Induction of Phytohormones and Differential Gene Expression in Citrus Flowers Infected by the Fungus Colletotrichum acutatum

Abstract

Colletotrichum acutatum infects citrus petals and induces premature fruit drop and the formation of persistent calyces. The accumulation of hormones and other growth regulators, and differential gene expression in affected flowers and young fruit, was examined following fungal infection. Ethylene evolution increased threefold and indole-3-acetic acid (IAA) accumulation was as much as 140 times. Abscisic acid (ABA) levels showed no significant response. After infection, both trans- and cis-12-oxo-phytodienoic acid increased 8- to 10-fold. No significant difference of transjasmonic acid (JA) was observed in citrus flower petals or pistils. However, a fivefold increase of cis-JA was detected. The amount of salicylic acid (SA) was elevated twofold in affected petals, but not in pistils. Northern blot analyses revealed that the genes encoding ACC oxidase or ACC synthase, and 12-oxo-phytodienoic acid (12-oxo-PDA) reductase, were highly expressed in affected flowers. The genes encoding auxin-related proteins also were upregulated. Application of 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate; a putative auxin inhibitor), 2,3,5-triiodobenzolic acid (an auxin transport inhibitor), or SA after inoculation significantly decreased the accumulation of the gene transcripts of auxin-responsive, GH3-like protein and 12-oxo-PDA reductase, but resulted in higher percentages of young fruit retention. The results indicate that imbalance of IAA, ethylene, and JA in C. acutatum-infected flowers may be involved in symptom development and young fruit drop.

Physiology of citrus fruiting

“…These observations exemplify the role of endogenous auxins antagonizing or delaying citrus fruit abscission, but as suggested above there are other instances showing inductive effects of auxins on abscission. Thus, the postbloom fruit drop disease provoked by fungus infection and characterized by premature fruit abscission has been associated with increases of ethylene production and IAA and jasmonic acid (JA) amounts in the infected petals (Lahey et al, 2004). Consistently, the application of both auxin transport and action inhibitors and JA biosynthesis inhibitors after infection improved fruit retention suggesting an inductive role of IAA and JA on young fruit abscission (Chen et al, 2006).…”

Modulating plant hormones by enzyme action

“…18 GH3 transcripts have also been reported in tobacco, pungent pepper and tomato. [18][19][20][21][22] In the genome of the model monocot Oryza sativa (rice) genome, there are 13 GH3 genes. 23 Transcripts have been detected, but not characterized, in other monocots, including wheat, maize, sorghum and barley.…”

ABA Increases Susceptibility of Pepper Fruits to Infection of Anthracnose by Collectotrichum acutatum

“…Indeed, certain plant pathogens are capable of producing plant hormones (Lahey et al, 2004;Tudzynski, 2005). Biosynthesis of the plant hormone gibberellin was originally observed in Fusarium species to cause rice bakanae disease (Malonek et al, 2005).…”