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GENOTYPIC TOLERANCE OF LETTUCE SEEDS TO THERMOINHIBITION IS ASSOCIATED WITH EARLY CELL WALL MODIFICATION IN THE MICROPYLAR ENDOSPERM AND EXPRESSION OF MULTIPLE MANNANASE GENES, FeiYian Yoong

Tue Sep 12, 2017
2:15 PM - 2:30 PM
Cypress 3&4, MPH

Description

GENOTYPIC TOLERANCE OF LETTUCE SEEDS TO THERMOINHIBITION IS ASSOCIATED WITH EARLY CELL WALL MODIFICATION IN THE MICROPYLAR ENDOSPERM AND EXPRESSION OF MULTIPLE MANNANASE GENES

Yoong, F.Y.1,2, Hill, T.1, Ma, S.3, Souza, G.A.1, Reyes Chin-Wo, S.4, Huo, H1,5, Froenicke, L.4, Elmore, M.4, Michelmore, R.W.1,4 and Bradford, K.J.1

1Department of Plant Sciences, Seed Biotechnology Center, University of California, Davis, California, USA
2Maastricht Science Program, Maastricht University, Maastricht, The Netherlands
3School of Life Sciences, University of Science and Technology of China, Hefei, China
4Genome Center, University of California, Davis, California, USA
5Mid-Florida Research and Education Center, University of Florida, Apopka, Florida, USA 

Contact: Fei-Yian Yoong, yfyian@gmail.com 

 

Lettuce (Lactuca sativa) seeds (e.g., cv. Salinas) usually exhibit thermoinhibition (failure to complete germination) when imbibed at temperatures above 28°C. Genetic and molecular studies with an accession of L. serriola (US96UC23), seeds of which can germinate up to 37°C, showed that LsNCED4 (9-cis-EPOXYCAROTENOID DIOXYGENASE 4) encoding an enzyme in the ABA biosynthetic pathway is a key regulator of thermoinhibition. Silencing of LsNCED4 in the Salinas background (LsNCED4-RNAi) enabled germination at high temperatures. Seeds of a primitive L. sativa accession (PI251246) also germinate at temperatures up to 33°C, which mapping and physiological studies attributed to modified regulation of the LsERF1 (ETHYLENE RESPONSE FACTOR 1) gene. Lettuce seeds (achenes) have a two-cell-layered endosperm and a fused testa/pericarp enclosing the embryo, of which the endosperm is the primary restraint on radicle emergence. Examination of developing and mature seeds revealed no anatomical alterations in the tissues enclosing the embryo of PI251246 and LsNCED4-RNAi seeds versus Salinas seeds. At permissive temperatures for germination, structural modifications of the micropylar endosperm immediately adjacent to the radicle tip were evident within 6 h of imbibition. Cell walls of this endosperm region stained brightly with calcofluor white (indicating cellulose), while adjacent lateral endosperm cell walls did not stain, indicating that galactomannan storage carbohydrates in the cell walls are degraded by mannanase prior to radicle emergence in the micropylar endosperm. This change in cell wall structure did not occur in Salinas seeds imbibed at a thermoinhibitory temperature. Previous studies had identified LsMAN1 encoding a mannanase that appeared only after radicle emergence in the lateral endosperm of lettuce seeds. However, biochemical studies had detected mannanase activity in the micropylar endosperm tightly bound to the cell walls prior to radicle emergence. Analysis of RNASeq data identified LsMAN1 with delayed expression following imbibition, but also a distinct gene encoding a mannanase (LsMAN2) whose expression is already elevated at 6 h of imbibition only in seeds imbibed at permissive temperatures for germination. Therefore, as also occurs in tomato seeds, distinct mannanase genes are expressed in a tissue-specific temporal sequence to regulate weakening of the micropylar endosperm and subsequent mobilization of reserves in the lateral endosperm. 


Speaker:

 
FeiYian Yoong
UC Davis

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