These findings underscore the critical role of this
NudC gene in orchestrating key aspects of embryonic development and its potential involvement in regulating auxin-related developmental pathways.
The importance of
NudC genes extend beyond embryonic development to postembryonic phases. The hypomorphic
bob1-3 allele introduces a broad spectrum of developmental abnormalities, including shorter roots, smaller serrated leaves, stunted branched inflorescences, and irregularities in inflorescence and floral meristem formation, leading to pin-formed meristems and abnormal floral organ numbers
[39]. It is noteworthy to mention that many of these phenotypic anomalies closely correspond to those seen in mutants associated with impaired auxin signaling or transport.
In addition, the genetic interplay of
BOB1 with
AS1 and
AS2 provides valuable insights into a developmental pathway that relies on the function of this
NudC gene.
AS1 and
AS2 serve as transcriptional regulators, contributing to the establishment of meristem boundaries by suppressing
KNOX gene expression and reinforcing adaxial polarity during leaf development
[74,75,76][58][59][60]. An allelic variant of
BOB1, known as
eal-1 and sharing the same mutation as
bob1-3, offers a unique perspective in this context. Remarkably,
eal-1 represents the sole viable allele of
BOB1 with well-documented phenotypes
[39]. When combined with
as1 and
as2 mutations,
eal-1 demonstrates leaf morphology characterized by abaxialized filamentous structures, along with an upregulation of
KNOTTED-like homeobox (
KNOX) and
ETTIN (
ETT)/
AUXIN RESPONSE FACTOR3 (
ARF3) genes.
ETT plays a role in enhancing abaxial identity and is directly regulated by the AS1–AS2 complex
[75][59]. The observed polarity abnormalities in
eal-1;
as2 plants are mitigated in an
ett genetic background, suggesting that
ETT acts downstream of
BOB1,
AS1, and AS2
[66][61].
3.3. Unveiling the Role of NudC Proteins in Plant Stress Response and Resilience
The structural similarities between the NudC domain and α-crystallin domain (ACD)/p23 proteins
[49] suggest the potential for shared functions with ACD-containing sHSPs
[77,78][62][63]. The ACD, initially identified in the chaperone of the vertebrate eye lens, spans approximately 80–100 amino acids and is primarily located in the C-terminal domain
[79][64]. It encompasses two conserved regions that form a pleated β-sheet sandwich, separated by a variable-length hydrophilic domain
[80,81][65][66]. These structural features closely resemble those observed in the human and mouse NudC homologs
[50], suggesting the possibility of common functions with sHSPs. In plants, ACD proteins serve diverse roles, from responding to abiotic stresses and hormones to regulating transcription, virus movement, and DNA demethylation
[82][67].
The Arabidopsis protein BOB1 demonstrates characteristics that align with typical sHSPs. It is induced under heat stress, possesses a NudC domain that shares structural homology with ACD-containing sHSPs, and exhibits in vitro chaperone activity, effectively preventing the aggregation of model protein substrates
[39,80,83][39][65][68]. Under normal conditions, BOB1 is primarily localized in the plant cell cytoplasm, but during heat stress, it translocates to heat shock granules in association with HSP17.6
[39,84,85][39][69][70].
BOB1, with its sHSP-like characteristics and crucial role in heat stress response, is not only involved in thermotolerance but also contributes to proteostasis. In conjunction with the 26S proteasome (26SP), it forms part of a genetic network linking proteostasis to the AS1-AS2 developmental pathway
[67][57]. The interactions within this network are believed to rely on BOB1 chaperone activities. Importantly, this network plays a vital role in repressing
KNOX gene expression, which is critical for normal plant development.
The second
NudC gene in Arabidopsis,
NMig1, also shares structural homology with ACD-containing sHSPs, and exhibits significant upregulation in response to various abiotic stressors, including heat shock
[40]. Constitutive overexpression of
NMig1 results in enhanced root growth and lateral root development, even under adverse abiotic stress conditions.
NMig1-overexpressing plants display reduced susceptibility to the inhibitory effects of abiotic stress on root morphology.
The involvement of tomato NudC homologs in the context of tomato immune responses is notably intriguing, given the broader context of the role of
NudC genes in enhancing plant stress resilience as discussed earlier. In a study conducted by Liu et al.
[68][71], a compelling connection between tomato orthologues of NudC domain proteins and SlSAP3, a member of the stress-associated protein family, came to light. SlSAP3 serves as a positive regulator of tomato immunity against
Pseudomonas syringae pv.
tomato (
Pst) DC3000. The three identified tomato SlBOB proteins, namely SlBOB1, SlBOB2, and SlBOB3, share a common NudC domain at their C termini, with variations in regions outside the NudC domain
[68][71]. Silencing
SlBOB1 or the simultaneous silencing of all three
SlBOB genes, which act as negative regulators of immunity, leads to enhanced resistance against
Pst DC3000. Hence, it appears that plant BOB proteins play diverse roles in responding to biotic and abiotic stress.
These findings further emphasize the vital role played by
NudC genes in modulating plant stress resilience, and provide valuable insights into their potential as targets for the development of stress-tolerant crops.