing in the plant defense response, and whether these changes are directly involved in plant defense against insects.

4. Molecular basis of insect adaptations to their host plant.

Both S. frugiperda and H. armigera belong to the family Noctuidae of the order Lepidoptera. They can be fed on a wide range of crops and harm the production of various crops. S. frugiperda is more common in maize fields, while H. armigera preferred on cotton plants and seriously affects cotton production. These phenomena suggest that H. armigera and S. frugiperda have different host preferences and different adaptation mechanisms. Research of the preference and adaptability of insects to host plants is helpful for the scientific, precise and individualized control of agricultural pests. The OS of insects are rich in signaling molecules involved in the plant-insect interactions, and the midgut is an important place for digestion and detoxification of chewing insects. The midgut and OS are rich in digestive enzymes and have diverse detoxification enzymes including P450, GST and esterase. Therefore, the mouthparts and midgut of chewing insects largely determine the range of host plants the insects choose. In this study, S. frugiperda and H. armigera are investigated to elucidate the molecular mechanisms of their adaptations to host plants.