Infection byRhodococcus fasciansmaintains cotyledons as a sink tissue for the pathogen

Abstract

Background and Aims Pisum sativum L. (pea) seed is a source of carbohydrate and protein for the developing plant. By studying pea seeds inoculated by the cytokinin-producing bacterium, Rhodococcus fascians, we sought to determine the impact of both an epiphytic (avirulent) strain and a pathogenic strain on source-sink activity within the cotyledons during and following germination.Methods Bacterial spread was monitored microscopically, and real-time reverse transcription-quantitative PCR was used to determine the expression of cytokinin biosynthesis, degradation and response regulator gene family members, along with expression of family members of SWEET, SUT, CWINV and AAP genes -gene families identified initially in pea by transcriptomic analysis. The endogenous cytokinin content was also determined.Key Results The cotyledons infected by the virulent strain remained intact and turned green, while multiple shoots were formed and root growth was reduced. The epiphytic strain had no such marked impact. Isopentenyl adenine was elevated in the cotyledons infected by the virulent strain. Strong expression of RfIPT, RfLOG and RfCKX was detected in the cotyledons infected by the virulent strain throughout the experiment, with elevated expression also observed for PsSWEET, PsSUT and PsINV gene family members. The epiphytic strain had some impact on the expression of these genes, especially at the later stages of reserve mobilization from the cotyledons.Conclusions The pathogenic strain retained the cotyledons as a sink tissue for the pathogen rather than the cotyledon converting completely to a source tissue for the germinating plant. We suggest that the interaction of cytokinins, CWINVs and SWEETs may lead to the loss of apical dominance and the appearance of multiple shoots.

Cytokinins and Expression of SWEET, SUT, CWINV and AAP Genes Increase as Pea Seeds Germinate

“…PsCKX2 remained constitutively expressed as roots and shoots developed, whereas PsCKX1 , 3 , 5 and 7 increased substantially in the developing shoots and roots (Figure 2). Of the four response regulators, which are putative Type As [17], PsRR9 was expressed within 4 hpi. All family members showed increased expression in cotyledons at 2 dpi relative to 4 hpi.…”

“…PsRR3 and 5 were strongly expressed in the developing shoot, but much less strongly expressed in the elongating root (Figure 2). In imbibing seeds at 4 hpi, only cytokinin free bases and ribosides were detected, including both trans zeatin ( t Z) and isopentenyladenine (iP) [17], but much lesser levels of dihydrozeatin (DZ) and c Z (Table 1). No O - or N -glucosides (Table 1) nor nucleotides ([17]; Table 1) were detected at 4 hpi.…”

“…In both the legume, Medicago truncatula , and in rice, the AAP s constitute larger gene families than in Arabidopsis [13,16]. We showed previously that of the 13 PsAAP gene family members detected in a pea transcriptome, the PsAAP s were represented in most of the clusters of AAP orthologues in Arabidopsis and other leguminous species [17]. Multiple sequences were identified in several family members, particularly for PsAAP2 (Cluster 3A) and PsAAP7 (Cluster 1), as previously noted [13].…”

“…Dhandapani et al (2016) [17] showed that the 13 PsSWEET gene sequences identified from a pea transcriptome had members in all four of the SWEET clades described recently [12,29]. SWEET s have been strongly linked to development in reproductive tissues, especially seeds [12,30,31].…”

“…Our aim was to determine if the gene families known to be involved in sink activity during seed development ( SWEET s, SUT s, CWINV s, IPT , LOG , CKX ) are also involved in mobilising reserves during germination. Based on gene family members detected in our pea transcriptome [17], we used RT-qPCR to monitor their expression. We show that genes actively expressed when the seed is a strong sink during its development are also actively expressed when the seed is in the reverse role of being an active source during germination.…”